Electrical and Electronics Engineering
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
Section
15
Electrical and Electronics
Engineering
BY
C. JAMES ERICKSON Retired Principal Consultant, E. I. du Pont de Nemours & Co., Inc.
CHARLES D. POTTS Retired Project Engineer, E. I. du Pont de Nemours & Co., Inc.
BYRON M. JONES Consulting Engineer, Assistant Professor of Electrical Engineering,
University of Wisconsin — Platteville.
15.1 ELECTRICAL ENGINEERING
by C. James Erickson
revised by Charles D. Potts
Electrical and Magnetic Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-2
Conductors and Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-4
Electrical Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-8
Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-8
Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-11
Dielectric Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-15
Transients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-16
Alternating Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-18
Electrical Instruments and Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 15-20
DC Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-25
DC Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-27
Synchronous Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-31
Induction Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-34
Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-34
Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-34
AC Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-37
AC-DC Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-41
Synchronous Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-42
Rating of Electrical Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-43
Electric Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-44
Switchboards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-44
Power Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-46
Power Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-51
Wiring Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-53
Interior Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-55
Resistor Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-61
Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-62
Automobile Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-66
15.2 ELECTRONICS
by Byron M. Jones
Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-68
Discrete-Component Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-71
Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-75
Linear Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-75
Digital Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-79
Computer Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-82
Computer Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-83
Digital Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-85
Power Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-85
Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-87
Telephone Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-90
Global Positioning Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-90
15-1
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
15.1
ELECTRICAL ENGINEERING
by C. James Erickson
revised by Charles D. Potts
REFERENCES: Knowlton, ‘‘Standard Handbook for Electrical Engineers,’’
McGraw-Hill. Pender and Del Mar, ‘‘Electrical Engineers’ Handbook,’’ Wiley.
Dawes, ‘‘Course in Electrical Engineering,’’ Vols. I and II, McGraw-Hill. Gray,
‘‘Principles and Practice of Electrical Engineering,’’ McGraw-Hill. Laws, ‘‘Electrical Measurements,’’ McGraw-Hill. Karapetoff-Dennison, ‘‘Experimental Electrical Engineering and Manual for Electrical Testing,’’ Wiley. Langsdorf, ‘‘Principles of Direct-current Machines,’’ McGraw-Hill. Hehre and Harness, ‘‘Electric
Circuits and Machinery,’’ Vols. I and II, Wiley. Timbie-Higbie, ‘‘Alternating
Current Electricity and Its Application to Industry,’’ Wiley. Lawrence, ‘‘Principles of Alternating-current Machinery,’’ McGraw-Hill. Puchstein and Lloyd,
‘‘Alternating-current Machinery,’’ Wiley. Lovell, ‘‘Generating Stations,’’
McGraw-Hill. Underhill, ‘‘Coils and Magnet Wire’’ and ‘‘Magnets,’’ McGrawHill. Abbott, ‘‘National Electrical Code Handbook,’’ McGraw-Hill. Dyke, ‘‘Automobile and Gasoline Engine Encyclopedia,’’ The Goodheart-Wilcox Co., Inc.
Fink and Carrol, ‘‘Standard Handbook for Electrical Engineers,’’ McGraw-Hill.
ELECTRICAL AND MAGNETIC UNITS
System of Units The International System of Units (SI) is being
adopted universally. The SI system has its roots in the metre, kilogram,
second (mks) system of units. Since a centimeter, gram, second (cgs)
system has been widely used, and will still be used in some instances,
Tables 15.1.1 and 15.1.2 are provided for conversion between the two
systems. Basic SI units are metre, kilogram (mass), second, ampere,
kelvin, mole (quantity), and candela (luminous intensity). Other SI units
are derived from these basic units.
Electrical Units
(See Table 15.1.1.)
Current (I, i) The SI unit of current is the ampere, which is equal to
one-tenth the absolute unit of current (abampere). The abampere of
current is defined as follows: if 0.01 metre (1 centimetre) of a circuit is
bent into an arc of 0.01 metre (1 centimetre) radius, the current is 1
abampere if the magnetic field intensity at the center is 0.01257 ampere
per metre (1 oersted), provided the remainder of the circuit produces no
magnetic effect at the center of the arc. One international ampere
(9.99835 amperes) (dc) will deposit 0.001118 gram per second of silver
from a standard silver solution.
Quantity (Q) The coulomb is the quantity of electricity transported
in one second by a current of one ampere.
Potential Difference or Electromotive Force (V, E, emf) The volt is
the difference of electric potential between two points of a conductor
carrying a constant current of one ampere, when the power dissipated
between these points is equal to one watt.
Resistance (R, r) The ohm is the electrical resistance between two
points of a conductor when a constant difference of potential of one
volt, applied between these two points, produces in this conductor a
current of one ampere, this conductor not being the source of any electromotive force.
Resistivity (␳) The resistivity of a material is the dc resistance between the opposite parallel faces of a portion of the material having unit
length and unit cross section.
Conductance (G, g) The siemens is the electrical conductance of a
conductor in which a current of one ampere is produced by an electric
potential difference of one volt. One siemens is the reciprocal of one
ohm.
Conductivity (␥ ) The conductivity of a material is the dc conductance between the opposite parallel faces of a portion of the material
having unit length and unit cross section.
Capacitance (C) is that property of a system of conductors and dielectrics which permits the storage of electricity when potential difference
15-2
exists between the conductors. Its value is expressed as a ratio of a
quantity of electricity to a potential difference. A capacitance value is
always positive. The farad is the capacitance of a capacitor between the
plates of which there appears a difference of potential of one volt when
it is charged by a quantity of electricity equal to one coulomb.
Permittivity or dielectric constant (␧ 0 ) is the electrostatic energy stored
per unit volume of a vacuum for unit potential gradient. The permittivity
of a vacuum or free space is 8.85 ⫻ 10⫺ 12 farads per metre.
Relative Permittivity or Dielectric Constant (␧ r ) is the ratio of electrostatic energy stored per unit volume of a dielectric for a unit potential
gradient to the permittivity (␧ 0 ) of a vacuum. The relative permittivity is
a number.
Self-inductance (L) is the property of an electric circuit which determines, for a given rate of change of current in the circuit, the emf
induced in the same circuit. Thus e1 ⫽ ⫺ Ldi1 /dt, where e1 and i1 are in
the same circuit and L is the coefficient of self-inductance.
The henry is the inductance of a closed circuit in which an electromotive force of one volt is produced when the electric current varies uniformly at a rate of one ampere per second.
Mutual inductance (M) is the common property of two associated
electric circuits which determines, for a given rate of change of current
in one of the circuits, the emf induced in the other. Thus e1 ⫽ ⫺ Mdi2 /dt
and e2 ⫽ ⫺ Mdi1 /dt, where e1 and i1 are in circuit 1; e2 and i2 are in
circuit 2; and M is the mutual inductance.
The henry is the mutual inductance of two separate circuits in which
an electromotive force of one volt is produced in one circuit when the
electric current in the other circuit varies uniformly at a rate of one
ampere per second.
If M is the mutual inductance of two circuits and k is the coefficient of
coupling, i.e., the proportion of flux produced by one circuit which links
the other, then M ⫽ k(L 1 L 2 )1/2, where L 1 and L 2 are the respective
self-inductances of the two circuits.
Energy (J ) in a system is measured by the amount of work which a
system is capable of doing. The joule is the work done when the point of
application of a force of one newton is displaced a distance of one metre
in the direction of the force.
Power (W) is the time rate of transferring or transforming energy. The
watt is the power which gives rise to the production of energy at the rate
of one joule per second.
Active power (P) at the points of entry of a single-phase, two-wire
circuit or of a polyphase circuit is the time average of the values of the
instantaneous power at the points of entry, the average being taken over
a complete cycle of the alternating current. The value of active power is
given in watts when the rms currents are in amperes and the rms potential differences are in volts. For sinusoidal emf and current, P ⫽ EI
cos ␪, where E and I are the rms values of volts and currents, and ␪ is the
phase difference of E and I.
Reactive power (Q) at the points of entry of a single-phase, two-wire
circuit, or for the special case of a sinusoidal current and sinusoidal
potential difference of the same frequency, is equal to the product obtained by multiplying the rms value of the current by the rms value of
the potential difference and by the sine of the angular phase difference
by which the current leads or lags the potential difference. Q ⫽ EI sin ␪.
The unit of Q is the var (volt-ampere-reactive). One kilovar ⫽ 103 var.
Apparent power (EI ) at the points of entry of a single-phase, two-wire
circuit is equal to the product of the rms current in one conductor multiplied by the rms potential difference between the two points of entry.
Apparent power ⫽ EI.
Power factor (pf ) is the ratio of power to apparent power. pf ⫽ P/EI ⫽
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
ELECTRICAL AND MAGNETIC UNITS
Table 15.1.1
15-3
Electrical Units
Quantity
Symbol
Current
Quantity
Electromotive force
Resistance
Resistivity
Conductance
Conductivity
Capacitance
Permittivity
Relative permittivity
Self-inductance
Mutual inductance
Energy
I, i
Q, q
E, e
R, r
␳
G, g
␥
C
␧
␧r
L
M
J
kWh
W
jQ
VA
pf
XL
XC
Z
G
B
Y
f
T
T
␻
Active power
Reactive power
Apparent power
Power factor
Reactance, inductive
Reactance, capacitive
Impedence
Conductance
Susceptance
Admittance
Frequency
Period
Time constant
Angular velocity
Equation
SI unit
I ⫽ E/R; I ⫽ E/Z; I ⫽ Q/t
Q ⫽ it; Q ⫽ CE
E ⫽ IR; E ⫽ W/Q
R ⫽ E/I; R ⫽ ␳ l /A
␳ ⫽ RA/l
G ⫽ ␥A /l; G ⫽ A/␳ l
␥ ⫽ 1/␳; ␥ ⫽ l/RA
C ⫽ Q/E
␧r ⫽ ␧/␧0
L ⫽ ⫺ N(d␾/dt)
M ⫽ K(L1L2 )1/2
J ⫽ eit
kWh ⫽ kW/3600; 3.6 M J
W ⫽ J/t; W ⫽ EI cos ␪
Q ⫽ EI sin ␪
VA ⫽ EI
pf ⫽ W/VA; pf ⫽ W/(W ⫹ jQ)
XL ⫽ 2␲ f L
XC ⫽ 1/(2␲ fC )
Z ⫽ E/I; Z ⫽ R ⫹ j(XL ⫺ XC )
G ⫽ R /Z 2
B ⫽ X/Z 2
Y ⫽ I/E; Y ⫽ G ⫹ jB
f ⫽ 1/ T
T ⫽ 1/ f
L /R; RC
␻ ⫽ 2␲ f
SI unit
symbol
Ampere
Coulomb
Volt
Ohm
Ohm-metre
Siemens
Siemens/meter
Farad
Farads/meter
Numerical
Henry
Henry
Joule
Kilowatthour
Watt
Var
Volt-ampere
A
C
V
⍀
⍀⭈m
S
S/m
F
F/m
H
H
J
kWh
W
var
VA
Ohm
Ohm
Ohm
Siemens
Siemens
Siemens
Hertz
Second
Second
Radians /second
⍀
⍀
⍀
S
S
S
Hz
s
s
rad/s
CGS unit
Abampere
Abcoulomb
Abvolt
Abohm
Abohm-cm
Abmho
Abmho/cm
Abfarad*
Stat farad*/cm
Numerical
Abhenry
Abhenry
Erg
Abwatt
Abvar
Abohm
Abohm
Abohm
Abmho
Abmho
Abmho
Cps, Hz
Second
Second
Radians /second
Ratio of magnitude
of SI to cgs unit
10⫺1
10⫺1
108
109
1011
10⫺9
10⫺11
10⫺9
8.85 ⫻ 10⫺12
1
109
109
107
36 ⫻ 1012
107
107
1
109
109
109
10⫺9
10⫺9
10⫺9
1
1
1
1
* 1 Abfarad (EMU Units) ⫽ 9 ⫻ 10⫺20 stat farads (ESU units).
cos ␪, where ␪ is the phase difference between E and I, both assumed to
be sinusoidal.
The reactance (X) of a portion of a circuit for a sinusoidal current and
potential difference of the same frequency is the product of the sine of
the angular phase difference between the current and potential difference times the ratio of the rms potential difference to the rms current,
there being no source of power in the portion of the circuit under consideration. X ⫽ (E/I ) sin ␪ ⫽ 2␲ fL ohms, where f is the frequency, and
L the inductance in henries; or X ⫽ 1/2␲ fC ohms, where C is the
capacitance in farads.
The impedance (Z) of a portion of an electric circuit to a completely
specified periodic current and potential difference is the ratio of the rms
value of the potential difference between the terminals to the rms value
of the current, there being no source of power in the portion under
consideration. Z ⫽ E/I ohms.
Admittance (Y ) is the reciprocal of impedance. Y ⫽ I/E siemens.
The susceptance (B) of a portion of a circuit for a sinusoidal current
and potential difference of the same frequency is the product of the sine
of the angular phase difference between the current and the potential
difference times the ratio of the rms current to the rms potential differTable 15.1.2
ence, there being no source of power in the portion of the circuit under
consideration. B ⫽ (I/E) sin ␪.
Magnetic Units
(See Table 15.1.2.)
Magnetic flux (⌽, ␾) is the magnetic flow that exists in any magnetic
circuit.
The weber is the magnetic flux which, linking a circuit of one turn,
produces in it an electromotive force of one volt as it is reduced to zero
at a uniform rate in one second.
Magnetic flux density (␤) is the ratio of the flux in any cross section to
the area of that cross section, the cross section being taken normal to the
direction of flux.
The tesla is the magnetic flux density given by a magnetic flux of one
weber per square metre.
Unit magnetic pole, when concentrated at a point and placed one metre
apart in a vacuum from a second unit magnetic pole, will repel or attract
the second unit pole with a force of one newton.
The weber is the magnetic flux produced by a unit pole.
Magnetic Units
Quantity
Symbol
Equation*
SI unit
Magnetic flux
Magnetic flux density
Pole strength
⌽, ␾
␤
Qm
␾ ⫽ F/R
␤ ⫽ ␾/A
Qm ⫽ F/␤; Qm ⫽ Fl /NI␮ 0␮r
Magnetomotive force
Magnetic field intensity
Permeability air
Relative permeability
Reluctivity
Permeance
Reluctance
Ᏺ
H
␮0
␮r
␥
P
R
Ᏺ ⫽ NI
H ⫽ Ᏺ/l
␮0 ⫽ ␤/H
␮ r ⫽ ␮ /␮ 0
␥ ⫽ 1/␮r
P ⫽ ␮ 0␮ r A/l
R ⫽ l/␮ 0␮ r A
Weber
Tesla
Ampere-turns-metre
Unit pole
Ampere-turns
Ampere-turns per metre
Henry per metre
Numeric
Numeric
Henry
1/ Henry
* l ⫽ length in metres; A ⫽ area in square metres; F ⫽ force in newtons; N ⫽ number of turns.
SI unit
symbol
wb
T
A⭈m
A
A /m
H /m
H
1/ H
CGS unit
Maxwell
Gauss
Unit pole
Gilbert
Oersted
Gilbert per oersted
Numeric
Numeric
Ratio of magnitude
of SI to cgs unit
108
104
0.7958 ⫻ 107
1.257
0.01257
1.257 ⫻ 10⫺6
1
1
7.96 ⫻ 107
1.257 ⫻ 10⫺8
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15-4
ELECTRICAL ENGINEERING
Magnetomotive force (Ᏺ, mmf ) produces magnetic flux and corresponds to electromotive force in an electric circuit.
The ampere (turn) is the unit of mmf.
Magnetic field intensity (H) at a point is the vector quantity which is
measured by a mechanical force which is exerted on a unit pole placed
at the point in a vacuum.
An ampere per metre is the unit of field intensity.
Permeability (␮) is the ratio of unit magnetic flux density to unit
magnetic field intensity in air (B/H). The permeability of air is 1.257 ⫻
10⫺ 6 henry per metre.
Relative permeability (␮r ) is the ratio of the magnetic flux in any
element of a medium to the flux that would exist if that element were
replaced with air, the magnetomotive force (mmf ) acting on the element
remaining unchanged (␮r ⫽ ␮ /␮0 ).
The relative permeability is a number.
Permeance (P) of a portion of a magnetic circuit bounded by two
equipotential surfaces, and by a third surface at every point of which
there is a tangent having the direction of the magnetic induction, is the
ratio of the flux through any cross section to the magnetic potential
difference between the surfaces when taken within the portion under
consideration. The equation for the permeance of the medium as defined
above is P ⫽ ␮ 0 ␮ r A/l. Permeance is the reciprocal of reluctance.
Reluctivity (␥) of a medium is the reciprocal of its permeability.
Reluctance (R) is the reciprocal of permeance. It is the resistance to
magnetic flow. In a homogeneous medium of uniform cross section,
reluctance is equal to the length divided by the product of the area and
permeability, the length and area being expressed in metre units. R ⫽
l/A␮ 0 ␮ r , where ␮ 0 ⫽ 1.257 ⫻ 10⫺ 6.
CONDUCTORS AND RESISTANCE
Resistivity, or specific resistance, is the resistance of a sample of the mate-
rial having both a length and cross section of unity. The two most
common resistivity samples are the centimetre cube and the cir mil ⭈ ft.
If l is the length of a conductor of uniform cross section a, then its
resistance is
R ⫽ ␳ l/a
(15.1.1)
where ␳ is the resistivity. With a cir mil ⭈ ft ␳ is the resistance of a cir
mil ⭈ ft and a is the cross section, cir mils. Since v ⫽ la is the volume of a
conductor,
R ⫽ ␳ l 2/v ⫽ ␳ v/a 2
(15.1.2)
A circular mil is a unit of area equal to that of a circle whose diameter
is 1 mil (0.001 in). It is the unit of area which is used almost entirely in
this country for wires and cables. To obtain the cir mils of a solid
cylindrical conductor, square its diameter expressed in mils. For example, the diameter of 000 AWG solid copper wire is 410 mils and its cross
section is (410)2 ⫽ 168,100 cir mils. The diameter in mils of a solid
cylindrical conductor is the square root of its cross section expressed in
cir mils.
A cir mil ⭈ ft is a conductor having a length of 1 ft and a uniform cross
section of 1 cir mil. In terms of the copper standard the resistance of a
cir mil ⭈ ft of copper at 20°C is 10.371 ⍀. As a first approximation 10 ⍀
may frequently be used.
At 60°C a cir mil ⭈ in of copper has a resistance of 1.0 ⍀. This is a
very convenient unit of resistivity for magnet coils since the resistance
is merely the length of copper in inches divided by its cross section in cir
mils.
Temperature Coefficient of Resistance The resistance of the pure
metals increases with temperature. The resistance at any temperature
t°C is
R ⫽ R 0 (1 ⫹ ␣t)
(15.1.3)
where R 0 is the resistance at 20°C and ␣ is the temperature coefficient of
resistance. For copper, ␣ ⫽ 0.00393.
With any initial temperature t1 , the resistance at temperature t°C is
R ⫽ R 1 [1 ⫹ ␣1(t ⫺ t1 )]
Table 15.1.3 Properties of Metals and Alloys
(See Table 15.1.27 for properties of resistor alloys)
Resistivity, 20°C
Metals
␮⍀ ⭈ cm
⍀ ⭈ cir mil /ft
Aluminum
Antimony
Bismuth
Brass
Carbon: amorphous
Retort (graphite)
Copper (drawn)
Gold
Iron: electrolytic
Cast
Wire
Lead
Molybdenum
Monel metal
Mercury
Nickel
Platinum
Platinum silver, 2Ag ⫹ 1Pt
Silver
Steel: soft
Glass hard
Silicon (4 percent)
Transformer
Trolley wire
Tin
Tungsten
Zinc
2.828
42.1
111.0
6.21
3,800 – 4,100
720 – 812*
1.724
2.44
10.1
75.2 – 98.8
97.8
22.0
5.78
43.5
96.8
8.54
10.72
24.6†
1.628
15.9
45.7
51.18
11.09
12.7
11.63
5.51
5.97
17.01
251.0
668.0
37.0
...........
...........
10.37
14.7
59.9
448 – 588
588
132
34.8
262
576
50.8
63.8
148.0
9.8
95.8
275
308
66.7
76.4
70
33.2
35.58
NOTE: Max working temperature: Cu, 260°C; Ni, 600°C; Pt, 1,500°C.
* Furnace electrodes, 3,000°C.
† 0°C.
Temperature coefficient
of resistance at 20°C
0.00403
0.0036
0.004
0.0015
(⫺)
(⫺)
0.00393
0.0034
0.0064
0.00387
0.0019
0.00089
0.0041
0.003
0.00031
0.0038
0.0016
0.0042
0.005
0.0037
(15.1.4)
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CONDUCTORS AND RESISTANCE
where R 1 is the resistance at temperature t1°C and ␣1 is the temperature
coefficient of resistance at temperature t1 [see Eq. (15.1.5)].
For any initial temperature t1 the value of ␣1 is
␣1 ⫽ 1/(234.5 ⫹ t1 )
(15.1.5)
Inferred Absolute Zero Between 100 and 0°C the resistance of cop-
per decreases at a rate which is practically uniform and which if continued would give a resistance of zero at ⫺ 234.5°C (an easy number to
remember). If the resistance at t1°C is R 1 and the resistance at t2°C is R 2 ,
then
R 2 /R 1 ⫽ (234.5 ⫹ t2 )/(234.5 ⫹ t1 )
(15.1.6)
EXAMPLE. The resistance of a copper coil at 25°C is 4.26 ⍀. Determine its
resistance at 45°C. Using Eq. (15.1.4) and ␣1 ⫽ 1/(234.5 ⫹ 25) ⫽ 0.00385, R ⫽
4.26[1 ⫹ 0.00385(45 ⫺ 25)] ⫽ 4.59 ⍀. Using Eq. (15.1.6) R ⫽ 4.26(234.5 ⫹
45)/(234.5 ⫹ 25) ⫽ 4.26 ⫻ 1.077 ⫽ 4.59 ⍀.
The inferred absolute zero for aluminum is ⫺ 228°C.
Materials The materials generally used for the transmission and
distribution of electrical energy are copper, aluminum, and sometimes
iron and steel. For resistors and heaters, iron, steel, commercial alloys,
and carbon are most used.
Copper is the most widely used electrical conductor. It has high conductivity, relatively low cost, good resistance to oxidation, is readily
soldered, and has good mechanical characteristics such as tensile
strength, toughness, and ductility. Its tensile strength together with its
low linear temperature coefficient of expansion are desirable characteristics in its use for overhead transmission lines. The international copper
standard for 100 percent conductivity annealed copper is a density of
8.89 g/cm3 (0.321 lb/in 3 ) and resistivity is given in Table 15.1.3.
ASTM specifications for minimum conductivities of copper wire are as
follows:
Conductor
diam, in
Soft or
annealed
Medium
hard drawn
Hard
drawn
0.040 – 0.324
0.325 – 0.460
98.16%
98.16%
96.60%
97.66%
96.16%
97.16%
Table 15.1.4 Working Table, Standard Annealed Copper Wire, Solid
[American Wire Gage (B & S)]
⍀ per 1,000 ft
Cross section
65°C
(⫽149°F)
⍀/mi at
25°C
(⫽77°F)
0.0500
0.0630
0.0795
0.100
0.126
0.0577
0.0727
0.0917
0.116
0.146
0.264
0.333
0.420
0.528
0.665
641.0
508.0
403.0
319.0
253.0
0.0521
0.0413
0.0328
0.0260
0.0206
0.159
0.201
0.253
0.319
0.403
0.184
0.232
0.292
0.369
0.465
0.839
1.061
1.335
1.685
2.13
201.0
159.0
126.0
100.0
79.5
20,800
16,500
13,100
10,400
8,230
0.0164
0.0130
0.0103
0.00815
0.00647
0.508
0.641
0.808
1.02
1.28
0.586
0.739
0.932
1.18
1.48
2.68
3.38
4.27
5.38
6.75
63.0
50.0
39.6
31.4
24.9
81.0
72.0
64.0
57.0
51.0
6,530
5,180
4,110
3,260
2,580
0.00513
0.00407
0.00323
0.00256
0.00203
1.62
2.04
2.58
3.25
4.09
1.87
2.36
2.97
3.75
4.73
8.55
10.77
13.62
17.16
21.6
19.8
15.7
12.4
9.86
7.82
17
18
19
20
21
45.0
40.0
36.0
32.0
28.5
2,050
1,620
1,290
1,020
810
0.00161
0.00128
0.00101
0.000802
0.000636
5.16
6.51
8.21
10.4
13.1
5.96
7.51
9.48
11.9
15.1
27.2
34.4
43.3
54.9
69.1
6.20
4.92
3.90
3.09
2.45
22
23
24
25
26
25.3
22.6
20.1
17.9
15.9
642
509
404
320
254
0.000505
0.000400
0.000317
0.000252
0.000200
16.5
20.8
26.2
33.0
41.6
19.0
24.0
30.2
38.1
48.0
87.1
109.8
138.3
174.1
220
1.94
1.54
1.22
0.970
0.769
27
28
29
30
31
14.2
12.6
11.3
10.0
8.9
202
160
127
101
79.7
0.000158
0.000126
0.0000995
0.0000789
0.0000626
52.5
66.2
83.4
105
133
60.6
76.4
96.3
121
153
277
350
440
554
702
0.610
0.484
0.384
0.304
0.241
32
33
34
35
36
8.0
7.1
6.3
5.6
5.0
63.2
50.1
39.8
31.5
25.0
0.0000496
0.0000394
0.0000312
0.0000248
0.0000196
167
211
266
335
423
193
243
307
387
488
882
1,114
1,404
1,769
2,230
0.191
0.152
0.120
0.0954
0.0757
37
38
39
40
4.5
4.0
3.5
3.1
19.8
15.7
12.5
9.9
0.0000156
0.0000123
0.0000098
0.0000078
533
673
848
1,070
616
776
979
1,230
2,810
3,550
4,480
5,650
0.0600
0.0476
0.0377
0.0200
Gage
no.
Diam,
mils
cir mils
in2
0000
000
00
0
1
460.0
410.0
365.0
325.0
289.0
212,000
168,000
133,000
106,000
83,700
0.166
0.132
0.105
0.0829
0.0657
2
3
4
5
6
258.0
229.0
204.0
182.0
162.0
66,400
52,600
41,700
33,100
26,300
7
8
9
10
11
144.0
128.0
114.0
102.0
91.0
12
13
14
15
16
15-5
25°C
(⫽77°F)
Weight
per 1,000
ft, lb
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15-6
ELECTRICAL ENGINEERING
Aluminum is used to considerable extent for high-voltage transmission lines, because its weight is one-half that of copper for the same
conductance. Moreover, the greater diameter reduces corona loss. As it
has 1.4 times the linear temperature coefficient of expansion, changes in
sag with temperature are greater. Because of its lower melting point,
spans may fail more readily with arc-overs. In aluminum cable steelreinforced (ACSR), the center strand is a steel cable, which gives added
tensile strength. Aluminum is used occasionally for bus bars because of
its large heat-dissipating surface for a given conductance. The greater
cross section for a given conductance requires a greater volume of insulation for a given voltage. When the ratio of the cost of aluminum to the
cost of copper becomes economically favorable, aluminum is often used
for insulated wires and cables. The international aluminum standard for
62 percent conductivity aluminum is a density of 2.70 g/cm3
(0.0976 lb/in 3 ) and resistivity as given in Table 15.1.3.
Steel, either galvanized or copper-covered (‘‘copperweld’’), is used for
high-voltage transmission spans where tensile strength is more important than high conductance. Steel is also used for third rails.
Copper alloys and bronzes are of increasing importance as electrical
conductors. They have lower electrical conductivity but greater tensile
strength and are resistant to corrosion. Hitenso, Calsum bronzes, Signal
bronze, Phono-electric, and Everdur are bronzes containing phosphorus,
silicon, manganese, or zinc. Their conductivities vary from 20 to 85
percent of 100 percent conductivity copper, and they have tensile
strengths up to 130,000 lb/in 2, about twice that of hard-drawn copper.
Such alloys were frequently used for trolley wires. Copper alloys having lower conductivity are usually classified as resistor materials.
In Table 15.1.3 are given the electrical properties of some of the pure
metals and alloys.
American Wire Gage (AWG) The AWG (formerly Brown & Sharpe
gage) is based on a constant ratio between diameters of successive gage
numbers (Table 15.1.4). The ratio of any diameter to the next smaller is
1.123, and the corresponding ratio of cross sections is (1.123)2 ⫽ 1.261,
or 11⁄4 approximately. (1.123)6 is 2.0050, so that diameters differing by
6 gage numbers have a ratio of approximately 2; cross sections differing
Table 15.1.5
by 3 gage numbers also have a ratio of approximately 2. The ratio of
cross sections differing by 2 numbers is (1.261)2 ⫽ 1.590, or 1.6 approximately. The ratio of cross sections differing by 10 numbers is
approximately 10. The gage ordinarily extends from no. 40 to 0000
(4/0). Wires larger than 0000 must be stranded, and their cross section is
given in cir mils.
The diameter of no. 10 wire is 102.0 mils. As an approximation this
may be considered as being 100 mils; the cross section is 10,000 cir
mils; the resistance is 1 ⍀ per 1,000 ft; and the weight of 1,000 ft is
31.4(10␲) lb. Also the weight of 1,000 ft of no. 2 is 200 lb. These facts
give many short cuts in estimating resistances and weights of various
gage numbers.
Lay Cables In order to obtain sufficient flexibility, wires larger than
0000 are stranded, and they are designated by their circular mils (Table
15.1.5). Smaller wires may be stranded also since sizes as small as no. 4
when insulated are usually too stiff for easy handling. Lay cables are
made up geometrically as shown in Fig. 15.1.1. Six strands will just fit
around the single central conductor; the number of strands in each succeeding layer increases by 6. The number of strands that can thus be laid
up are 1, 7, 19, 37, 61, 91, 127, etc. In order to obtain sufficient flexibility with large cables, the strands themselves frequently consist of
stranded cable.
Fig. 15.1.1
Makeup of a 19-strand cable.
The resistance of cables is readily computed from Eq. (15.1.1), using
the cir mil ft as the unit of resistivity.
EXAMPLE. Determine the resistance of 3,500 ft of 800,000 cir mil cable at
20°C. Answer: ␳ (of a cir mil ⭈ ft) ⫽ 10.37. R ⫽ 10.37 ⫻ 3,500/800,000 ⫽
0.0454 ⍀.
␳ ⫽ 10 ⍀/cir mil ⭈ ft is often sufficiently accurate for practical purposes.
Bare Concentric Lay Cables of Standard Annealed Copper
⍀ per 1,000 ft
Standard concentric standing
cir mils
25°C
(⫽77°F)
65°C
(⫽149°F)
Weight
per 1,000
ft, lb
No.
of wires
Diam of
wires, mils
Outside
diam, mils
2,000,000
1,700,000
1,500,000
1,200,000
1,000,000
0.00539
0.00634
0.00719
0.00899
0.0108
0.00622
0.00732
0.00830
0.0104
0.0124
6,180
5,250
4,630
3,710
3,090
127
127
91
91
61
125.5
115.7
128.4
114.8
128.0
1,631
1,504
1,412
1,263
1,152
900,000
850,000
750,000
650,000
600,000
0.0120
0.0127
0.0144
0.0166
0.0180
0.0138
0.0146
0.0166
0.0192
0.0207
2,780
2,620
2,320
2,010
1,850
61
61
61
61
61
121.5
118.0
110.9
103.2
99.2
1,093
1,062
998
929
893
550,000
500,000
450,000
400,000
0.0196
0.0216
0.0240
0.0270
0.0226
0.0249
0.0277
0.0311
1,700
1,540
1,390
1,240
61
37
37
37
95.0
116.2
110.3
104.0
855
814
772
728
0000
000
350,000
300,000
250,000
212,000
168,000
0.0308
0.0360
0.0431
0.0509
0.0642
0.0356
0.0415
0.0498
0.0587
0.0741
1,080
926
772
653
518
37
37
37
19
19
97.3
90.0
82.2
105.5
94.0
681
630
575
528
470
00
0
1
2
3
133,000
106,000
83,700
66,400
52,600
0.0811
0.102
0.129
0.162
0.205
0.0936
0.117
0.149
0.187
0.237
411
326
258
205
163
19
19
19
7
7
83.7
74.5
66.4
97.4
86.7
418
373
332
292
260
4
41,700
0.259
0.299
129
7
77.2
232
AWG
no.
NOTE: See Table 15.1.21 for the carrying capacity of wires.
SOURCE: From NBS Cir. 31.
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CONDUCTORS AND RESISTANCE
Fig. 15.1.2 Diagrammatic symbols for electrical machinery and apparatus. (American Standard, ‘‘Graphic Symbols
for Electrical and Electronic Diagrams,’’ ANS/IEEE, 315, 1975.)
15-7
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15-8
ELECTRICAL ENGINEERING
ELECTRICAL CIRCUITS
Applying Kirchhoff’s second law to circuit abcdea,
Figure 15.1.2 shows standard symbols for electrical circuit diagrams.
Ohm’s law states that, with a steady current, the current in a circuit is
directly proportional to the total emf acting in the circuit and is inversely
proportional to the total resistance of the circuit. The law may be expressed by the following three equations:
I ⫽ E/R
E ⫽ IR
R ⫽ E/I
(15.1.7)
(15.1.8)
(15.1.9)
where E is the emf, V; R the resistance, ⍀; and I the current, A.
Series Circuits The combined resistance of a number of series-connected resistors is the sum of their separate resistances. When batteries
or other sources of emf are connected in series, the total emf of the
combination is the sum of the separate emfs. The open-circuit emf of a
battery is the total generated emf and can be measured at the battery
terminals only when no current is being delivered by the battery. The
internal resistance is the resistance of the battery alone. The current in a
circuit connected in series with a source of emf is I ⫽ E/(R ⫹ r), where
E is the open-circuit emf, R the external resistance, and r the internal
resistance of the source of emf.
Parallel Circuits The combined conductance of a number of parallel-connected resistors is the sum of their separate conductances.
G ⫽ G1 ⫹ G2 ⫹ G3 ⫹ ⭈ ⭈ ⭈
1
1
1
1
⫹
⫹
⫹⭈⭈⭈
⫽
R
R1
R2
R3
(15.1.11)
(15.1.13)
and for four parallel resistors having resistances R 1 , R 2 , R 3 , R 4
R⫽
R1R2R3R4
R1R2R3 ⫹ R2R3R4 ⫹ R3R4R1 ⫹ R4R1R2
(I)
⫺2 ⫹ 0.1I2 ⫹ 3I2 ⫹ I3 ⫹ 3 ⫹ 0.3I3 ⫽ 0
⫹1 ⫹ 3.1I2 ⫹ 1.3I3 ⫽ 0
(II)
and for edcfge.
or
Applying Kirchhoff’s first law to junction c,
⫺I1 ⫺ I2 ⫹ I3 ⫽ 0
(III)
Solving (I), (II), and (III) simultaneously gives I1 ⫽ ⫺2.56, I2 ⫽ ⫹0.53, and
I3 ⫽ ⫺2.03. The minus signs before I1 and I3 show that the actual directions of
these two currents are opposite the assumed directions.
Fig. 15.1.3
Electric network and Kirchhoff’s laws.
Electrical Power With direct currents the electrical power is given
by the product of the volts and amperes. That is,
P ⫽ EI
(15.1.12)
The equivalent resistance for three parallel resistors having resistances R 1 , R 2 , R 3 is
R1R2R3
R⫽
R1R2 ⫹ R2R3 ⫹ R3R1
⫹4 ⫹ 0.2 I1 ⫹ 0.5I1 ⫺ 3I2 ⫹ 2 ⫺ 0.1I2 ⫹ I1 ⫽ 0
⫹6 ⫹ 1.7I1 ⫺ 3.1I2 ⫽ 0
(15.1.10)
The equivalent resistance for two parallel resistors having resistances
R 1 , R 2 is
R ⫽ R 1 R 2 /(R 1 ⫹ R 2 )
or
(15.1.14)
To obtain the resistance of combined series and parallel resistors, the
equivalent resistance of each parallel portion is obtained separately and
then these equivalent resistances are added to the series resistances
according to the principles stated above.
Kirchhoff’s laws (derived from Ohm’s law) make it possible to solve
many circuit networks that would otherwise be difficult to solve. The
first law states that: In any branching network of wires the algebraic
sum of the currents in all the wires that meet at a point is zero. The
second law states that: The sum of all the electromotive forces acting
around a complete circuit is equal to the sum of the resistances of its
separate parts multiplied each by the strength of the current in it, or the
total change of potential around any closed circuit is zero.
In applying Kirchhoff’s laws the following rules should be observed.
Currents going toward a junction should be preceded by a plus sign.
Currents going away from a junction should be preceded by a minus
sign. A rise in potential should be preceded by a plus sign. (This occurs
in going through a source of emf from the negative to the positive
terminal, and in going through resistance in opposition to the direction
of current.) A drop in potential should be preceded by a minus sign.
(This occurs in going through a source of emf from the positive to the
negative terminal and in going through resistance in conjunction with
the current.)
The application of Kirchhoff’s laws is illustrated by the following
example.
EXAMPLE. Determine the three currents I1 , I2 , and I3 in the circuit network
(Fig. 15.1.3). The arrows show the assumed directions of the three currents.
W
(15.1.15)
Also, by substituting for E and I Eqs. (8) and (7),
P ⫽ I 2R
P ⫽ E 2/R
W
W
(15.1.16)
(15.1.17)
The watt is too small a unit for many purposes. Hence, the kilowatt
(kW) is used. 746 watts ⫽ 1 hp ⫽ 0.746 kW; 1 kW ⫽ 1.340 hp. The
kilowatthour (kWh) is the common engineering unit of electrical energy.
Joule’s Law When an electric current flows through resistance, the
number of heat units developed is proportional to the square of the
current, directly proportional to the resistance, and directly proportional
to the time that the current flows. h ⫽ i 2rt, where h ⫽ number of joules;
i ⫽ current, A; r ⫽ resistance, ⍀; and t ⫽ time, s. h (in Btu) ⫽
0.0009478i 2rt.
MAGNETISM
Magnetic Circuit The magnetic circuit is analogous to the electric
circuit in that the flux ⌽ is proportional to the magnetomotive force Ᏺ
and inversely proportional to the reluctance ᏾ or magnetic resistance.
Thus
⌽ ⫽ Ᏺ/᏾
(15.1.18)
Compare with Eq. (15.1.7). ⌽ is in webers, where the weber is the SI
unit of flux, Ᏺ in ampere-turns, and ᏾ in SI reluctance units. In the cgs
system, ␾ is in maxwells, Ᏺ is in gilberts, and ᏾ is in cgs reluctance
units.
᏾ ⫽ l/␮ r ␮ v A
(15.1.19)
where ␮ r is relative permeability (commonly called permeability, ␮), a
property of the magnetic material, and ␮ v is the permeability of evacuated space ⫽ 4␲ ⫻ 10⫺7, and A is in square metres. In the cgs system
␮v ⫽ 1
᏾⫽
l
l
⫽
␮ r (4␲ ⫻ 10⫺ 7 )A
␮ r (1.257 ⫻ 10⫺ 6 )A
l is in metres and A in square metres.
(15.1.20)
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MAGNETISM
The unit of flux density in the SI system is the tesla, which is equal to
the number of webers per square metre taken perpendicular to their
direction. One ampere-turn between opposite faces of a metre cube of a
magnetic medium produces ␮ r tesla. For air, ␮ r ⫽ 4␲ ⫻ 10⫺7. In the
cgs system the unit of flux density is gauss ⫽ 10 4 T (see Table 15.1.2).
Magnetic-circuit calculations cannot be made with the same degree
of accuracy as electric-circuit calculations because of several factors.
The cross-sectional dimensions of the magnetic circuit are large relative
to its length; magnetic paths are irregular, and their geometry can only
be approximated as with the air gap of electric machines, which usually
have slots on one or both sides of the gap.
Magnetic flux cannot be confined to definite magnetic paths, but a
considerable proportion usually takes paths external to the circuit giving
magnetic leakage (see Fig. 15.1.7). The relative permeability of iron
varies over wide ranges with the flux density and with the previous
magnetic condition (see Fig. 15.1.5). These variations of relative permeability cannot be expressed by any simple equation. Although the
foregoing factors prevent the obtaining of extremely high accuracy in
magnetic calculations, yet, with experience, it is possible to design
magnetic circuits with a precision that is satisfactory for all practical
purposes.
The magnetomotive force Ᏺ in Eq. (15.1.18) is expressed in ampereturns ⫽ NI, where N is the number of turns linked with the circuit and I
is the current, A. The unit of reluctance is the reluctance of a 1-m cube
of air. The total reluctance is proportional to the length and inversely
proportional to the cross-sectional area of the magnetic circuit, which is
analogous to electrical resistance. Hence the reluctance of any given
path of uniform cross section A is l/A␮, where l ⫽ length of path,
cm; A ⫽ its cross section, cm2; and ␮ ⫽ permeability. Reluctances in
series are added to obtain their combined reluctance. Ohm’s law of the
magnetic circuit becomes
⌽⫽
NI
Mx
l1 /A1 ␮1 ⫹ l2 /A2 ␮2 ⫹ l3 /A3 ␮3⭈ ⭈ ⭈
(15.1.21)
where l1 , A1 , ␮ 1 , etc., are the lengths, cross sections, and relative permeabilities of each series part of the circuit.
15-9
Magnetization and Permeability Curves The magnetic permeability of air is a constant and is taken as unity. The relative permeability of
iron and other magnetic substances varies with the flux density. In Fig.
15.1.5 is shown a magnetization curve for cast steel in which the flux
density B in tesla is plotted as a function of the field intensity, amperes
Fig. 15.1.5
Magnetization and relative-permeability curves for cast steel.
per metre, H. Also the relative permeability ␮ r ⫽ B/H is plotted as a
function of the flux density B. Note the wide range over which the
relative permeability varies. No satisfactory equation has been found to
express the relation between magnetizing force and flux density and
between relative permeability and flux density. If an attempt is made to
solve Eq. (15.1.21) for flux, the factors ␮ 1 , ␮ 2 , etc., are unknown since
they are functions of the flux density, which is being determined. The
simplest method is one of trial and error, i.e., a value of flux, and the
corresponding permeability, is first assumed, the equation solved for the
flux, and if the computed flux differs widely from the assumed flux, a
second approximation is made, etc. In nearly all magnetic designs either
the flux or flux density is the independent variable, and it is required to
find the necessary ampere-turns to produce them. Let the flux ⌽ ⫽ BA
where B is the flux density, G. Then
and
⌽ ⫽ BA ⫽ 0.4␲NI(l/A ␮ r )
NI ⫽ Bl/␮ 0 ␮ r ⫽ 0.796Bl/␮ r ⫻ 10 6
(15.1.22)
Equation (15.1.22) shows that the necessary ampere-turns are proportional to the flux density and the length of path and are inversely proportional to the relative permeability.
With air and nonmagnetic substances ␮ r [Eq. (15.1.22)] becomes
unity, and
NI ⫽ 0.796Bl ⫻ 10 6
(15.1.23)
in metre units. With inch units
NI ⫽ 0.313B⬘l⬘
Fig. 15.1.4 Magnetic circuit.
EXAMPLE. In Fig. 15.1.4 is shown a magnetic circuit of cast steel with a
0.4-cm air gap. The cross section of the core is 4 cm square. There are 425 turns
wound on the core and the current is 10 A. The relative permeability of the steel at
the operating flux density is 1,100. Assume that the path of the flux is as shown,
the average path at the corners being quarter circles. Neglect fringing at the air gap
and any leakage. Determine the flux and the flux density.
Using the SI system, the length of the iron is 0.522 m, the length of the air gap is
0.004 m, and the cross section of the iron and air gap is 0.0016 m2.
⌽⫽
425 ⫻ 10
0.004
0.522
⫹
1,100 ⫻ 4␲ ⫻ 10⫺ 7 ⫻ 0.0016
4␲ ⫻ 10⫺ 7 ⫻ 0.0016
⫽ 0.00191 Wb
Using the cgs system, the length of the magnetic path in the iron ⫽ 12 ⫹ 8 ⫹
8 ⫹ 5.8 ⫹ 5.8 ⫹ 4␲ ⫽ 52.2 cm. From Eq. (15.1.21),
⌽⫽
0.4␲ ⫻ 425 ⫻ 10
⫽ 191,000 Mx
[52.2/(16 ⫻ 1,100)] ⫹ (0.4/16)
B⫽
191,000
⫽ 11,940 G
16
(15.1.24)
where B⬘ is the flux density, Mx/in 2 ; and l⬘ the length of the magnetic
path, in.
EXAMPLE. The average flux density in the air gap of a generator is
40,000 Mx/in 2, and the effective length of the gap is 0.2 in. How many ampereturns per pole are necessary for the gap?
NI ⫽ 0.313 ⫻ 40,000 ⫻ 0.2 ⫽ 2,500
Since the relation of ␮ r to flux density B in Eq. (15.1.22) is not
simple, the relation of ampere-turns per unit length of magnetic circuit
to flux density is ordinarily shown graphically. Typical curves of this
character are shown in Fig. 15.1.6, inch units being used although scales
of tesla, and ampere turns per metre are also given. To determine the
number of ampere-turns necessary to produce a given total flux in a
magnetic circuit composed of several parts in series having various
lengths, cross sections, and relative permeabilities, determine the flux
density if the cross section is fixed, or otherwise choose a cross section
to give a suitable flux density. From the magnetization curve obtain the
ampere-turns necessary to drive this flux density through a unit length of
the portion of the circuit considered and multiply by the length. Add
together the ampere-turns required for each series part of the magnetic
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15-10
ELECTRICAL ENGINEERING
circuit to obtain the total ampere-turns necessary to give the assumed
flux.
It is desirable to operate magnetic circuits at as high flux densities as
is practicable in order to reduce the amount of iron and copper. The air
gaps of dynamos are operated at average densities of 40,000 to
OD, called the coercive force, is required to bring the flux density to zero.
If the magnetizing force is increased negatively to OA⬘, the flux density
will be given by the curve DE. If the magnetizing force is increased
positively from A⬘ to A, the flux density will be given by the curve
EFGB, which is similar to the curve BCDE. OF is the negative remanence and OG again is the coercive force. The complete curve is called a
Fig. 15.1.6 Typical magnetization curves.
50,000 Mx/in 2. Higher densities increase the exciting ampere-turns and
tooth losses. At 45,000 Mx/in 2 the flux density in the teeth may be as
high as 120,000 to 130,000 Mx/in 2. The flux densities in transformer
cores are limited as a rule by the permissible losses. At 60 Hz and with
silicon steel the maximum density is 60,000 to 70,000 Mx/in 2, at 25 Hz
the density may run as high as 75,000 to 90,000 Mx/in 2. With laminated
cores, the net iron is approximately 0.9 the gross cross section.
Magnetic Leakage It is impossible to confine all magnetic flux to
any desired path since there is no known insulator of magnetic flux.
Figure 15.1.7 shows the magnetic circuit of a modern four-pole dy-
Fig. 15.1.8
Hysteresis loop for dynamo steel.
hysteresis loop. When the normal curve reaches the point K, if the magnetizing force is then decreased, another hysteresis loop, a portion of
which is shown at KL, will be obtained. It is seen that the flux density
lags the magnetizing force throughout.
The energy dissipated per cycle is proportional to the area of the loop
and is equal to (1/4 ␲)兰H dB ergs/(Hz)(cm3 ). For moderately high densities the energy loss per cycle varies according to the Steinmetz law
W ⫽ 10 ␩ B 1.6
m
W ⭈ s/m3
(15.1.25)
where Bm is the maximum value of the flux density, T (Fig. 15.1.8).
Table 15.1.6 gives values of the Steinmetz coefficient ␩ for common
magnetic steels.
Table 15.1.6
Fig. 15.1.7 Magnetic circuit of a four-pole dynamo with leakage flux.
namo. A considerable proportion of the useful magnetic flux leaks between the pole shoes and cores, rather than across the air gap. The ratio
of the maximum flux, which exists in the field cores, to the useful flux,
i.e., the flux that crosses the air gap, is the coefficient of leakage. This
coefficient must always be greater than unity and in carefully designed
dynamos may be as low as 1.15. It is frequently as high as 1.30. Although the geometry of the leakage-flux paths is not simple, the leakage
flux may be determined by approximations with a fair degree of accuracy.
Magnetic Hysteresis The magnetization curves shown in Figs.
15.1.5 and 15.1.6 are called normal curves. They are taken with the
magnetizing force continuously increased from zero. If at any point the
magnetizing force be decreased, a greater value of flux density for any
given magnetizing force will result. The effect of carrying iron through
a complete cycle of magnetization, both positive and negative, is shown
in Fig. 15.1.8.
The curve OKB, taken with increasing values of magnetizing force
per centimeter H, is the normal induction curve. If after the magnetizing
force has reached the value OA, it is decreased, the magnetic flux density B will decrease in accordance with curve BCD, between A and O the
values being much greater than those given by the normal curve, i.e., the
flux density lags the magnetizing force. At zero magnetizing force, the
flux density is OC, call the remanence. A negative magnetizing force
Steinmetz Coefficients
Hard tungsten steel
Hard cast steel
Forged steel
Cast iron
Electrolytic iron
Soft machine steel
Annealed cast steel
0.058
0.025
0.020
0.013
0.009
0.009
0.008
Ordinary sheet iron
Pure iron
Annealed iron sheet
Best annealed sheet
Silicon steel sheet
Permalloy
0.004
0.003
0.002
0.001
0.00046
0.0001
A permanent increase in the hysteresis constant occurs if the temperature of operation remains for some time above 80°C. This phenomenon
is known as aging and may be much reduced by proper annealing of the
iron. Silicon steels containing about 3 percent silicon have a lower
hysteresis loss, somewhat larger eddy-current loss, and are practically
nonaging.
Eddy-current losses, also known as Foucault-current losses, occur in
iron subjected to cyclic magnetization. Eddy-current losses are reduced
by laminating the iron, which subdivides the emf and increases greatly
the length of path of the parasitic currents. Eddy currents also have a
screening effect, which tends to prevent the flux penetrating the iron.
Hence laminating also allows the full cross section of the iron to be
utilized unless the frequency is too high.
Eddy-current loss in sheets is given by
Pe ⫽ (␲ tf Bm )2/6␳1016
W/cm3
(15.1.26)
where t ⫽ thickness, cm; f ⫽ frequency, Hz; Bm ⫽ the maximum flux
density, G; ␳ ⫽ the resistivity, ⍀ ⭈ cm.
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BATTERIES
Relations of Direction of Magnetic Flux to Current Direction The
direction of the magnetizing force of a current is at right angles to its
direction of flow. Magnetic lines about a cylindrical conductor carrying
current exist in circular planes concentric with and normal to the conductor. This is illustrated in Fig. 15.1.9a. The 丣 sign, corresponding to
the feathered end of the arrow, indicates a direction of current away
from the observer; a 䉺 sign, corresponding to the tip of an arrow,
indicates a direction of current toward the observer.
Fig. 15.1.9 Currents in (a) opposite directions, (b) in the same direction.
Corkscrew Rule The direction of the current and that of the resulting magnetic field are related to each other as the forward travel of a
corkscrew and the direction in which it is rotated.
Hand Rule Grasp the conductor in the right hand with the thumb
pointing in the direction of the current. The fingers will then point in the
direction of the lines of flux.
The applications of these rules are illustrated in Fig. 15.1.9. If the
currents in parallel conductors are in opposite directions (Fig. 15.1.9a),
the conductors tend to move apart; if the currents in parallel conductors
are in the same direction (Fig. 15.1.9b), the conductors tend to come
together. The magnetic lines act like stretched rubber bands and, in
attempting to contract, tend to pull the two conductors together.
The relation of the direction of current in a solenoid helix to the
direction of flux is shown in Fig. 15.1.10. Figure 15.1.11 shows the
Fig. 15.1.10 Direction of current and poles in a solenoid.
Fig. 15.1.11 Effect of a current on a uniform magnetic field.
effect on a uniform field of placing a conductor carrying current in that
field and normal to it. In (a) the direction of the current is toward the
observer. By applying the corkscrew rule it is seen that the current
weakens the field immediately above it and strengthens the field immediately below it. The reverse is true in (b), where the direction of the
current is away from the observer.
Figure 15.1.11 is illustrative of the force developed on a conductor
carrying current in a magnetic field. In (a) the conductor will tend to
move upward owing to the stretching of the magnetic lines beneath it.
Similarly, the conductor in (b) will tend to move downward. This principle is the basis of motor action. (See also ‘‘Magnets.’’)
BATTERIES
In an electric cell, or battery, chemical energy is converted into electrical
energy. The word battery may be used for a single cell or for an assembly of cells connected in series or parallel. A battery utilizes the potential difference which exists between different elements. When two different elements are immersed in electrolyte an emf exists tending to
send current within the cell from the negative pole, which is the more
15-11
highly electropositive, to the positive pole. The poles, or electrodes of a
battery form the junction with the external circuit.
If the external circuit is closed, current flows from the battery at the
positive electrode, or anode, and enters the battery at the negative electrode,
or cathode.
In a primary battery the chemically reacting parts require renewal; in a
secondary battery, the electrochemical processes are reversible to a high
degree and the chemically reacting parts are restored after partial or
complete discharge by reversing the direction of current through the
battery. See Table 15.1.7 for a summary of battery types and applications.
Electromotive force of a battery is the total potential difference existing between the electrodes on open circuit. When current flows, the
potential difference across the terminal drops because of the resistance
drop within the cell and because of polarization.
Polarization When current flows in a battery, hydrogen is deposited
on the cathode. This produces two effects, both of which reduce the
terminal voltage of the battery. The hydrogen in contact with the cathode constitutes a hydrogen battery which opposes the emf of the battery;
the hydrogen bubbles reduce the contact area of the electrolyte with the
cathode, thus increasing the battery resistance. The most satisfactory
method of reducing polarization is to have present at the cathode some
compound that supplies negative ions to combine with the positive hydrogen ions at the plate. In the Leclanché cell, manganese peroxide in
contact with the carbon cathode serves as a depolarizer, its oxygen ion
combining with the hydrogen ion to form water.
If E is the emf of the cell, Ep the emf of polarization, r the internal
resistance, V the terminal voltage, when current I flows, then
V ⫽ (E ⫺ Ep ) ⫺ Ir
(15.1.27)
Primary Batteries
Dry Cells A dry cell is one in which the electrolyte exists in the form
of a jelly, is absorbed in a porous medium, or is otherwise restrained
from flowing from its intended position, such a cell being completely
portable and the electrolyte nonspillable. The Leclanché cell consists of
a cylindrical zinc container which serves as the negative electrode and is
lined with specially prepared paper, or some similar absorbent material,
to prevent the mixture of carbon and manganese dioxide, which is
tamped tightly around the positive carbon electrode, from coming in
contact with the zinc. The absorbent lining and the mixture are moistened with a solution of zinc chloride and sal ammoniac. In smaller cells
(Fig. 15.1.12) the manganese-carbon mixture is often molded into a
cylinder around the carbon electrode, the whole is then set into the zinc
cup, and the space between the molded mixture and the zinc is filled
with electrolyte made into a paste in such a manner that it can be
solidified by either standing or heating. The top of the cell is closed with
a sealing compound, and the cell is placed in a cardboard container. The
emf of a dry cell when new is 1.4 to 1.6 V.
In block assembly the dry cells, especially in the smaller sizes, are
assembled in series and sealed in blocks of insulating compound with
only two terminals and, sometimes, intermediate taps brought out. This
type of battery is used for radio B and C batteries. Another construction
is to build the battery up of layers in somewhat the manner of the old
voltaic pile. Each cell consists of a layer of zinc, a layer of treated paper,
and a flat cake of the manganese-carbon mixture. The cells are separated by layers of a special material which conducts electricity but
which is impervious to electrolyte. A sufficient number of such cells are
built up to give the required voltage and the whole battery is sealed into
the carton.
Leclanché cells are generally available in sizes ranging from small,
thin penlight batteries to large assemblies of cells in series or parallel for
special high-voltage or high-current applications.
The efficiency of a standard-size dry battery depends on the rate at
which it is discharged. Up to a certain rate the lower the discharge rate,
the greater the efficiency. Above this rate the efficiency decreases (see
Natl. Bur. Stand. Circ. 79, p. 39).
When used efficiently, a 6-in dry cell will give over 30 A ⭈ h of ser-
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15-12
ELECTRICAL ENGINEERING
Table 15.1.7
Battery Types and Applications
Battery type
Cell
type
Nominal
cell voltage
Capacity,
wH / kg
Applications
Primary
Leclanché (zinc-carbon)
Zinc-mercury (Ruben)
Dry
Dry
1.5
1.34
22 – 44
90 – 110
Zinc-alkaline-manganese
dioxide
Silver or cuprous chloridemagnesium
Dry
1.5
66
Lead-acid
Wet
2
Lead-calcium
Edison (nickel-iron)
Nickel-cadmium
Wet
Wet
Wet
2
1.2
1.2
28
Silver oxide-cadmium
Silver-zinc
Wet
Wet
1.4
1.55
45 – 65
90 – 155
Wet
55 – 120
Flashlights, emergency lights, radios
Medical, marine, space, laboratory, and
emergency devices
Models, cameras, shavers, lights
Disposable devices: torpedoes, rescue beacons, meteorological balloons
Secondary
vice. As ordinarily used, however, the dry cell give no more than 8 to 10
A ⭈ h of service and at times even less. The 11⁄4 by 21⁄4 in flashlight
battery is usually employed with a lamp taking 0.25 to 0.35 A. Under
these conditions 3 A ⭈ h or thereabouts may be expected if the battery is
used for not more than an hour or so a day. The so-called ‘‘heavy-duty’’
radio battery will give about 8 to 10 A ⭈ h when efficiently used.
Fig. 15.1.12 Cross section of a standard round zinc-carbon cell. (From ‘‘Standard Handbook for Electrical Engineers,’’ Fink and Carrol, McGraw-Hill, NY,
copyright 1968.)
For the best results 6-in dry cells should not be used for current drains
of over 0.5 A except for very short periods of time. Flashlight batteries
should not be used for higher than the preceding current drain, and
heavy-duty radio batteries will give best results if the current drain is
kept below 25 mA.
Dry cells should be stored in a cool, dry place. Extreme heat during
storage will shorten their life. The cell will not be injured by being
frozen but will be as good as new after being brought back to normal
temperature. In extreme cold weather dry cells may not give more than
half of their normal service. At a temperature of about ⫺ 30°F they
freeze solid and give neither voltage nor current.
The amperage of a dry cell by definition is the current that it will give
when it is short-circuited (at about 70°F) through an ammeter which
with its leads has a resistance of 0.01 ⍀.
The Ruben cell (Ruben, Balanced Alkaline Dry Cells, Trans. Electrochem. Soc., 92, 1947) was developed jointly by the Ruben Laboratories
Automotive, industrial trucks, railway, station service
Standby
Industrial trucks; boat and train lights
Engine starting, emergency lighting, station
service
Space
Models, photographic equipment, missiles
and P. R. Mallory & Company during World War II for the operation of
radar equipment and other electronic devices which require a high ratio
of ampere-hour capacity to the volume of the cell at higher current
densities than were considered practicable for the Leclanché type. The
anode is of amalgamated zinc, and the cathode is a mercuric oxide
depolarizing material intimately mixed with graphite in order to reduce
its electrical resistivity. The electrolyte is a solution of potassium hydroxide (KOH) containing potassium zincate. The cell is made in three
forms as shown in Fig. 15.1.13, the wound-anode type (a), the button
type (b), and the cylindrical type (c).
The no-load emf of the cell is 1.34 V and remains essentially constant
irrespective of time and temperature. Advantages of the cell are long
shelf life, which enables them to be stored indefinitely; long service life,
about four times that of the Leclanché dry cell of equivalent volume;
small weight; a flat voltage characteristic which is advantageous for
electronic uses in which the characteristics of tubes vary widely with
voltage; adaptability to operating at high temperatures without deterioration; high resistance to shock.
The zinc-alkaline-manganese dioxide cell is a cell especially useful in
applications that require a dry cell with relatively heavy or continuous
drain. The anode is of amalgamated zinc, and the cathode is a manganese dioxide depolarizing material mixed with graphite for conductivity. The electrolyte is a solution of highly alkaline potassium hydroxide
immobilized in cellulosic-type separators. These cells are available in
standard-size cylindrical construction and wafer (flat) construction for
cassette and tape recorder applications.
Wet Cells The silver or cuprous chloride-magnesium cell is a one-shot
battery with a life of days after the electrolyte is added. A wet cell may
be stored for years in a dry state. The cathode is either compacted
copper chloride and graphite or sheet silver chloride, while the cathode
is a thin magnesium sheet. The electrolyte is a solution of sodium chloride. The silver chloride cells are more expensive and are available in
more and larger ratings.
The Weston cell is a primary cell used as a standard of emf. It consists
of a glass H tube in the bottom of one leg of which is mercury which
forms the cathode; in the bottom of the other leg is cadmium amalgam
forming the anode. The electrolytes consist of mercurous sulfate and
cadmium sulfate. There are two forms of the Weston cell: the saturated
or normal cell, and the unsaturated cell. In the normal cell the electrolyte
is saturated. This is the official standard since it is more permanent than
the unsaturated type and can be reproduced with far greater accuracy.
When carefully made, the emfs of cells agree within a few parts per
million. There is, however, a small temperature coefficient. Although
the unsaturated cell is not so reliable as the normal cell and must be
standardized, it has a negligible temperature coefficient and is more
convenient for general use. The manufacturers recommend that the
temperature be not less than 4°C and not more than 40°C and the current
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BATTERIES
15-13
electrically formed of pure lead by repeated reversals of the charging
current. In the Faure, or pasted-plate, type, the positive and negative
plates are formed by applying a paste, largely of lead oxides (PbO2 ,
Pb3O4 ), to lead-antimony or lead calcium supporting grids. A current is
passed through the plates while they are immersed in weak sulfuric acid,
the positive plates being connected as anodes and the negative ones as
cathodes. The paste on the positive plates is converted into lead peroxide while that on the negative plate is reduced to spongy lead. The
tubular plate (iron-clad) type has lead-alloy rods surrounded by perforated dielectric tubes with powdered-lead oxides packed between the rod
and tube for the positive plate.
In order to obtain high capacity per unit weight it is necessary to
expose a large plate area to the action of the acid. This is done in the
Planté plate by ‘‘ploughing’’ with sharp steel disks, and by using corrugated helical inserts as active positive material (Manchester plate). In
the pasted plate a large area of the material is necessarily exposed to the
action of the acid.
The chemical reactions in a lead cell may be expressed by the following equation, based on the double sulfation theory:
charge
sulfuric
acid
positive and
negative plates
discharge
冎
negative
plate



positive
plate
冎
Pb
冎
冎
PbO2 ⫹
;
⫹ 2H2SO4 ⫽ 2PbSO4 ⫹ 2H2O
water
:
Between the extremes of complete charge and discharge, complex
combinations of lead and sulfate are formed. After complete discharge a
hard insoluble sulfate forms slowly on the plates, and this is reducible
only by slow charging. This sulfation is objectionable and should be
avoided.
Specific Gravity Water is formed with discharge and sulfuric acid is
formed on charge, consequently the specific gravity must decrease on
discharge and increase on charge. The variation of the specific gravity
for a stationary battery is shown in Fig. 15.1.14. With starting and
Fig. 15.1.14
Fig. 15.1.13 Ruben cells. (a) Wound-anode; (b) button; (c) cylindrical. (From
‘‘Standard Handbook for Electrical Engineers,’’ Fink and Carrol, McGraw-Hill,
NY, copyright 1968.)
should not exceed 0.0001 A. The emf is between 1.0185 and 1.0190 V.
Since no appreciable current can be taken from the cell, a null method
must be used to utilize its emf.
Storage (Secondary) Batteries
In a storage battery the electrolytic action must be reversible to a high
degree. There are three types of storage batteries; the lead-lead-acid
type, the nickel-iron-alkaline type ( Edison battery), and the nickelcadmium-alkali type ( Nicad ). In addition, there are various specialized types of cells for scientific and military purposes, and there is
continuous development work in the search for higher capacities.
In the manufacture of the lead-lead-acid cells there are three general
types of plates, or electrodes. In the Planté type the active material is
Variations of specific gravity in a stationary battery.
vehicle batteries it is necessary to operate the electrolyte from between
1.280 to 1.300 when fully charged to as low as 1.100 when completely
discharged. The condition of charge of a battery can be determined by
its specific gravity.
Battery electrolyte may be made from concentrated sulfuric acid (oil
of vitriol, sp gr 1.84) by pouring the acid into the water in the following
proportions:
Parts Water to 1 Part Acid
Specific gravity
Volume
Weight
1.200
4.3
2.4
1.210
4.0
2.2
1.240
3.4
1.9
1.280
2.75
1.5
Freezing Temperature of Sulfuric Acid and Water Mixtures
Specific gravity
Freezing temp, °F
1.180
⫺6
1.200
⫺ 16
1.240
⫺ 51
1.280
⫺ 90
Voltage The emf of a lead cell when fully charged and idle is 2.05 to
2.10 V. Discharge lowers the voltage in proportion to the current. When
charging at constant current and normal rate, the terminal voltage gradually increases from 2.14 to 2.3 V, then increases rapidly to between 2.5
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15-14
ELECTRICAL ENGINEERING
and 2.6 V (Fig. 15.1.15). This latter interval is known as the gassing
period. When this period is reached, the charging rate should be reduced
in order to avoid waste of power and unnecessary erosion of the plates.
Practically all batteries have a normal rating based on the 8-h rate of
Fig. 15.1.15 Voltage curves on charge and discharge for a lead cell.
discharge. Thus a 320 A ⭈ h battery would have a normal rate of 40 A.
The ampere-hour capacity of batteries falls off rapidly with increase in
discharge rate.
Effect of Discharge Rate on Battery Capacity
Discharge rate, h
Percentage of
rated capacity,
Planté type
Pasted type
8
5
3
1
100
100
88
93
75
83
55.8
63
⁄
13
37
41
⁄
1 10
19.5
25.5
The following rule may be observed in charging a lead battery. The
charging rate in amperes should be less than the number of amperehours out of the battery. For example, if 200 A ⭈ h are out of a battery, a
charging rate of 200 A may be used until the ampere-hours out of the
battery are reduced appreciably.
There are two common methods of charging: the constant-current
method and the constant-potential method. Figure 15.1.16a shows a common method of charging with constant current, provided a low-voltage
dc power supply is available. The resistor connected in series may be
adjusted to give the required current. Several batteries may be connected in series. Figure 15.1.16b shows a more common method, using
a copper oxide or silicon rectifier, since ac power supply is more common than dc. The rectifier disks, mounted in a stack, are bridge-connected, the directions of rectification being indicated. The polarity of
the two wires can readily be determined by means of a dc voltmeter.
The constant-potential method is to be preferred since the rate automatically tapers off as the cell approaches the charged condition. Without resistance the terminal voltage should be 2.3 V per cell, but it is
preferable to use 2.4 to 2.5 V per cell with low resistance in series.
When a battery is being charged, its terminal voltage
V ⫽ E ⫹ Ir
(15.1.28)
Compare with Eq. (15.1.27).
When a battery is fully charged, any rate will produce gassing, but the
rate may be reduced to such a low value that gassing is practically
harmless. This is called the finishing rate.
Portable batteries for automobile starting and lighting, airplanes, industrial trucks, electric locomotives, train lighting, and power boats
employ the pasted-type plates because of their high discharge rates for a
given weight and size. The separators are either of treated grooved
wood; perforated hard rubber; glass-wool mats; perforated rubber, and
grooved wood; ribbed microporous rubber. In low-priced short-lived
batteries for automobiles, grooved wood alone is used; in the better
types, the wood is reinforced with perforated hard rubber. Containers
for the low-priced short-lived automobile-type starting batteries are of
asphaltic compound; for other portable types they are usually of hard
rubber.
The Exide iron-clad battery is a portable type designed for propelling
electric vehicles. The positive plate consists of a lead-antimony frame
supporting perforated hard-rubber tubes. An irregular lead-antimony
core runs down the center of each tube, and the lead peroxide paste is
packed into these tubes so that shedding of active material from the
positive plate cannot occur. Pasted negative plates are used. The separators are flat microporous rubber.
Stationary Batteries The tanks of stationary batteries are made of
hard rubber or plastics. When the battery is used for regulating or cycling duty, the positive plates may be of the Planté type because of their
long life. However, in most modern installations thick pasted plates are
used. Because of the tight fit of the plate assembly within the container
and the resulting pressure of the separator against the plate surfaces,
shedding of active material is reduced to a minimum and long life is
obtained. Pasted negative plates are used in almost all batteries.
A lead storage battery removed from service for less than 9 months
should be charged once a month if possible; if not, it should be given a
heavy overcharge before discontinuing service. If removed for a longer
period, siphon off acid (which may be used again) and fill with fresh
water. Allow to stand 15 h and siphon off water. Remove and throw
away the wood separators. The battery will now stand indefinitely. To
put in service again, install new separators, fill with acid (sp gr 1.210)
and charge at normal rate 35 h or until gravity has ceased to rise over a
period of 5 h. Charge at a low rate a few hours longer.
The ampere-hour efficiency of lead batteries is 85 to 90 percent. The
watthour efficiency obtained from full charge to discharge at the normal
rate and at rated amp-hour is 75 to 80 percent. Batteries which do
regulating duty only may have a much higher watthour efficiency.
The Edison storage cell when fully charged has a positive plate of
nickel pencils filled with a higher nickel oxide and a negative plate of
flat nickel-plated-steel stampings containing metallic iron in finely divided form. The active material for the positive plate is nickel hydrate
and for the negative plate, iron oxide. The electrolyte is a 21 percent
solution of potassium hydrate with lithium hydroxides. The initial emf
is about 1.4 V and the average emf about 1.1 V throughout discharge. In
Fig. 15.1.16 Connections for charging a storage battery from (a) 110-V dc mains, (b) copper oxide rectifier.
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DIELECTRIC CIRCUITS
Fig. 15.1.17 are shown typical voltage characteristics on charge and
discharge for an Edison cell. On account of the higher internal resistance of the cell the battery is not so efficient from the energy standpoint
as the lead cell. The jar is welded nickel-plated steel. The battery is
compact and extremely light and strong and for these reasons is particularly adapted for propelling electric vehicles and for boat- and trainlighting systems. The battery is rugged, and since there is no opportunity for the growth of active material on the plates or flaking of active
material, the battery has long life.
15-15
type, the cylindrical type, and the prismatic type. Their ratings range
from 20 mA ⭈ h to 23 A ⭈ h. Their average discharge voltage is 1.22 V,
and they require 14 h of charge at the normal rate (one-tenth A ⭈ h rating), which for a 3.5 A ⭈ h cell is 0.35 A.
Precautions in the care of storage batteries: An ammeter should not be
connected directly across the terminals to test the condition of a cell; a
battery should not be left to stand in a discharged condition; a flame
should not be brought in the vicinity of a battery that is being charged;
the battery should not be allowed to become heated when charging;
water should never be added to the concentrated acid — always acid to
the water; acid should never be equalized except when the battery is in a
charged condition; a battery should never be exposed to the influence of
external heat; voltmeter tests should be made when the current is flowing; batteries should always be kept clean. To replace acid lost through
slopping, use a solution of 2 parts concentrated sulfuric acid in 5 parts
water by weight, unless a hydrometer is at hand to enable the solution to
be made up according to the specifications of the makers of the cell.
Fig. 15.1.17 Voltage during charge and discharge of an Edison cell.
DIELECTRIC CIRCUITS
The positive active material is nickelic (black) hydroxide mixed with graphite to give it high
conductivity. The negative active material is cadmium oxide. Both materials are used in powdered form and are contained within flat perforated steel pockets. These pockets are locked into steel plates, the positive and negative being alike in construction. All steel parts are
nickel-plated. A complete plate group consists of a number of positive
and negative plates assembled on bolts and terminal posts common to
plates of the same polarity. The separators are thin strips of polystyrene,
and all other battery insulation is also polystyrene. The entire plate
assembly is contained within a welded-steel tank. The electrolyte is
potassium hydroxide (KOH), specific gravity 1.210 at 72°F (22°C); it
does not enter into any chemical reactions with the electrode materials,
and its specific gravity remains constant during charge and discharge,
neglecting any slight change due to the small amount of gassing. On
charge, the voltage is 1.4 to 1.5 V until near the end when it rises to
1.8 V. On discharge, the voltage is nearly constant at 1.2 V.
Nicad batteries are strong mechanically and are not damaged by
overcharge; they hold their charge over long periods of idleness, the
active material cannot flake off, the internal resistance is low, there is no
corrosion, and the battery has an indefinitely long life. It is a generalpurpose battery.
In the Sonotone nickel-cadmium battery the positive plates are nickel
oxide when the battery is charged, and the negative plates are metallic
cadmium. On discharge the positive plates are reduced to a state of
lower oxidation, and the negative plates regain oxygen. The electrolyte
is a 30 percent solution of potassium hydroxide, the specific gravity of
which is 1.29 at room temperature. The case is a transparent plastic. The
terminal voltage at the normal discharge rate is 1.2 V per cell.
Rechargeable batteries, exemplified by Gould Nicad cells (Alkaline
Battery Division, Gould National Batteries, Inc.), are hermetically
sealed nickel-cadmium cells that contain no free alkaline electrolyte.
Since there is no spillage or leakage, they can operate in any position,
have long life, and require no maintenance or servicing, and their
weight is small for their output. They are thus well adapted to power
many types of cordless appliances such as tools, hedge shears, cameras,
dictating equipment, electric razors, radios, and television sets. The
electrodes consist of a plaque of microporous sintered nickel having an
extremely high surface area. The electrochemical reactions differ from
those of the conventional vented-type alkaline battery, a type which at
the end of a charge liberates both oxygen and hydrogen gases as well as
electrolytic fumes that must be vented through a valve in the top of the
cell. In the sealed nickel-cadmium cell, the negative electrode (at the
time that the cell is sealed) never becomes fully charged, and the evolution of hydrogen is completely suppressed. On charging, when the positive electrode has reached its full capacity, the oxygen which has
evolved is channeled through the porous separator to the negative electrode and oxidizes the finely divided cadmium of the microporous plate
to cadmium hydroxide, which at the same time is reduced to metallic
cadmium. The cells are constructed in three different forms: the button
Nickel-Cadmium-Alkali (Nicad) Battery
Dynamic and Static Electricity Electricity in motion such as an
electric current is dynamic electricity; electricity at rest is static electricity. The two are identical physically. Since static electricity is frequently produced at high voltage and small quantity, the two are frequently considered as being two different types of electricity.
Capacitors
Capacitors (formerly condensers) Two conducting bodies, or electrodes, separated by a dielectric constitute a capacitor. If a positive
charge is placed on one electrode of a capacitor, an equal negative
charge is induced on the other. The medium between the capacitor
plates is called a dielectric. The dielectric properties of a medium relate
to its ability to conduct dielectric lines. This is in distinction to its insulating properties which relate to its property to conduct electric current. For
example, air is an excellent insulator but ruptures dielectrically at low
voltage. It is not a good dielectric so far as breakdown strength is concerned.
With capacitors
Q ⫽ CE
(15.1.29)
C ⫽ Q/E
(15.1.30)
E ⫽ Q/C
(15.1.31)
where Q ⫽ quantity, C; C ⫽ capacitance, F; and E ⫽ voltage. The unit
of capacitance in the practical system is the farad. The farad is too large
a unit for practical purposes, so that either the microfarad ( ␮F) or the
picofarad (pF) are used. However, in voltage, current, and energy relations the capacitance must be expressed in farads.
The energy stored in a capacitor is
W ⫽ 1⁄2 QE ⫽ 1⁄2 CE 2 ⫽ 1⁄2 Q 2 /C
J
(15.1.32)
Capacitance of Capacitors The capacitance of a parallel-electrode
capacitor (Fig. 15.1.18) is
C ⫽ ␧ r A/(4␲d ⫻ 9 ⫻ 103 )
␮F
(15.1.33)
where ␧r ⫽ relative capacitivity; A ⫽ area of one electrode, m2; and d ⫽
distance between electrodes, m.
Fig. 15.1.18
capacitor.
Parallel-electrode
Fig. 15.1.19
capacitor.
Coaxial-cylinder
The capacitance of coaxial cylindrical capacitors (Fig. 15.1.19) is
C ⫽ 0.2171␧r l/[9 ⫻ 105 log (R2 /R1 )]
␮F
(15.1.34)
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15-16
ELECTRICAL ENGINEERING
where ␧r is the relative capacitivity and l the length, m. Also
C ⫽ 0.03882␧r /log (R2 /R1 )
␮F/mi
(15.1.35)
Equation (15.1.35) is useful in that it is applicable to cables.
The capacitance of two parallel cylindrical conductors D m between
centers and having radii of r m is
C ⫽ 0.01941/log (D/r)
␮F/mi
(15.1.36)
In practice, the capacitance to neutral or to an infinite conducting
plane midway between the conductors and perpendicular to their plane
is usually used. The capacitance to neutral is
C ⫽ 0.03882/log (D/r)
␮F/mi
(15.1.37)
Equations (15.1.36) and (15.1.37) are used for calculating the capacitance of overhead transmission lines. When computing charging
current, use voltage between lines in (15.1.36) and to neutral in
(15.1.37).
If the capacitances are not leaky, the charge Q is the same on each. Q ⫽
CE, E1 ⫽ Q/C1 , E2 ⫽ Q/C2 , etc.
Insulators and Dielectrics Insulating materials are applied to electric circuits to prevent the leakage of current. Insulating materials used
with high voltage must not only have a high resistance to leakage
current, but must also be able to resist dielectric puncture; i.e., in addition to being a good insulator, the material must be a good dielectric.
Insulation resistance is usually expressed in M⍀ and the resistivity
given in M⍀ ⭈ cm. The dielectric strength is usually given in terms of
voltage gradient, common units being V/mil, V/mm, and kV/cm. Insulation resistance decreases very rapidly with increase in temperature.
Absorbed moisture reduces the insulation resistance, and moisture and
humidity have a large effect on surface leakage. In Table 15.1.8 are
given the insulating and dielectric properties of several common insulating materials (see also Sec. 6). Dielectric heating of materials is described in Sec. 7.
TRANSIENTS
Induced EMF If a flux ␾ webers linking N turns of conductor
changes, an emf
e ⫽ ⫺N(d␾/dt)
Fig. 15.1.20 Capacitances in parallel.
V
(15.1.40)
is induced.
Capacitances in Parallel The equivalent capacitance of capacitances in parallel (Fig. 15.1.20) is
C ⫽ C1 ⫹ C2 ⫹ C3
(15.1.38)
Capacitances in parallel are all across the same voltage. If the voltage is
E, then the total quantity Q ⫽ CE and Q1 ⫽ C1E, etc.
Self-inductance Let a flux ␾ link N turns. The linkages of the circuit are N␾ weber-turns. If the permeability of the circuit is assumed
constant, the number of these linkages per ampere is the self-inductance
or inductance of the circuit. The unit of inductance is the henry. The
inductance is
H
(15.1.41)
L ⫽ N␾ /(i)
If the permeability changes with the current
L ⫽ N(d␾ /di)
H
(15.1.42)
The energy stored in the magnetic field
W ⫽ 1⁄2 Li 2
J
(15.1.43)
EMF of Self-induction If Eq. (15.1.41) is written Li ⫽ N␾ and dif-
Fig. 15.1.21 Capacitances in series.
Capacitances in Series The equivalent capacitance C of capacitances in series (Fig. 15.1.21) is found as follows:
1/C ⫽ 1/C1 ⫹ 1/C2 ⫹ 1/C3
Table 15.1.8
(15.1.39)
ferentiated with respect to the time t, L(di/dt) ⫽ N(d␾/dt) and from Eq.
(15.1.40)
e ⫽ ⫺L(di/dt)
V
(15.1.44)
e is the emf of self-induction. If a rate of change of current of 1 A/s
induces an emf of 1 V, the inductance is then 1 H.
Electrical Properties of Insulating Materials
Material
Asbestos board (ebonized)
Bakelite
Epoxy
Fluorocarbons:
Fluorinated ethylene propylene
Polytetrafluoroethylene
Glass
Magnesium oxide
Mica
Nylon
Neoprene
Oils:
Mineral
Paraffin
Paper
Paper, treated
Phenolic (glass filled)
Polyethylene
Polyimide
Polyvinyl chloride (flexible)
Porcelain
Rubber
Rubber (butyl)
Volume
resistivity,
M ⍀ ⭈ cm
107
5 – 30 ⫻ 1011
1014
1018
1018
17 ⫻ 109
1014 – 1017
1014 – 1017
21 ⫻ 106
1015
1012 – 1013
1015 – 1018
1016 – 1017
1011 – 1015
3 ⫻ 108
1014 – 1016
1018
Dielectric
constant,
60Hz
Dielectric strength
V/mil
V/mm
4.5 – 5.5
3.5 – 5
55
450 – 1,400
300 – 400
2 ⫻ 103
(17 – 55) ⫻ 103
(12 – 16) ⫻ 103
2.1
2.1
5.4 – 9.9
2.2
4.5 – 7.5
4 – 7.6
7.5
500
400
760 – 3,800
300 – 700
1,000 – 4,000
300 – 400
600
20 ⫻ 103
16 ⫻ 103
(3 – 15) ⫻ 104
(12 – 27) ⫻ 103
(4 – 16) ⫻ 104
(12 – 16) ⫻ 103
23.5 ⫻ 103
2 – 4.7
2.41
1.7 – 2.6
2.5 – 4
5–9
2.3
3.5
5–9
5.7 – 6.8
2 – 3.5
2.1
300 – 400
410 – 550
110 – 230
500 – 750
140 – 400
450 – 1,000
400
300 – 1,000
240 – 300
500 – 700
(12 – 16) ⫻ 103
(16 – 22) ⫻ 103
(4 – 9) ⫻ 103
(20 – 30) ⫻ 103
(5.5 – 16) ⫻ 103
(17 – 40) ⫻ 103
16 ⫻ 103
(12 – 40) ⫻ 103
(9.5 – 12) ⫻ 103
(20 – 27) ⫻ 103
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TRANSIENTS
Current in Inductive Circuit If a circuit containing resistance R and
inductance L in series is connected across a steady voltage E, the voltage E must supply the iR drop in the circuit and at the same time
overcome the emf of self-induction. That is E ⫽ Ri ⫹ L di/dt. A solution
of this differential equation gives
i ⫽ (E/R) (1 ⫺ e⫺ Rt/L )
A
(15.1.45)
15-17
If a capacitor charged to voltage E is discharged through resistance R,
the current is
i ⫽ ⫺ (E/R)e⫺t/CR
A
(15.1.51)
Except for sign, these two equations are identical and are of the same
form as Eq. (15.1.46).
where e is the base of the natural system of logarithms.
Fig. 15.1.22 Rise of current in an inductive circuit.
Fig. 15.1.24
Figure 15.1.22 shows this equation plotted when E ⫽ 10 V, R ⫽
20 ⍀, L ⫽ 0.6 H. It is to be noted that inductance causes the current to
rise slowly to its Ohm’s law value, I0 ⫽ E/R ⫽ 10⁄20 ⫽ 0.5 A. When t ⫽
L/R, the current has reached 63.2 percent of its Ohm’s law value. L/R is
the time constant of the circuit. In the foregoing circuit, the time constant
L/R ⫽ 0.6/20 ⫽ 0.03 s. The initial rate of rise of current is tan ␣ ⫽ E/L.
If current continued at this rate, it would reach ␣ ⫽ E/R in L/R s
[(E/L) ⫻ (L/R) ⫽ E/R].
If a circuit containing inductance and resistance in series is short-circuited when the current is I0 , the equation of current becomes
In Fig. 15.1.24 is shown the transient current to a capacitor in series
with a resistor when E ⫽ 200 V, C ⫽ 4.0 ␮F, R ⫽ 2 k⍀. When t ⫽ CR,
the current has reached 1/e ⫽ 0.368 its initial value. CR is the time
constant of the circuit. The initial rate of decrease of current is tan ␣ ⫽
⫺E/CR 2. If the current continued at this rate it would reach zero when
the time is CR s. If, in its fully charged condition, the capacitor of Fig.
15.1.24 is discharged through the resistor R, the curve will be the negative of that shown in Fig. 15.1.24.
Resistance, Inductance, and Capacitance in Series If a circuit
having resistance, inductance, and capacitance in series is connected
across a source of steady voltage, a transient condition results. If R ⬎
√4L/C, the circuit is nonoscillatory or overdamped.
The current is
i ⫽ I0 e⫺ Rt/L
A
(15.1.46)
i⫽
Transient current to a capacitor.
EC
√R 2C 2 ⫺ 4LC
冉
e(⫺ ␣ ⫹ ␤ )t ⫺ e(⫺ ␣ ⫺ ␤)t
冊
A
(15.1.52)
where ␣ ⫽ R/2L and ␤ ⫽ (√R 2C 2 ⫺ 4LC)/2LC.
Fig. 15.1.23 Decay of current in an inductive circuit.
Figure 15.1.23 shows this equation plotted when I0 ⫽ 0.5 A, R ⫽ 20 ⍀,
L ⫽ 0.6 H. It is seen that inductance opposes the decay of current.
Inductance always opposes change of current.
Mutual Inductance If two circuits having inductances L1 and L2
henrys are so related to each other geometrically that any portion of the
flux produced by the current in one circuit links the other circuit, the two
circuits possess mutual inductance. It follows that a change of current in
one circuit causes an emf to be induced in the other. Let ␧ 2 be induced in
circuit 2 by a change di1 /dt in circuit 1. Then
e2 ⫽ ⫺M di1 /dt
V
(15.1.47)
M is the mutual inductance of the two circuits.
M ⫽ k √L 1 L 2
(15.1.48)
where k is the coefficient of coupling of the two circuits, or the proportion
of the flux in one circuit which links the other. Also a change of current
di2 /dt in circuit 2 induces an emf e1 in circuit 1, e1 ⫽ ⫺M di2 /dt.
The stored energy is
W ⫽ 1⁄2 L 1 I 21 ⫹ 1⁄2 L 2 I 22 ⫹ MI1 I2
J
(15.1.49)
where I1 and I2 are the currents in circuits 1 and 2.
Current in Capacitive Circuit If capacitance C farads and resistance
R ohms are connected in series across the steady voltage E, the current is
i ⫽ (E/R)e⫺t/CR
A
(15.1.50)
Fig. 15.1.25
Transient current in nonoscillatory circuits.
In Fig. 15.1.25 is shown the curve corresponding to Eq. (15.1.52).
When R ⫽ √4L/C, the system is critically damped and the transient dies
out rapidly without oscillation. The current is
i ⫽ (E/L)te⫺ Rt/2L
A
(15.1.53)
Figure 15.1.25 shows also the curve corresponding to Eq. (15.1.53).
If R ⬍ √4L/C, the transient is oscillatory, being a logarithmically
damped sine wave. The current is
i⫽
2EC
√4LC ⫺ R 2C 2
e⫺ Rt/2L sin
√4LC ⫺ R 2C 2
t
2LC
A (15.1.54)
The transient oscillates at a frequency very nearly equal to
1/(2␲ √LC) Hz. This is the natural frequency of the circuit.
In Fig. 15.1.26 is shown the curve corresponding to Eq. (15.1.54). If
the capacitor, after being charged to E V, is discharged into the foregoing series circuits, the currents are given by Eqs. (15.1.52) to (15.1.54)
multiplied by ⫺1. Equations (15.1.52) to (15.1.54) are the same types
obtained with dynamic mechanical systems with friction, mass, and
elasticity.
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15-18
ELECTRICAL ENGINEERING
This current lags the voltage by 90 electrical degrees. Inductance absorbs no energy. The energy stored in the magnetic field during each
half cycle is returned to the source during the same half cycle.
Capacitive reactance is 1/(2␲ fC) ⫽ 1/␻ C and is denoted by X C , where
C is in F. If C is given in ␮ F, XC ⫽ 106 2␲ fC. The current in a capacitive
reactance X C when connected across voltage E is
I ⫽ E/XC ⫽ 2␲ fCE
A
Fig. 15.1.26 Transient current in an oscillatory circuit.
Z ⫽ √R 2 ⫹ X 2L ⫽ √R 2 ⫹ (2␲ fL)2
ALTERNATING CURRENTS
⍀
(15.1.59)
With resistance and capacitance in series
Sine Waves In the following discussion of alternating currents, sine
waves of voltage and current will be assumed. That is, e ⫽ E m sin ␻t and
i ⫽ Im sin (␻t ⫺ ␪ ), where Em and Im are maximum values of voltage and
current; ␻, the angular velocity, in rad/s, is equal to 2␲ f, where f is the
frequency; ␪ is the angle of phase difference.
Cycle; Frequency When any given armature coil has passed a pair
of poles, the emf or current has gone through 360 electrical degrees, or 1
cycle. An alternation is one-half cycle. The frequency of a synchronous
machine in cycles per second (hertz) is
f ⫽ NP/120
Hz
(15.1.55)
where N is the speed in r/min and P the number of poles. In the United
States and Canada the frequency of 60 Hz is almost universal for general lighting and power. For the ac power supply to dc transit systems,
and for railroad electrification, a frequency of 25 Hz is used in many
installations. In most of Europe and Latin America the frequency of
50 Hz is in general use. In aircraft the frequency of 400 Hz has become
standard.
Static inverters make it possible to obtain high and variable frequencies to drive motors at greater than the 3,600 r/min limitation on 60-Hz
circuits, and to vary speeds. The textile industry has small motors operating at 12,000 r/min (200 Hz), and larger motors have been run at
6,000 r/min (100 Hz). Many large mainframe computers have been
powered at 400 Hz.
The root-mean-square (rms), or effective, value of a current wave produces the same heating in a given resistance as a direct current of the
same ampere value. Since the heating effect of a current is proportional
to i 2r, the rms value is obtained by squaring the ordinates, finding their
average value, and extracting the square root, i.e., the rms value is
I⫽
(15.1.58)
This current leads the voltage by 90 electrical degrees. Pure capacitance
absorbs no energy. The energy stored in the dielectric field during each
half cycle is returned to the source during the same half cycle.
Impedance opposes the flow of alternating current and is expressed in
⍀. It is denoted by Z. With resistance and inductance in series
√1/T 冕 i dt
T
2
A
(15.1.56)
0
where T is the time of a cycle. The rms value I of a sine wave equals
(1/√2)Im ⫽ 0.707Im .
Average Value of a Wave The average value of a sine wave over a
complete cycle is zero. For a half cycle the average is (2/␲)Im , or
0.637 Im , where Im is the maximum value of the sine wave. The average
value is of importance only occasionally. A dc measuring instrument
gives the average value of a pulsating wave. The average value is of use
(1) when the effects of the current are proportional to the number of
coulombs, as in electrolytic work and (2) when converting alternating to
direct current.
Form Factor The form factor of a wave is the ratio of rms value to
average value. For a sine wave this is ␲/(2√2) ⫽ 1.11. This factor is
important in that it enters equations for induced emf.
Inductive reactance, 2␲ fL or ␻ L, opposes an alternating current in
inductance L. It is expressed in ⍀. Reactance is usually denoted by the
symbol X. Inductive reactance is denoted by X L .
The current in an inductive reactance X L when connected across the
voltage E is
I ⫽ E/X L ⫽ E/(2␲ fL)
A
(15.1.57)
Z ⫽ √R 2 ⫹ X 2C ⫽ √R 2 ⫹ [1/(2␲ fC)]2
⍀
(15.1.60)
With resistance, inductance, and capacitance in series
Z ⫽ √R 2 ⫹ (X L ⫺ X C )2
⫽ √R 2 ⫹ [2␲ fL ⫺ 1/(2␲ fC)]2
⍀
(15.1.61)
The current is
I ⫽ E/√R 2 ⫹ [2␲ fL ⫺ 1/(2␲ fC)]2
A
(15.1.62)
Phasor or Vector Representation Sine waves of voltage and
current can be represented by phasors, these phasors being proportional
in magnitude to the waves that they represent. The angle between two
phasors is also equal to the time angle existing between the two waves
that they represent.
Phasors may be combined as forces are combined in mechanics. Both
graphical methods and the methods of complex algebra are used. Impedances and also admittances may be similarly combined, either
graphically or symbolically. The usual method is to resolve series impedances into their component resistances and reactances, then combine
all resistances and all reactances, from which the resultant impedance is
obtained. Thus Z1 ⫹ Z2 ⫽ √(r1 ⫹ r2 )2 ⫹ (x 1 ⫹ x 2 )2, where r1 and x 1 are
the components of Z 1 , etc.
Phase Difference With resistance only in the circuit, the current
and the voltage are in phase with each other; with inductance only in the
circuit, the current lags the voltage by 90 electrical degrees; with capacitance only in the circuit, the current leads the voltage by 90 electrical
degrees.
With resistance and inductance in series, the voltage leads the current
by angle ␪ where tan ␪ ⫽ X L /R. With resistance and capacitance in
series, the voltage lags the current by angle ␪ where tan ␪ ⫽ ⫺X C /R.
With resistance, inductance, and capacitance in series, the voltage
may lag, lead, or be in phase with the current.
tan ␪ ⫽ (X L ⫺ X C )/R ⫽ (2␲ fL ⫺ 1/2␲ fC)/R
(15.1.63)
If X L ⬎ X C the voltage leads; if X L ⬍ X C the voltage lags; if X L ⫽ X C the
current and voltage are in phase and the circuit is in resonance.
Power Factor In ac circuits the power P ⫽ I 2R where I is the current
and R the effective resistance (see below). Also the power
P ⫽ EI cos ␪
W
(15.1.64)
where ␪ is the phase angle between E and I. Cos ␪ is the power factor
(pf ) of the circuit. It can never exceed unity and is usually less than
unity.
cos ␪ ⫽ P/EI
(15.1.65)
P is often called the true power. The product EI is the volt-amp (V ⭈ A)
and is often called the apparent power.
Active or energy current is the projection of the total current on the
voltage phasor. Ie ⫽ I cos ␪. Power ⫽ EI e .
Reactive, quadrature, or wattless current Iq ⫽ I sin ␪ and is the component of the current that contributes no power but increases the I 2R losses
of the system. In power systems it should ordinarily be made low.
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ALTERNATING CURRENTS
The vars (volt-amp-reactive) are equal to the product of the voltage
and reactive current. Vars ⫽ EIq . Kilovars ⫽ EIq /1,000.
Effective Resistance When alternating current flows in a circuit,
the losses are ordinarily greater than are given by the losses in the ohmic
resistance alone. For example, alternating current tends to flow near the
surface of conductors (skin effect). If iron is associated with the circuit,
eddy-current and hysteresis losses result. These power losses may be
accounted for by increasing the ohmic resistance to a value R, where R
is the effective resistance, R ⫽ P/I 2. Since the iron losses vary as I 1.8 to I 2,
little error results from this assumption.
15-19
is the natural frequency of the circuit and is the frequency at which it will
oscillate if the circuit is not acted upon by some external frequency.
This is the principle of radio sending and receiving circuits. Resonant
conditions of this type should be avoided in power circuits, as the piling
up of voltage may endanger apparatus and insulation.
EXAMPLE. For what value of the inductance in the circuit (Fig. 15.1.27) will
the circuit be in resonance, and what is the voltage across the inductor and capacitor under these conditions?
From Eq. (15.1.66) L ⫽ 1/(2␲ f )2C ⫽ 0.1173 H. I ⫽ E/R ⫽ 120/10 ⫽ 12 A.
L␻I ⫽ I/C␻ ⫽ 0.1173 ⫻ 377 ⫻ 12 ⫽ 530 V. This voltage is over four times the
line voltage.
Parallel Circuits Parallel circuits are used for nearly all power distribution. With several series circuits in parallel it is merely necessary to
find the current in each and add all the current phasors vectorially to
find the total current. Parallel circuits may be solved analytically.
A series circuit has resistance r1 and inductive reactance x 1 . The conductance is
Fig. 15.1.27 Resistor, inductor, and capacitor in series.
g1 ⫽ r1 /(r21 ⫹ x 21 ) ⫽ r1 /Z 21
SOLUTION OF SERIES-CIRCUIT PROBLEM. Let a resistor R of 10 ⍀, an inductor L of 0.06 H, and a capacitor C of 60 ␮F be connected in series across
120-V 60-Hz mains (Fig. 15.1.27). Determine (1) the impedance, (2) the current,
(3) the voltage across the resistance, the inductance, the capacitance, (4) the power
factor, (5) the power, (6) the angle of phase difference.
(1) ␻ ⫽ 2␲ 60 ⫽ 377. X L ⫽ 0.06 ⫻ 377 ⫽ 22.6 ⍀; X C ⫽ 1/(377 ⫻ 0.000060) ⫽
44.2 ⍀; Z ⫽ √(10)2 ⫹ (22.6 ⫺ 44.2)2 ⫽ 23.8 ⍀; (2) I ⫽ 120/23.8 ⫽ 5.04 A;
(3) E R ⫽ IR ⫽ 5.04 ⫻ 10 ⫽ 50.4 V; E L ⫽ IX L ⫽ 5.04 ⫻ 22.6 ⫽ 114.0 V; E C ⫽
IX C ⫽ 5.04 ⫻ 44.2 ⫽ 223 V; (4) tan ␪ ⫽ (X L ⫺ X C )/R ⫽ ⫺21.6/10 ⫽ ⫺2.16,
␪ ⫽ ⫺65.2°, cos ␪ ⫽ pf ⫽ 0.420; (5) P ⫽ 120 ⫻ 5.04 ⫻ 0.420 ⫽ 254 W; P ⫽
I 2R ⫽ (5.04)2 ⫻ 10 ⫽ 254 W (check); (6) From (4) ␪ ⫽ ⫺65.2°. Voltage
lags. The phasor diagram to scale of this circuit is shown in Fig. 15.1.28. Since the
current is common for all elements of the circuit, its phasor is laid horizontally
along the axis of reference.
S
(15.1.67)
S
(15.1.68)
and the susceptance is
b1 ⫽ x 1 /(r 21 ⫹ x 21 ) ⫽ x 1 /Z 21
Conductance is not the reciprocal of resistance unless the reactance is
zero; susceptance is not the reciprocal of reactance unless the resistance
is zero. With inductive reactance the susceptance is negative; with capacitive reactance the susceptance is positive.
If a second circuit has resistance r2 and capacitive reactance x 2 in
series, g 2 ⫽ r2 /(r 22 ⫹ x 22 ) ⫽ r2 /Z 22 ; b2 ⫽ x 2 /(r 22 ⫹ x 22 ) ⫽ x 2 /Z 22 . The total
conductance G ⫽ g1 ⫹ g2 ; the total susceptance B ⫽ ⫺b1 ⫹ b2 . The
admittance is
Y ⫽ √G 2 ⫹ B2 ⫽ 1/Z
S
(15.1.69)
The energy current is EG; the reactive current is EB; the power is
P ⫽ E 2G
vars ⫽ E 2B
W
W
(15.1.70)
(15.1.71)
The power factor is
pf ⫽ G/Y
(15.1.72)
Also the following relations hold:
r ⫽ g/(g 2 ⫹ b2 ) ⫽ g/Y 2
x ⫽ b/(g 2 ⫹ b2 ) ⫽ b/Y 2
Fig. 15.1.28 Phasor diagram for a series circuit.
Resonance If the voltage E and the resistance R [Eq. (15.1.62)] are
fixed, the maximum value of current occurs when 2␲ fL ⫺ 1/2␲ fC ⫽ 0.
The circuit so far as its terminals are concerned behaves like a noninductive resistor. The current I ⫽ E/R, the power P ⫽ EI, and the power
factor is unity.
The voltage across the inductor and the voltage across the capacitor
are opposite and equal and may be many times greater than the circuit
voltage. The frequency
f ⫽ 1/(2␲ √LC)
Hz
(15.1.66)
Fig. 15.1.29 Parallel circuit and phasor diagrams.
⍀
⍀
(15.1.73)
(15.1.74)
SOLUTION OF A PARALLEL-CIRCUIT PROBLEM. In the parallel circuit of
Fig. 15.1.29 it is desired to find the joint impedance, the total current, the power in
each branch, the total power, and the power factor, when E ⫽ 100, f ⫽ 60, R1 ⫽
2 ⍀, R2 ⫽ 4 ⍀, L 1 ⫽ 0.00795 H, X 1 ⫽ 2␲ f L 1 ⫽ 3 ⍀, C 2 ⫽ 1,326 ␮F, X 2 ⫽
1/2␲ fC 2 ⫽ 2 ⍀, Z 1 ⫽ √22 ⫹ 32 ⫽ 3.6 ⍀, and Y1 ⫽ 1/3.6 ⫽ 0.278 S.
Solution: g1 ⫽ R1 /(R 21 ⫹ X 12 ) ⫽ 2/13 ⫽ 0.154; b1 ⫽ ⫺3/13 ⫽ ⫺0.231;
Z 2 ⫽ √16 ⫹ 4 ⫽ 4.47; Y2 ⫽ 1/4.47 ⫽ 0.224; g 2 ⫽ R2 /(R 22 ⫹ X 22 ) ⫽ 4/(16 ⫹ 4) ⫽
0.2 S; b2 ⫽ 2/20 ⫽ 0.1 S; G ⫽ g1 ⫹ g2 ⫽ 0.154 ⫹ 0.2 ⫽ 0.354 S; B ⫽ b1 ⫹ b2 ⫽
⫺0.231 ⫹ 0.1 ⫽ ⫺0.131 S; Y ⫽ √G2 ⫹ B2 ⫽ √0.3542 ⫹ (⫺0.131)2 ⫽ 0.377 S,
and joint impedance Z ⫽ 1/0.377 ⫽ 2.65 ⍀. Phase angle ␪ ⫽ tan⫺ 1 (⫺0.131/
0.354) ⫽ ⫺20.3°. I ⫽ EY ⫽ 100 ⫻ 0.377 ⫽ 37.7 A; P1 ⫽ E 2g1 ⫽ 1002 ⫻ 0.154 ⫽
1,540 W; P2 ⫽ E 2g2 ⫽ 1002 ⫻ 0.2 ⫽ 2,000 W; total power ⫽ E 2G ⫽ 1002 ⫻
0.354 ⫽ 3,540 W. Power factor ⫽ cos ␪ ⫽ 3,540/(100 ⫻ 37.7) ⫽ 93.8 percent.
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15-20
ELECTRICAL ENGINEERING
With parallel circuits, unity power factor is obtained when the algebraic sum of the quadrature currents is zero. That is, b1 ⫹ b2 ⫹
b3 ⭈ ⭈ ⭈ ⫽ 0.
Three-Phase Circuits Ac generators are usually wound with three
armature circuits which are spaced 120 electrical degrees apart on the
armature. Hence these coils generate emfs 120 electrical degrees apart.
The coils are connected either in Y (star) or in ⌬ (mesh) as shown in Fig.
15.1.30. Whether Y- or ⌬-connected, with a balanced load, the three
ductor OO⬘. If Ec is the voltage across OA or OB, √2 E c will be the
voltage across AB. The power of a two-phase circuit is twice the power
in either coil if the load is balanced. Normally, the voltages OA and OB
are equal, and the current is the same in both coils. Owing to nonsymmetry and the high degree of unbalancing of this system even under
balanced loads, it is not used at the present time for transmission and is
little used for distribution.
Four-Phase Circuit A four-phase or quarter-phase circuit is shown
in Fig. 15.1.32. The windings AC and BD may be independent or con-
Fig. 15.1.30 Three-phase connections. (a) Y connection; (b) ⌬ connection.
coil emfs Ec and the three coil currents Ic are equal. In the Y connection
the line and coil currents are equal, but the line emfs EAB , EBC , ECA are
√3 times in magnitude the coil emfs EOA , EOB , EOC , since each is the
phasor difference of two coil emfs. In the delta connection the line and
coil emfs are equal, but I, the line current, is √3 Ic , the coil current, i.e., it
is the phasor difference of the currents in the two coils connected to the
line. The power of a coil is Ec Ic cos ␪, so that the total power is 3Ec Ic cos
␪. If ␪ is the angle between coil current and coil voltage, the angle
between line current and line voltage will be 30° ⫾ ␪. In terms of line
current and emf, the power is √3 EI cos ␪. A fourth or neutral conductor
connected to O is frequently used with the Y connection. The neutral
point O is frequently grounded in transmission and distribution circuits.
The coil emfs are assumed to be sine waves. Under these conditions
they balance, so that in the delta connection the sum of the two coil emfs
at each instant is balanced by the third coil emf. Even though the third,
ninth, fifteenth . . . harmonics, 3(2n ⫹ 1)f, where n ⫽ 0 or an integer,
exist in the coil emfs, they cannot appear between the three external line
conductors of the three-phase Y-connected circuit. In the delta circuit,
the same harmonics 3(2n ⫹ 1)f cause local currents to circulate around
the mesh. This may cause a very appreciable heating. In a three-phase
system the power
P ⫽ √3 EI cos ␪
W
(15.1.75)
the power factor is
P/√3 EI
(15.1.76)
√3 EI/1,000
(15.1.77)
and the kV ⭈ A
where E and I are line voltages and currents.
Two-Phase Circuits Two-phase generators have two windings
spaced 90 electrical degrees apart on the armature. These windings
generate emfs differing in time phase by 90°. The two windings may be
independent and power transmitted to the receiver though the two single-phase circuits are entirely insulated from each other. The two circuits may be combined into a two-phase three-wire circuit such as is
shown in Fig. 15.1.31, where OA and OB are the generator circuits (or
Fig. 15.1.31 Two-phase, three-wire circuit.
transformer secondaries) and A⬘O⬘ and B⬘O⬘ are the load circuits. The
wire OO⬘ is the common wire and under balanced conditions carries a
current √2 times the current wires AA⬘ and BB⬘. For example, if Ic is the
coil current, √2 Ic will be the value of the current in the common con-
Fig. 15.1.32
Four-phase or quarter-phase circuit.
nected at O. The voltages AC and BD are 90 electrical degrees apart as
in two-phase circuits. If a neutral wire O-O⬘ is added, three different
voltages can be obtained. Let E1 ⫽ voltage between O-A, O-B, O-C,
O-D. Voltages between A-B, B-C, C-D, D-A ⫽ √2 E 1 . Voltages between
A-C, B-D ⫽ 2E 1 . Because of this multiplicity of voltages and the fact
that polyphase power apparatus and lamps may be connected at the
same time, this system is still used to some extent in distribution.
Advantages of Polyphase Power The advantages of polyphase
power over single-phase power are as follows. The output of synchronous generators and most other rotating machinery is from 60 to 90
percent greater when operated polyphase than when operated single
phase; pulsating fluxes and corresponding iron losses which occur in
many common types of machinery when operated single phase are negligible when operated polyphase; with balanced polyphase loads polyphase power is constant whereas with single phase the power fluctuates
over wide limits during the cycle. Because of its minimum number of
wires and the fact that it is not easily unbalanced, the three-phase system
has for the most part superseded other polyphase systems.
ELECTRICAL INSTRUMENTS AND
MEASUREMENTS
Electrical measuring devices that merely indicate, such as ammeters and
voltmeters, are called instruments; devices that totalize with time such as
watthour meters and ampere-hour meters are called meters. (See also
Sec. 16.) Most types of electrical instruments are available with digital
read out.
DC Instruments Direct current and voltage are both measured with
an indicating instrument based on the principle of the D’Arsonval galvanometer. A coil with steel pivots and turning in jewel bearings is
mounted in a magnetic field produced by permanent magnets. The motion is restrained by two small flat coiled springs, which also serve to
conduct the current to the coil. The deflections of the coil are read with a
light aluminum pointer attached to the coil and moving over a graduated
scale. The same instrument may be used for either current or voltage,
but the method of connecting in circuit is different in the two cases.
Usually, however, the coil of an instrument to be used as an ammeter is
wound with fewer turns of coarser wire than an instrument to be used as
a voltmeter and so has lower resistance. The instrument itself is frequently called a millivoltmeter. It cannot be used alone to measure voltage of any magnitude since its resistance is so low that it would be
burned out if connected across the line. Hence a resistance r⬘ in series
with the coil is necessary as indicated in Fig. 15.1.33a in which rc is the
resistance of the coil. From 0.2 to 750 V this resistance is usually within
the instrument. For higher voltages an external resistance R, called an
extension coil or multiplier (Fig. 15.1.33b), is necessary. Let e be the
reading of the instrument, in volts (Fig. 15.1.33b), r the internal resis-
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ELECTRICAL INSTRUMENTS AND MEASUREMENTS
tance of the instrument, including r⬘ and rc in Eq. (15.1.33a), R the
resistance of the multiplier. Then the total voltage is
E ⫽ e(R ⫹ r)/r
(15.1.78)
It is clear that by using suitable values of R a voltmeter can be made to
have several scales.
Fig. 15.1.33 Voltmeters. (a) Internal resistance; (b) with multiplier.
Instruments themselves can only carry currents of the magnitudes of
0.01 to 0.06 A. To measure larger values of current the instrument is
provided with a shunt R (Fig. 15.1.34). The current divides inversely as
the resistances r and R of the instrument and the shunt. A low resistance
r⬘ within the instrument is connected in series with the coil. This permits some adjustment to the deflection so that the instrument can be
15-21
general, indicate so accurately on direct as on alternating current because of the effects of external stray magnetic fields. Also reversed
readings should be taken. Iron vane instruments consist of a fixed coil
which actuates magnetically a light movable iron vane mounted on a
spindle; they are rugged, inexpensive, and may be had in ranges of 30 to
750 V and 0.05 to 100 A. They measure rms values and tend to have
compressed scales as in the case of electrodynamometer instruments.
The compressed part of the scale may, however, be extended by
changing the shape of the vanes. Such instruments operate with direct
current and are accurate to within 1 percent or so. AC instruments of the
induction type (Westinghouse Electric Corp.) must be used on ac circuits
of the frequency for which they have been designed. They are rugged
and relatively inexpensive and are used principally for switchboards
where a long-scale range and a strong deflecting torque are of particular
advantage. Thermocouple instruments operate on the Seebeck effect. The
current to be measured is conducted through a heater wire, and a thermojunction is either in thermal contact with the heater or is very close to
it. The emf developed in the thermojunction is measured by a permanent-magnet dc type of instrument. By controlling the shape of the air
gap, a nearly uniform scale is obtained. This type of instrument is well
adapted to the measurement of high-frequency currents or voltages, and
since it operates on the heating effect of current, it is convenient as a
transfer instrument between direct current and alternating current.
In the rectifier-type instrument the ac voltage or current is rectified,
usually by means of a small copper oxide or a selenium-type rectifier,
connected in a bridge circuit to give full-wave rectification (Fig.
15.1.35). The rectified current is measured with a dc permanent-mag-
Fig. 15.1.34 Millivoltmeter with shunt.
adapted to its shunt. Usually most of the current flows through the
shunt, and the current in the instrument is negligible in comparison. Up
to 50 and 75 A the shunt can be incorporated within the instrument. For
larger currents it is usually necessary to have the shunt external to the
instrument and connect the instrument to the potential terminals of the
shunt by means of leads. Any given instrument may have any number of
ranges by providing it with a sufficient number of shunts. The range of
the usual instrument of this type is approximately 50 mV. Although the
same instrument may be used for voltmeters or ammeters, the moving
coils of voltmeters are usually wound with more turns of finer wire.
They take approximately 0.01 A so that their resistance is approximately 100 ⍀/V. Instruments used as ammeters alone operate with 0.01
to 0.06 A.
Permanent-magnet moving-coil instruments may be used to measure
unidirectional pulsating currents or voltages and in such cases will indicate the average value of the periodically varying current or voltage.
AC Instruments Instruments generally used for alternating currents
may be divided into five types: electrodynamometer, iron-vane, thermocouple, rectifier, and electronic. Instruments of the electrodynamometer
type, the most precise, operate on the principle of one coil carrying
current, turning in the magnetic field produced by a second coil carrying
current taken from the same circuit. If these circuits or coils are connected in series, the torque exerted on the moving system for a given
relative position of the coil system is proportional to the square of the
current and is not dependent on the direction of the current. Consequently, the instrument will have a compressed scale at the lower end
and will usually have only the upper two-thirds of the scale range useful
for accurate measurement. Instruments of this type ordinarily require
0.04 to 0.08 A or more in the moving-coil circuit for full-scale deflection. They read the rms value of the alternating or pulsating current. The
wattmeter operates on the electrodynamometer principle. The fixed
coil, however, is energized by the current of the circuit, and the moving
coil is connected across the potential in series with high resistance.
Unless shielded magnetically the foregoing instruments will not, in
Fig. 15.1.35
Rectifier-type instrument.
net-type instrument M. The instrument measures the average value of
the half waves that have been rectified, and with the sine waves, the
average value is 0.9 the rms value. The scale is calibrated to indicate
rms values. With nonsinusoidal waves the ratio of average to rms may
vary considerably from 0.9 so that the instrument may be in error up to
⫾ 5 percent from this cause. This type of instrument is widely used in
the measurement of high-frequency voltages and currents. Electronic
voltmeters operate on the principle of the amplification which can be
obtained with a transistor. Since the emf to be measured is applied to the
base, the instruments take practically no current and hence are adapted
to measure potential differences which would change radically were any
appreciable current taken by the measuring device. This type of instrument can measure voltages from a few tenths of a volt to several
hundred volts, and with a potential divider, up to thousands of volts.
They are also adapted to frequencies up to 100 MHz.
Particular care must be used in selecting instruments for measuring
the nonsinusoidal waves of rectifier and controlled rectifier circuits. The
electrodynamic, iron vane, and thermocouple instruments will read rms
values. The rectifier instrument will read average values, while the electronic instrument may read either rms or average value, depending on
the type.
Power Measurement in Single-Phase Circuits Wattmeters are not
rated primarily in W, but in A and V. For example, with a low power
factor the current and voltage coils may be overloaded and yet the
needle be well on the scale. The current coil may be carrying several
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15-22
ELECTRICAL ENGINEERING
times its rated current, and yet the instrument reads zero because the
potential circuit is not closed, etc. Hence it is desirable to use both an
ammeter and a voltmeter in conjunction with a wattmeter when measuring power (Fig. 15.1.36a). The instruments themselves consume appreciable power, and correction is often necessary unless these losses are
negligible compared with the power being measured. For example, in
Fig. 15.1.36a, the wattmeter measures the I 2 R loss in its own current
coil and in the ammeter (1 to 2 W each), as well as the loss in the
Fig. 15.1.38
Fig. 15.1.36 Connections of instruments to single-phase load.
voltmeter (⫽ E 2/R where R is the resistance of the voltmeter). The
losses in the ammeter and voltmeter may be eliminated by short-circuiting the ammeter and disconnecting the voltmeter when reading the
wattmeter. If the wattmeter is connected as shown in Fig. 15.1.36b,
it measures the power taken by its own potential coil (E 2/Rp ) which at
110 V is 5 to 7 W. (Rp is the resistance of the potential circuit.) Frequently correction must be made for this power.
Power measurement in an n-wire system.
The thermal watt converter is also used to measure power. This instrument produces a dc voltage proportional to three-phase ac power.
Three-Phase Systems The three-wattmeter method (Fig. 15.1.37)
is applicable to any three-phase system. It is commonly used with the
three-phase four-wire system. If the loads are balanced, P1 ⫽ P2 ⫽ P3
and the power P ⫽ 3P1 .
The two-wattmeter method is most commonly used with three-phase
three-wire systems (Fig. 15.1.39). The current coils may be connected
in any two wires, the potential circuits being connected to the third. It
will be recognized that this is adapting the method of Fig. 15.1.38
to three wires. With balanced loads the readings of the wattmeters are
Power Measurement in Polyphase Circuits; Three-Wattmeter
Method Let ao, bo, and co be any Y-connected three-phase load (Fig.
15.1.37). Three wattmeters with their current coils in each line and their
potential circuits connected to neutral measure the total power, since the
power in each load is measured by one of the wattmeters. The connection oo⬘ may, however, be broken, and the total power is still the sum of
the three readings; i.e., the power P ⫽ P1 ⫹ P2 ⫹ P3 . This method is
Fig. 15.1.39
Two-wattmeter method.
P1 ⫽ Ei cos (30° ⫹ ␪ ), P2 ⫽ Ei cos (30° ⫺ ␪ ), and P ⫽ P2 ⫾ P1 . ␪ is the
angle of phase difference between coil voltage and current. Since
P1 /P2 ⫽ cos (30° ⫹ ␪ )/cos (30° ⫺ ␪ )
(15.1.79)
the power factor is a function of P1 /P2 . Table 15.1.9 gives values of
power factor for different ratios of P1 /P2 .
P ⫽ P2 ⫹ P1 when ␪ ⬍ 60°.
When ␪ ⫽ 60°, pf ⫽ cos 60° ⫽ 0.5, P1 ⫽ cos (30° ⫹ 60°) ⫽ 0, P ⫽
P2 . When ␪ ⬎ 60°, pf ⬍ 0.5, P ⫽ P2 ⫺ P1 . Also,
Fig. 15.1.37 Three-wattmeter method.
applicable to any system of n wires. The current coil of one wattmeter is
connected in each of the n wires. The potential circuit of each wattmeter
is connected between its own phase wire and a junction in common with
all the other potential circuits. The wattmeters must be connected symmetrically, and the readings of any that read negative must be given the
negative sign.
In the general case any system of n wires requires at least n ⫺ 1
wattmeters to measure the power correctly. The n ⫺ 1 wattmeters are
connected in series with n ⫺ 1 wires. The potential circuit of each is
connected between its own phase wire and the wire in which no wattmeter is connected (Fig. 15.1.38).
Table 15.1.9
tan ␪ ⫽ √3 (P2 ⫺ P1)/(P2 ⫹ P1)
Ratio P1 /P2 and Power Factor
P1 /P2
Power
factor
⫹ 1.0
⫹ 0.9
⫹ 0.8
⫹ 0.7
⫹ 0.6
⫹ 0.5
1.000
0.996
0.982
0.956
0.918
0.866
(15.1.80)
In a polyphase wattmeter the two single-phase wattmeter elements are
combined to act on a single spindle. Hence the adding and subtracting of
the individual readings are done automatically. The total power is indicated on one scale. This type of instrument is almost always used on
switchboards. The connections of a portable type are shown in Fig.
15.1.40.
In the foregoing instrument connections, Y-connected loads are
shown. These methods are equally applicable to delta-connected loads.
The two-wattmeter method (Fig. 15.1.39) is obviously adapted to the
two-phase three-wire system (Fig. 15.1.31).
P1 /P2
Power
factor
P1 /P2
Power
factor
P1 /P2
Power
factor
⫹ 0.4
⫹ 0.3
⫹ 0.2
⫹ 0.1
0.0
0.804
0.732
0.656
0.576
0.50
⫺ 0.1
⫺ 0.2
⫺ 0.3
⫺ 0.4
⫺ 0.5
0.427
0.360
0.296
0.240
0.188
⫺ 0.6
⫺ 0.7
⫺ 0.8
⫺ 0.9
⫺ 1.0
0.142
0.102
0.064
0.030
0.000
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ELECTRICAL INSTRUMENTS AND MEASUREMENTS
15-23
Power-Factor Measurement The usual method of determining
power factor is by the use of voltmeter, ammeter, and wattmeter. The
wattmeter gives the watts of the circuit, and the product of the voltmeter
reading and the ammeter reading gives the volt-amperes. The power
factor is the ratio of the two [see Eqs. (15.1.65) and (15.1.76)]. Also
single-phase and three-phase power-factor indicators, which can be
connected directly in circuit, are on the market.
Fig. 15.1.40 Connections for a polyphase wattmeter in a three-phase circuit.
Measurement of Energy
Watthour meters record the energy taken by a circuit over some interval
of time. Correct registration occurs if the angular velocity of the rotating
element at every instant is proportional to the power. The method of
accomplishing this with dc meters is illustrated in Fig. 15.1.41. The
meter is in reality a small motor. The field coils FF are in series with the
line. The armature A is connected across the line, usually in series with a
resistor R. The movable field coil F⬘ is in series with the armature A and
serves to compensate for friction. C is a small commutator, either of
Fig. 15.1.41 DC watthour meter.
copper or of silver, and the two small brushes are usually of silver. An
aluminum disk, rotating between the poles of permanent magnets M,
acts as a magnetic brake the retarding torque of which is proportional to
the angular velocity of the disk. A small worm and the gears G actuate
the recording dials.
The following relation, or an equivalent, holds with most types of
meter. With each revolution of the disk, K Wh are recorded, where K is
the meter constant found usually on the disk. It follows that the average
watts P over any period of time t sec is
P ⫽ 3,600KN/t
(15.1.81)
where N is the revolutions of the disk during that period. Hence, the
meter may be calibrated by connecting standardized instruments to
measure the average power taken by the load and by counting the revolutions N for t s. Near full load, if the meter registers fast, the magnets M
should be moved outward radially; if it registers slow, the magnets
should be moved inward. If the meter registers fast at light (5 to 10
percent) load, the starting coil F⬘ should be moved further away from
the armature; if it registers slow, F⬘ should be moved nearer the armature. A meter should not register more than 1.5 percent fast or slow, and
with calibrated standards it can be made to register to within 1 percent
of correct.
The induction watthour meter is used with alternating current. Although the dc meter registers correctly with alternating current, it is
more expensive than the induction type, the commutator and brushes
may cause trouble, and at low power factors compensation is necessary.
In the induction watthour meter the driving torque is developed in the
aluminum disk by the joint action of the alternating magnetic flux produced by the potential circuit and by the load current. The driving
torque and the retarding torque are both developed in the same aluminum disk, hence no commutator and brushes are necessary. The rotating
element is very light, and hence the friction torque is small. Equation
(15.1.81) applies to this type of meter. When calibrating, the average
power W for t s is determined with a calibrated wattmeter. The friction
compensation is made at light loads by changing the position of a small
hollow stamping with respect to the potential lug. The meter should also
be adjusted at low power factor (0.5 is customary). If the meter is slow
with lagging current, resistance should be cut out of the compensating
circuit; if slow with leading current, resistance should be inserted.
Instrument Transformers
With voltages higher than 600 V, and even at 600 V, it becomes dangerous and inaccurate to connect instruments and meters directly into
power lines. It is also difficult to make potential instruments for voltages in excess of 600 V and ammeters in excess of 60-A ratings. To
insulate such instruments from high voltage and at the same time to
permit the use of low-range instruments, instrument transformers are
used. Potential transformers are identical with power transformers except that their volt-ampere rating is low, being 40 to 500 W. Their
primaries are wound for line voltage and their secondaries for 110 V.
Current transformers are designed to go in series with the line, and the
rated secondary current is 5 A. The secondary of a current transformer
should always be closed when current is flowing; it should never be allowed to become open circuited under these conditions. When open-circuited the voltage across the secondary becomes so high as to be dangerous and the flux becomes so large in magnitude that the transformer
overheats. Semiconductors that break down at safe voltages and short
current-transformer secondaries are available to ensure that the secondary is closed. The secondaries of both potential and current transformers
should be well grounded at one point (Figs. 15.1.42 and 15.1.43). Instrument transformers introduce slight errors because of small variations in their ratio with load. Also there is slight phase displacement in
both current and potential transformers. The readings of the instruments
must be multiplied by the instrument transformer ratios. The scales of
switchboard instruments are usually calibrated to take these ratios into
account.
Fig. 15.1.42
Single-phase connections of instruments with transformers.
Figure 15.1.42 shows the use of instrument transformers to measure
the voltage, current, power, and kilowatthours of a single-phase load.
Figure 15.1.43 shows the connections that would be used to measure the
voltage, current, and power of a 26,400-V 600-A three-phase load.
Fig. 15.1.43
formers.
Three-phase connections of instruments and instrument trans-
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15-24
ELECTRICAL ENGINEERING
Measurement of High Voltages Potential transformers such as
those shown in Figs. 15.1.42 and 15.1.43 may be used even for very
high voltages, but for voltages above 132 kV they become so large and
expensive that they are used only sparingly. A convenient method used
with testing transformers is the employment of a voltmeter coil, which
consists of a coil of a few turns interwoven in the high-voltage winding
and insulated from it. The voltage ratio is the ratio of the turns in the
high-voltage winding to those in the voltmeter coil. A capacitance voltage divider consists of two or more capacitors connected in series across
the high voltage to be measured. A high-impedance voltmeter, such as
an electronic one, is connected across the capacitor at the grounded end.
The high voltage V ⫽ VmCm /C V, where Vm is the voltmeter reading, C
the capacitance (in ␮F) of the entire divider, and Cm the equivalent
capacitance (in ␮F) of the capacitor at the grounded end. A bushing
potential device consists of a high-voltage-transformer bushing having
a capacitance tap brought out from one of the metallic electrodes within
the bushing which is near ground potential. This device is obviously a
capacitance voltage divider. For testing, sphere gaps are used for the
very high voltages. Calibration data for sphere gaps are given in the
ANSI/IEEE Std. 4-1978 Standard Techniques for High Voltage Testing. Even when it is not being used for the measurement of voltage, it is
frequently advisable to connect a sphere gap in parallel with the specimen being tested so as to prevent overvoltages. The gap is set to a
slightly higher voltage than that which is desired.
Measurement of Resistance
Voltmeter-Ammeter Method A common method of measuring resistance, known as the voltmeter-ammeter or fall-in-potential method,
makes use of an ammeter and a voltmeter. In Fig. 15.1.44, the resistance
to be measured is R. The current in the resistor R is I A, which is
measured by the ammeter A in series. The drop in potential across the
resistor R is measured by the voltmeter V. The current shunted by the
voltmeter is so small that it may generally be neglected. A correction
tion with the switch connecting S and A is E/(R ⫹ r), where r is the
resistance of the voltmeter. A high-resistance voltmeter is necessary,
since the method is in reality a comparison of the unknown insulation
resistance R with the known resistance r of the voltmeter. Hence, the
Fig. 15.1.45
Voltmeter method for insulation resistance measurement.
resistance of the voltmeter must be comparable with the unknown resistance, or the deflection of the instrument will be so small that the
results will be inaccurate. To determine the impressed voltage E, the
same voltmeter is used. The switch S connects S and B for this purpose.
With these two readings, the unknown resistance is
R ⫽ r(E ⫺ e)/e
(15.1.82)
where e is the deflection of the voltmeter when in series with the resistance to be measured as when S is at A. If a special voltmeter, having a
resistance of 100 k⍀ per 150 V, is available, a resistance of the order of
2 to 3 M⍀ may be measured very accurately.
When the insulation resistance is too high to be measured with a
voltmeter, a sensitive galvanometer may be used. The connections for
measuring the insulation resistance of a cable are shown in Fig. 15.1.46.
The battery should have an emf of at least 100 V. Radio B batteries are
convenient for this purpose. The method involves comparing the unknown resistance with a standard 0.1 M⍀. To calibrate the galvanometer the cable is short-circuited (dotted line) and the switch S is thrown to
Fig. 15.1.44 Voltmeter-ammeter method for resistance measurement.
may be applied if necessary, for the resistance of the voltmeter is generally given with the instrument. The potential difference divided by the
current gives the resistance included between the voltmeter leads. As a
check, determinations are generally made with several values of current,
which may be varied by means of the controlling resistor r. If the resistance to be measured is that of the armature of a dc machine and the
voltmeter leads are placed on the brush holders, the resistance determined will include that of the brush contacts. To measure the resistance
of the armature alone, the voltmeter leads should be placed directly on
the commutator segments on which the brushes rest but not under the
brushes.
Insulation Resistance Insulation resistance is so high that it is usually given in megohms (106 ohms, M⍀) rather than in ohms. Insulation
resistance tests are important, for although they may not be conclusive
they frequently reveal flaws in insulation, poor insulating material,
presence of moisture, etc. Such tests are applied to the insulation of
electrical machinery from the windings to the frame, to underground
cables, to insulators, capacitors, etc.
For moderately low resistances, 1 to 10 M⍀, the voltmeter method
given in Fig. 15.1.45, which shows insulation measurement to the frame
of the field winding of a generator, may be used. To measure the current
when a voltage E is impressed across the resistor R, a high-reading
voltmeter V is connected in series with R. The current under this condi-
Fig. 15.1.46
Measurement of insulation resistance with a galvanometer.
position (a). Let the galvanometer deflection be D1 and the reading of
the Ayrton shunt S1 . The short circuit is then removed. The 0.1 M⍀ is
left in circuit since it is usually negligible in comparison with the unknown resistance X. Let the reading of the galvanometer now be D2 and
the reading of the shunt S2 . Then
X ⫽ 0.1 S2 D1 /S1 D2
M⍀
(15.1.83)
When the switch S is thrown to position (b), the cable is short-circuited through the 0.1 M⍀ and becomes discharged.
The Megger insulation tester is an instrument that indicates insulation
resistance directly on a scale. It consists of a small hand or motor-driven
generator which generates 500 V, 1,000 V, 2,500 V, or 5,000 V. A
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DC GENERATORS
clutch slips when the voltage exceeds the rated value. The current
through the unknown resistance flows through a moving element consisting of two coils fastened rigidly together, but which move in different portions of the magnetic field. A pointer attached to the spindle of
the moving element indicates the insulation resistance directly. These
instruments have a range up to 10,000 M⍀ and are very convenient
where portability and convenience are desirable.
The insulation resistance of electrical machinery may be of doubtful
significance as far as dielectric strength is concerned. It varies widely
with temperature, humidity, and cleanliness of the parts. When the insulation resistance falls below the prescribed value, it can (in most cases
of good design) be brought to the required standard by cleaning and
drying the machine. Hence it may be useful in determining whether or
not the insulation is in proper condition for a dielectric test. IEEE Std.
62-1978 specifies minimum values of insulation resistance in M⍀ ⫽
(rated voltage)/(rating in kW ⫹ 1,000). If the operating voltage is higher
than the rated voltage, the operating voltage should be used. The rule
specifies that a dc voltage of 500 be used in testing. If not, the voltage
should be specified.
Wheatstone Bridge Resistors from a fraction of an ⍀ to 100 k⍀
and more may be measured with a high degree of precision with the
Wheatstone bridge (Fig. 15.1.47). The bridge consists of four resistors
ABCX connected as shown. X is the unknown resistance; A and B are
ratio arms, the resistance units of which are in even decimal ⍀ as 1, 10,
100, etc. C is the rheostat arm. A battery or low-voltage source of direct
current is connected across ab. A galvanometer G of moderate sensitivity is connected across cd. The values of A and B are so chosen that three
Fig. 15.1.48
or four significant figures in the value of C are obtained. As a first
approximation it is well to make A and B equal. When the bridge is in
balance,
X/C ⫽ A/B
(15.1.84)
The positions of the battery and galvanometer are interchangeable.
There are many modifications of the bridge which adapt it to measurements of very low resistances and also to ac measurements.
Kelvin Double Bridge The simple Wheatstone bridge is not adapted
to measuring very low resistances since the contact resistances of the
test specimen become comparable with the specimen resistance. This
error is avoided in the Kelvin double bridge, the diagram of which is
shown in Fig. 15.1.48. The specimen X, which may be a short length of
copper wire or bus bar, is connected in series with an adjustable calibrated resistor R whose resistance is comparable with that of the specimen. The arms A and B of the bridge are ratio arms usually with decimal
values of 1, 10, 100 ⍀. One terminal of the galvanometer is connected to
X and R by means of two resistors a and b. If these resistors are set so
that a/b ⫽ A/B, the contact resistance r between X and R is eliminated in
the measurement. The contact resistances at c and d have no effect since
at balance the galvanometer current is zero. The contact resistances at f
and e need only be negligible compared with the resistances of arms A
and B both of which are reasonably high. By means of the variable
resistor Rh the value of current, as indicated by ammeter A, may be
adjusted to give the necessary sensitivity. When the bridge is in balance,
X/R ⫽ A/B
(15.1.85)
Kelvin double bridge.
Potentiometer The principle of the potentiometer is shown in Fig.
15.1.49. ab is a slide wire, and bc consists of a number of equal individual resistors between contacts. A battery Ba the emf of which is approximately 2 V supplies current to this wire through the adjustable rheostat
R. A slider m makes contact with ab, and a contactor m⬘ connects with
the contacts in bc. A galvanometer G is in series with the wire connecting to m. By means of the double-throw double-pole switch Sw, either
Fig. 15.1.49
Fig. 15.1.47 Wheatstone bridge.
15-25
Potentiometer principle.
the standard cell or the unknown emf (EMF) may be connected to mm⬘
through the galvanometer G. The potentiometer is standardized by
throwing Sw to the standard-cell side, setting mm⬘ so that their positions
on ab and bc correspond to the emf of the standard cell. The rheostat R
is then adjusted until G reads zero. (In commercial potentiometers a dial
which may be set directly to the emf of the standard cell is usually
provided.) The unknown emf is measured by throwing Sw to EMF and
adjusting m and m⬘ until G reads zero. The advantage of this method of
measuring emf is that when the potentiometer is in balance no current is
taken from either the standard cell or the source of emf. Potentiometers
seldom exceed 1.6 V in range. To measure voltage in excess of this, a
volt box which acts as a multiplier is used. To measure current, the
voltage drop across a standard resistor of suitable value is measured
with the potentiometer. For example, with 50 A a 0.01-⍀ standard
resistance gives a voltage drop of 0.5 V which is well within the range
of the potentiometer.
Potentiometers of low range are used extensively with thermocouple
pyrometers. Figure 15.1.49 merely illustrates the principle of the potentiometer. There are many modifications, conveniences, etc., not shown
in Fig. 15.1.49.
DC GENERATORS
All electrical machines are comprised of a magnetic circuit of iron (or
steel) and an electric circuit of copper. In a generator the armature
conductors are rotated so that they cut the magnetic flux coming from
and entering the field poles. In the dc generator (except the unipolar
type) the emf induced in the individual conductors is alternating, but
this is rectified by the commutator and brushes, so that the current to the
external circuit is unidirectional.
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15-26
ELECTRICAL ENGINEERING
The induced emf in a generator (or motor)
V
E ⫽ ␾ ZNP/60P⬘108
(15.1.86)
where ␾ ⫽ flux in webers entering the armature from one north pole;
Z ⫽ total number of conductors on the armature; N ⫽ speed, r/min;
P ⫽ number of poles; and P⬘ ⫽ number of parallel paths through the
armature.
Since with a given generator, Z, P, P⬘ are fixed, the induced emf
V
(15.1.87)
E ⫽ K␾N
where K is a constant. When the armature delivers current, the terminal
volts are
V ⫽ E ⫺ Ia R a
(15.1.88)
where Ia is the armature current and R a the armature resistance including the brush and contact resistance, which vary somewhat.
There are three standard types of dc generators: the shunt generator,
the series generator, and the compound generator.
Shunt Generator The field of the shunt generator in series with its
rheostat is connected directly across the armature as shown in Fig.
15.1.50. This machine maintains approximately constant terminal voltage over its working range of load. An external characteristic of the
generator is shown in Fig. 15.1.51. As load is applied the terminal
voltage drops owing to the armature-resistance drop [Eq. (15.1.88)] and
armature reaction which decreases the flux. The drop in terminal voltage reduces the field current which in turn reduces the flux, hence the
induced emf, etc. At some point B, usually well above rated current, the
foregoing reactions become cumulative and the generator starts to break
Fig. 15.1.50 Shunt generator.
down. The current reaches a maximum value and then decreases to
nearly zero at short circuit. With large machines, point B is well above
rated current, the operating range being between O and A. The voltage
may be maintained constant by means of the field rheostat. Automatic
regulators which operate through field resistance are frequently used to
maintain constant voltage.
Shunt generators are used in systems which are all tied together
where their stability when in parallel is an advantage. If a generator fails
to build up, (1) the load may be connected; (2) the field resistance may
be too high; (3) the field circuit may be open; (4) the residual magnetism
may be insufficient; (5) the field connection may be reversed.
Series Generator In the series generator (Fig. 15.1.52) the entire
load current flows through the field winding, which consists of relatively few turns of wire of sufficient size to carry the entire load current
without undue heating. The field excitation, and hence the terminal
voltage, depends on the magnitude of the load current. The generator
Fig. 15.1.51 Shunt generator
characteristic.
Fig. 15.1.52
Compound-Wound Generators By the addition of a series winding
to a shunt generator the terminal voltage may be automatically maintained very nearly constant, or, by properly proportioning the series
turns, the terminal voltage may be made to increase with load to compensate for loss of voltage in the line, so that approximately constant
voltage is maintained at the load. If the shunt field is connected outside
the series field (Fig. 15.1.53), the machine is long shunt; if the shunt
field is connected inside the series field, i.e., directly to the armature
terminals, it is short shunt. So far as the operating characteristic is concerned, it makes little difference which way a machine is connected.
Table 15.1.10 gives performance characteristics.
Series generator.
supplies an essentially constant current and for years was used to supply
series arc lamps for street lighting requiring direct current. Except for
some special applications, the series generator is now obsolete.
Fig. 15.1.53
Compound-wound dc generator.
Compound-wound generators are chiefly used for small isolated
plants and for generators supplying a purely motor load subject to rapid
fluctuations such as in railway work. When first putting a compound
generator in service, the shunt field must be so connected that the machine builds up. The series field is then connected so that it aids the
shunt field. Figure 15.1.54 gives the characteristics of an overcompounded 200-kW 600-V compound-wound generator.
Amplidynes The amplidyne is a dc generator in which a small
amount of power supplied to a control field controls the generator output, the response being nearly proportional to the control field input.
The amplidyne is a dc amplifier which can supply large amounts of
power. The amplifier operates on the principle of armature reaction. In
Fig. 15.1.55, NN and SS are the conventional north and south poles of a
dc generator with central cavities. BB are the usual brushes placed at
right angles to the pole axes of NN and SS. A control winding CC of
Fig. 15.1.54
Characteristics of a 200-kW compound-wound dc generator.
Table 15.1.10 Approximate Test Performance of
Compound-Wound DC Generators with Commutating Poles
Efficiencies, percent
kW
Speed,
r/min
Volts
Amperes
5
10
25
50
100
200
400
1,000
1,750
1,750
1,750
1,750
1,750
1,750
1,750
1,750
125
125
125
125
125
125
250
250
40
80
200
400
800
1,600
1,600
4,000
SOURCE: Westinghouse Electric Corp.
⁄ load
14
77.0
80.0
84.0
83.0
87.0
88.0
91.7
92.1
⁄ load
Full load
80.5
83.0
86.5
86.0
88.5
90.5
91.9
92.6
82.0
85.0
88.0
88.0
90.0
91.0
91.7
92.1
12
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DC MOTORS
small rating, as low as 100 W, is wound on the field poles. In Fig.
15.1.55, for simplicity, the control winding is shown as being wound on
one pole only. The brushes BB are short-circuited, so that a small excitation mmf in the control field produces a large short-circuit current
along the brush axis BB. This large short-circuit current produces a
large armature-reaction flux AA along brush axis BB. The armature
rotating in this field produces a large voltage along the brush axis B⬘B⬘.
The load or working current is taken from brushes B⬘B⬘ as shown. In
Fig. 15.1.55 the working current only is shown by the crosses and dots
in the circles. The short-circuit current would be shown by crosses in the
conductors to the left of brushes BB and by dots in the conductors to the
right of brushes BB.
15-27
which connects the terminals of the generator at the junctions of the
series fields. This connection is of low resistance so that any increase of
current divides proportionately between the series fields of the two
machines. The equalizer switch (E.S.) should be closed first and opened
last, if possible. In practice, the equalizer switch is often one blade of a
Fig. 15.1.56
parallel.
Connections for compound-wound generators operating in
three-pole switch, the other two being the bus switch S, as in Fig.
15.1.56. When compound generators are used on a three-wire system,
two series fields — one at each armature terminal — and two equalizers
are necessary. It is possible to operate any number of compound generators in parallel provided their characteristics are not too different and
the equalizer connection is used.
DC MOTORS
Fig. 15.1.55 Amplidyne.
A small current in the control winding produces a high output voltage
and current as a result of the large short-circuit current in brushes BB.
In order that the brushes B⬘B⬘ shall not be short-circuiting conductors
which are cutting the flux of poles NN and SS, cavities are cut in these
poles. Also the load current from brushes B⬘B⬘ produces an armature
reaction mmf in opposition to flux A⬘A⬘ produced by the control field
CC. Were this mmf not compensated, the flux A⬘A⬘ and the output of
the machine would no longer be determined entirely by the control field.
Hence there is a compensating field FF⬘ in series with the armature,
which neutralizes the armature-reaction mmf which the load current
produces. For simplicity the compensating field is shown on one field
pole only.
The amplidyne is capable of controlling and regulating speed, voltage, current, and power with accurate and rapid response. The amplification is from 10,000 to 250,000 times in machines rated from 1 to 50
kW. Amplidynes are frequently used in connection with selsyns and are
employed for gun and turret control and for accurate controls in many
industrial power applications.
Parallel Operation of Shunt Generators It is desirable to operate
generators in parallel so that the station capacity can be adapted to the
load. Shunt generators, because of their drooping characteristics (Fig.
15.1.51), are inherently stable when in parallel. To connect shunt generators in parallel it is necessary that the switches be so connected that like
poles are connected to the same bus bars when the switches are closed.
Assume one generator to be in operation; to connect another generator
in parallel with it, the incoming generator is first brought up to speed
and its terminal voltage adjusted to a value slightly greater than the
bus-bar voltage. This generator may then be connected in parallel with
the other without difficulty. The proper division of load between them is
adjusted by means of the field rheostats and is maintained automatically
if the machines have similar voltage-regulation characteristics.
Parallel Operation of Compound Generators As a rule, compound
generators have either flat or rising voltage characteristics. Therefore,
when connected in parallel, they are inherently unstable. Stability may,
however, be obtained by using an equalizer connection, Fig. 15.1.56,
Motors operate on the principle that a conductor carrying current in a
magnetic field tends to move at right angles to that field (see Fig.
15.1.11). The ordinary dc generator will operate entirely satisfactorily
as a motor and will have the same rating. The conductors of the motor
rotate in a magnetic field and therefore must generate an emf just as
does the generator. The induced emf
E ⫽ K␾ N
(15.1.89)
where K ⫽ constant, ␾ flux entering the armature from one north pole,
and N ⫽ r/min [see Eq. (15.1.87)]. This emf is in opposition to the
terminal voltage and tends to oppose current entering the armature. Its
value is
E ⫽ V ⫺ Ia Ra
(15.1.90)
where V ⫽ terminal voltage, Ia ⫽ armature current, and Ra ⫽ armature
resistance [compare with Eq. (15.1.88)]. From Eq. (15.1.89) it is seen
that the speed
N ⫽ Ks E/␾
(15.1.91)
when Ks ⫽ 1/K. This is the fundamental speed equation for a motor. By
substituting in Eq. (15.1.90)
N ⫽ Ks(V ⫺ Ia Ra)/␾
(15.1.92)
which is the general equation for the speed of a motor.
The internal or electromagnetic torque developed by an armature is
proportional to the flux and to the armature current; i.e.,
Tt ⫽ Kt ␾ Ia
(15.1.93)
when Kt is a constant. The torque at the pulley is slightly less than the
internal torque by the torque necessary to overcome the rotational
losses, such as friction, windage, eddy-current and hysteresis losses in
the armature iron and in the pole faces.
The total mechanical power developed internally
Pm ⫽ EIa
W
(⫽ EIa /746
hp)
(15.1.94)
The internal torque thus becomes
T ⫽ EIa 33,000/(2 ␲ ⫻ 746N ) ⫽ 7.04EIa /N
(15.1.95)
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15-28
ELECTRICAL ENGINEERING
Fig. 15.1.57 Connections for shunt dc motors and starters. (a) Three-point box; (b) four-point box.
Let VI be the motor input. The output is VI␩ where ␩ is the efficiency.
The horsepower is
PH ⫽ VI␩/746
(15.1.96)
and the torque is
T ⫽ 33,000PH /2␲ N ⫽ 5,260PH /N
lb ⭈ ft
(15.1.97)
where N is r/min.
Shunt Motor In the shunt motor (Fig. 15.1.57) the flux is substantially constant and Ia R a is 2 to 6 percent of V. Hence from Eq. (15.1.92),
the speed varies only slightly with load (Fig. 15.1.58), so that the motor
is adapted to work requiring constant speed. The speed regulation of
constant-speed motors is defined by ANSI/IEEE Std. 100-1992 Standard Dictionary of Electrical and Electronic terms as follows:
The speed regulation of a constant-speed direct-current motor is the change in
speed when the load is reduced gradually from the rated value to zero with constant applied voltage and field rheostat setting expressed as a percent of speed at
rated load.
In Fig. 15.1.61 the speed regulation under each condition is 100(ac ⫺
bc)/bc (Fig. 15.1.58a). Also from Eq. (15.1.93) it is seen that the torque
is practically proportional to the armature current (see Fig. 15.1.58b).
The motor is able to develop full-load torque and more on starting, but
Fig. 15.1.58 Speed and torque characteristics of dc motors. (1) Shunt motor
(2) cumulative compound motor; (3) differential compound motor; (4) series
motor.
the ordinary starter is not designed to carry the current necessary for
starting under load. If a motor is to be started under load, the starter
should be provided with resistors adapted to carry the required current
without overheating. A controller is also adapted for starting duty under
load.
Commutating poles have so improved commutation in dc machines
that it is possible to use a much shorter air gap than formerly. Since,
with the shorter air gap, fewer field ampere-turns are required, the armature becomes magnetically strong with respect to the field. Hence, a
sudden overload might weaken the field through armature reaction, thus
causing an increase in speed; the effect may become cumulative and the
motor run away. To prevent this, modern shunt motors are usually provided with a stabilizing winding, consisting of a few turns of the field in
series with the armature and aiding the shunt field. The resulting increase of field ampere-turns with load will more than compensate for
any weakening of the field through armature reaction. The series turns
are so few that they have no appreciable compounding effect. The shunt
motor is used to drive constant-speed line shafting, for machine tools,
etc. Since its speed may be efficiently varied, it is very useful when
adjustable speeds are necessary, such as individual drive for machine
tools.
Shunt-Motor Starters At standstill the counter emf of the motor is
zero and the armature resistance is very low. Hence, except in motors of
very small size, series resistance in the armature circuit is necessary on
starting. The field must, however, be connected across the line so that it
may obtain full excitation.
Figure 15.1.57 shows the two common types of starting boxes used
for starting shunt motors. The armature resistance remains in circuit
only during starting. In the three-point box (Fig. 15.1.57a) the starting
lever is held, against the force of a spring, in the running position, by an
electromagnet in series with the field circuit, so that, if the field circuit is
interrupted or the line voltage becomes too low, the lever is released and
the armature circuit is opened automatically. In the four-point starting
box the electromagnet is connected directly across the line, as shown in
Fig. 15.1.57b. In this type the arm is released instantly upon failure of
the line voltage. In the three-point type some time elapses before the
field current drops enough to effect the release. Some starting rheostats
are provided with an overload device so that the circuit is automatically
interrupted if too large a current is taken by the armature. The four-point
box is used where a wide speed range is obtained by means of the field
rheostat. The electromagnet is not then affected by changes in field
current.
In large motors and in many small motors, automatic starters are
widely used. The advantages of the automatic starter are that the current
is held between certain maximum and minimum values so that the circuit does not become opened by too rapid starting as may occur with
manual operation; the acceleration is smooth and nearly uniform. Since
workers can stop and start a motor merely by the pushing of a button,
there results considerable saving by the shutting down of the motor
when it is not needed. Automatic controls are essential to elevator
motors so that smooth rapid acceleration with frequent starting and
stopping may be obtained. Also automatic starting is very necessary
with multiple-unit operation of electric-railway cars and with rollingmill motors which are continually subjected to rapid acceleration, stopping, and reversing.
Series Motor In the series motor the armature and field are in series.
Hence, if saturation is neglected, the flux is proportional to the current
and the torque [Eq. (15.1.93)] varies as the current squared. Therefore
any increase in current will produce a much greater proportionate increase in torque (see Fig. 15.1.58b). This makes the motor particularly
well adapted to traction work, cranes, hoists, fork-lift trucks, and other
types of work which require large starting torques. A study of Eq.
(15.1.92) shows that with increase in current the numerator changes
only slightly, whereas the change in the denominator is nearly proportional to the change in current. Hence the speed of the series motor is
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DC MOTORS
practically inversely proportional to the current. With overloads the
speed drops to very low values (see Fig. 15.1.58a). With decrease in
load the speed approaches infinity, theoretically. Hence the series motor
should always be connected to its load by a direct drive, such as gears,
so that it cannot reach unsafe speeds (see Speed Control of Motors). A
series-motor starting box with no-voltage release is shown in Fig.
15.1.59.
Differential Compound Motors The cumulative compound winding
of a generator becomes a differential compound winding when the machine is used as a motor. Its speed may be made more nearly constant
than that of a shunt motor, or, if desired, it may be adjusted to increase
with increasing load.
15-29
self-induction ⫺L di/dt tends to prolong the current flow which produces sparking. In a generator, armature reaction distorts the flux in the
direction of rotation and the brushes should be advanced. In order to
neutralize the emf of self-induction the brushes should be set a little
ahead of the neutral plane so that the emf induced in the short-circuited
coils by the cutting of the flux at the fringe of the next pole is opposite to
this emf of self-induction. In a motor the brushes are correspondingly
moved backward in the direction opposite rotation.
Theoretically, the brushes should be shifted with every change in
load. However, practically all dc generators and motors now have commutating poles (or interpoles) and with these the brushes can remain in
the no-load neutral plane, and good commutation can be obtained over
the entire range of load. Commutating poles are small poles between the
main poles (Fig. 15.1.60) and are excited by a winding in series with the
armature. Their function is to neutralize the flux distortion in the neutral
plane caused by armature reaction and also to supply a flux that will
cause an emf to be induced in the conductors undergoing commutation,
opposite and equal to the emf of self-induction. Since armature reaction
Fig. 15.1.59 Series motor starter, no-voltage release.
The speed as a function of armature current is shown in Fig. 15.1.58a
and the torque as a function of armature current is shown in Fig.
15.1.58b.
Since the speed of the shunt motor is sufficiently constant for most
purposes and the differential motor tends toward instability, particularly
in starting and on overloads, the differential motor is little used.
Cumulative compound motors develop a more rapid increase in torque
with load than shunt motors (Fig. 15.1.58b); on the other hand, they
have much poorer speed regulation (Fig. 15.1.58a). Hence they are used
where larger starting torque than that developed by the shunt motor is
necessary, as in some industrial drives. They are particularly useful
where large and intermittent increases of torque occur as in drives for
shears, punches, rolling mills, etc. In addition to the sudden increase in
torque which the motor develops with sudden applications of load, the
fact that it slows down rapidly and hence causes the rotating parts to
give up some of their kinetic energy is another important advantage in
that it reduces the peaks on the power plant. Performance data for compound motors are given in Table 15.1.11.
Commutation The brushes on the commutator of either a motor or
generator should be set in such a position that the induced emf in the
armature coils undergoing commutation and hence short-circuited by
the brushes, is zero. In practice, this condition can at best be only approximately realized. Frequently conditions are such that it is far from
being realized. At no load, the brushes should be set in a position corresponding to the geometrical neutral of the machine, for under these
conditions the induced emf in the coils short-circuited by the brushes is
zero. As load is applied, two factors cause sparking under the brushes.
The mmf of the armature, or armature reaction, distorts the flux; when
the current in the coils undergoing commutation reverses, an emf of
Table 15.1.11
Fig. 15.1.60
Commutating poles in motor.
and the emf of self-induction are both proportional to the armature
current, saturation being neglected, they are neutralized theoretically at
every load. Commutating poles have made possible dc generators and
motors of very much higher voltage, greater speeds, and larger kW
ratings than would otherwise be possible.
Occasionally, the commutating poles may be connected incorrectly.
In a motor, passing from an N main pole in the direction of rotation of
the armature, an N commutating pole should be encountered as shown
in Fig. 15.1.60. In a generator under these conditions an S commutating
pole should be encountered. The test can easily be made with a compass. If poor commutation is caused by too strong interpoles, the winding may be shunted. If the poles are too weak and the shunting cannot be
reduced, they may be strengthened by inserting sheet-iron shims between the pole and the yoke thus reducing the air gap.
Although the emfs induced in the coils undergoing commutation are
relatively small, the resistance of the coils themselves is low so that
unless further resistance is introduced, the short-circuit currents would
be large. Hence, with the exception of certain low-voltage generators,
carbon brushes that have relatively large contact resistance are almost
always used. Moreover, the graphite in the brushes has a lubricating
action, and the usual carbon brush does not score the commutator.
Test Performance of Compound-Wound DC Motors
115 V
230 V
Power,
hp
Speed,
r/min
Current,
A
Full-load
efficiency,
%
1
2
5
10
25
50
100
200
1,750
1,750
1,750
1,750
1,750
850
850
1,750
8.4
16.0
40.0
75.0
182.0
—
—
—
78
80
82
85.6
87.3
—
—
—
SOURCE: Westinghouse Electric Corp.
550 V
Current,
A
Full-load
efficiency,
%
Current,
A
Full-load
efficiency,
%
4.3
8.0
20.0
37.5
91.7
180.0
350.0
700.0
79
81
83
85
87.5
89
90.5
91
1.86
3.21
8.40
15.4
38.1
73.1
149.0
295.0
73.0
82.0
81.0
86.5
88.5
90.0
91.0
92.0
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15-30
ELECTRICAL ENGINEERING
Speed Control of Motors
Shunt Motors In Eq. (15.1.91) the speed of a shunt motor N ⫽
Ks E/␾, where Ks is a constant involving the design of the motor such as
conductors on armature surface and number of poles. Obviously, in
order to change the speed of a motor, without changing its construction,
two factors may be varied, the counter emf E and the flux ␾.
Armature-Resistance Control The counter emf E ⫽ V ⫺ Ia Ra ,
where V is the terminal voltage, assumed constant. Ra must be small so
that the armature heating can be maintained within permissible limits.
Under these conditions the speed change with load is small. By inserting
an external resistor, however, into the armature circuit the counter emf
E may be made to decrease rapidly with increase in load; that is, E ⫽
V ⫺ Ia(Ra ⫹ R) [see Eq. (15.1.90)] where R is the resistance of the
external resistor. The resistor R must be inserted in the armature circuit
only. The advantages of this method are its simplicity, the full torque of
the motor is developed at any speed, and the method introduces no
commutating difficulties. Its disadvantages are the increased speed regulation with change of load (Fig. 15.1.61), the low efficiency, particularly at the lower speeds, and the fact that provision must be made to
dissipate the comparatively large power losses in the series resistor.
Figure 15.1.61 shows typical speed-load curves without and with series
resistors in the armature circuit. The armature efficiency is nearly equal
Fig. 15.1.61 Speed-load characteristics with armature resistance control.
to the ratio of the operating speed to the no-load speed. Hence at 25
percent speed the armature efficiency is practically 25 percent. Frequently the controlling and starting resistors are one, and the device is
called a controller. Starting rheostats themselves are not designed to
carry the armature current continuously and must not be used as controllers. The armature-resistance method of speed control is frequently
used to regulate the speed of ventilating fans where the power demand
diminishes rapidly with decrease in speed.
Control by Changing Impressed Voltage From Eq. (15.1.92) it is
evident that the speed of a motor may be changed if V is changed by
connecting the armature across different voltages. Speed control by this
method is accomplished by having mains (usually four), which are
maintained at different voltages, available at the motor.
The shunt field of the motor is generally permanently connected to
one pair of mains, and the armature circuit is provided with a controller
by means of which the operator can readily connect the armature to any
pair of mains. Such a system gives a series of distinct and widely separated speeds and generally necessitates the use of field-resistance control, in combination, to obtain intermediate speeds. This method, known
as the multivoltage method, has the disadvantage that the system is expensive, for it requires several generating machines, a somewhat complicated switchboard, and a number of service wires. The system is used
somewhat in machine shops and is extensively used for dc elevator
starting and speed control.
In the Ward Leonard system, the variable voltage is obtained from a
separately excited generator whose armature terminals are connected
directed to the armature terminals of the working motor. The generator
is driven at essentially constant speed by a dc shunt motor if the power
supply is direct current, or by an induction motor or a synchronous
motor if the power supply is alternating current. The field circuit of the
generator and that of the motor are connected across a constant-voltage
dc supply. The terminal voltage of the generator, and hence the voltage
applied to the armature of the motor, is varied by changing the generator-field current with a field rheostat. The rheostat has a wide range of
resistance so that the speed of the motor may be varied smoothly from 0
to 100 percent. Since three machines are involved the system is costly,
somewhat complicated, and has low power efficiency. However, because the system is flexible and the speed can be smoothly varied over
wide ranges, it has been used in many applications, such as elevators,
mine hoists, large printing presses, paper machines, and electric locomotives.
The Ward-Leonard system has been largely replaced by a static converter system where a silicon-controlled-rectifier bridge is used to convert three-phase alternating current, or single-phase on smaller drives,
to dc voltage that can be smoothly varied by phase-angle firing of the
rectifiers from full voltage to zero. This system is smaller, lighter, and
less expensive than the motor-generator system. Care must be taken to
assure that the dc motor will accept the harmonics present in the dc
output without overheating.
Control by Changing Field Flux Equation (15.1.91) shows that the
speed of a motor is inversely proportional to the flux ␾. The flux can be
changed either by varying the shunt-field current or by varying the
reluctance of the magnetic circuit. The variation of the shunt-field
current is the simplest and most efficient of all the methods of speed
control.
With the ordinary motor, speed variation of 1.5 to 1.0 is obtainable
with this method. If attempt is made to obtain greater ratios, severe
sparking at the brushes results, owing to the field distortion caused by
the armature mmf becoming large in comparison with the weakened
field of the motor. Speed ratios of 5 : 1 and higher are, however, obtainable with motors which have commutating poles. Since the field current
is a small proportion of the total current (1 to 3 percent), the rheostat
losses in the field circuit are always small. This method is efficient.
Also for any given speed adjustment the speed regulation is excellent,
which is another advantage. Because of its simplicity, efficiency, and
excellent speed regulation, the control of speed by means of the field
current is by far the most common method. Output power remains constant when the field is weakened, so output torque varies inversely with
motor speed.
Speed Control of Series Motors The series motor is fundamentally
a variable-speed motor, the speed varying widely from light load to full
load and more (see Fig. 15.1.58a). From Eq. (15.1.92) the speed for any
value of ␾, or current, can be changed by varying the impressed voltage.
Hence the speed can be controlled by inserting resistance in series with
the motor. This method, which is practically the same as the armatureresistance control method for shunt motors, has the same objections of
low efficiency and poor regulation with fluctuating loads. It is extensively used in controlling the speed of hoist and crane motors.
The series-parallel system of series-motor speed control is almost universally used in electric traction. At least two motors are necessary. The
two motors are first connected in series with each other and with the
starting resistor. The starting resistor is gradually cut out and, since each
motor then operates at half line voltage, the speed of each is approximately half speed. Both motors take the same current, and each can
develop full torque. This condition of operation is efficient since there is
no external resistance in circuit. When the controller is moved to the
next position, the motors are connected in parallel with each other and
each in series with starting resistors. Full speed of the motors is obtained
by gradually cutting out these resistors. Connecting the two motors in
series on starting reduces the current to one-half the value that would be
required for a given torque were both motors connected in parallel on
starting. The power taken from the trolley is halved, and an intermediate
running speed is efficiently obtained.
In the multiple-unit method of speed control which is used for electric
railway trains, the starting contactors, reverser, etc., for each car are
located under that car. The relays operating these control devices are
actuated by energy taken from the train line consisting usually of seven
wires. The train line runs the entire length of the train, the connections
between the individual cars being made through the couplers. The train
line is energized by the action of the motorman operating any one of the
small master controllers which are located in each car. Hence corresponding relays, contactors, etc., in every car all operate simultaneously. High accelerations may be reached with this system because of
the large tractive effort exerted by the wheels on every car.
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
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SYNCHRONOUS GENERATORS
SYNCHRONOUS GENERATORS
The synchronous generator is the only type of ac generator now in
general use at power stations.
Construction In the usual synchronous generator the armature or
stator is the stationary member. This construction has many advantages.
It is possible to make the slots any reasonable depth, since the tooth
necks increase in cross section with increase in depth of slot; this is not
true of the rotor. The large slot section which is thus obtainable gives
ample space for copper and insulation. The conductors from the armature to the bus bars can be insulated throughout their entire lengths,
since no rotating or sliding contacts are necessary. The insulation in a
stationary member does not deteriorate as rapidly as that on a rotating
member, for it is not subjected to centrifugal force or to any considerable vibration.
The rotating member is ordinarily the field. There are two general
types of field construction: the salient-pole type and the cylindrical, or
nonsalient-pole, type. The salient-pole type is used almost entirely for
slow and moderate-speed generators since this construction is the least
expensive and permits ample space for the field ampere-turns.
It is not practicable to employ salient poles in high-speed turboalternators because of the excessive windage and the difficulty of obtaining
sufficient mechanical strength. The cylindrical type consists of a cylindrical steel forging with radial slots in which the field copper, usually in
strip form, is placed. The fields are ordinarily excited at low voltage,
125 and 250 V, the current being conducted to the rotating member by
means of slip rings and brushes. An ac generator armature to supply
field voltage can be mounted on the generator shaft and supply dc to the
motor field through a static rectifier bridge, also mounted on the shaft,
eliminating all slip rings and brushes. The field power is ordinarily only
Table 15.1.12
15-31
1.5 percent and less of the rated power of the machine (see Table
15.1.12).
Classes of Synchronous Generators Synchronous generators may
be divided into three general classes: (1) the slow-speed engine-driven
type; (2) the moderate-speed waterwheel-driven type; and (3) the highspeed turbine-driven type. In (1) a hollow box frame is used as the stator
support, and the field consists of a spider to which a larger number of
salient poles are attached, usually bolted. The speed seldom exceeds 75
to 90 r/min, although it may run as high as 150 r/min. Waterwheel
generators also have salient poles which are usually dovetailed to a
cylindrical spider consisting of steel plates riveted together. Their
speeds range from 80 to 900 r/min and sometimes higher, although the
9,000-kVA Keokuk synchronous generators rotate at only 58 r/min,
operating at a very low head. The speed rating of direct-connected
waterwheel generators decreases with decrease in head. It is desirable to
operate synchronous generators at the highest permissible speed since
the weight and costs diminish with increase in speed. Waterwheeldriven generators must be able to run at double speed, as a precaution
against accident, should the governor fail to shut the gate sufficiently
rapidly in case the circuit breakers open or should the governing mechanism become inoperative.
Turbine-driven generators operate at speeds of 720 to 3,600 r/min.
Direct-connected exciters, belt-driven exciters from the generator shaft,
and separately driven exciters are used. In large stations separately
driven (usually motor) exciters may supply the excitation energy to
excitation bus bars. Steam-driven exciters and storage batteries are frequently held in reserve. With slow-speed synchronous generators, the
belt-driven exciter is frequently used because it can be driven at higher
speed, thus reducing the cost.
Performance Data for Synchronous Generators
80% pf, 3 phase, 60 Hz, 240 to 2,400 V, horizontal-coupled or belted-type engine
kVA
Poles
Speed,
r/min
25
93.8
250
500
1,000
3,125
4
8
12
18
24
48
1,800
900
600
400
300
150
Excitation,
kW
0.8
2
5
8
14.5
40
⁄ load
Full load
Approx.
net weight,
lb
85.7
89.5
91.3
92.6
93.4
94.2
87.6
90.9
92.2
93.2
93.9
94.6
900
2,700
6,000
10,000
16,100
52,000
Efficiencies, %
⁄ load
12
34
81.5
87
90
91.7
92.6
93.4
Industrial-size turbine generators, direct-connected type, 80% pf, 3 phase, 60 Hz, air-cooled
Efficiency, %
kVA
Poles
Speed,
r/min
kW
V
⁄
load
⁄
load
Full
load
Volume
of air,
ft 3/min
1,875
2,500
3,125
3,750
5,000
6,250
7,500
9,375
2
2
2
2
2
2
2
2
3,600
3,600
3,600
3,600
3,600
3,600
3,600
3,600
18
22
24
24
29
38
42
47
125
125
125
125 – 250
125 – 250
125 – 250
125 – 250
125 – 250
95.3
95.3
95.3
95.3
95.3
95.3
95.5
95.5
96.1
96.1
96.3
96.3
96.3
96.3
96.5
96.5
96.3
96.3
96.5
96.6
96.6
96.7
96.9
96.9
3,500
5,000
5,500
6,500
11,000
12,000
15,000
16,500
Excitation
12
34
Voltage
Approx.
weight,
including
exciter, lb
480 – 6,900
2,400 – 6,900
2,400 – 6,900
2,400 – 6,900
2,400 – 6,900
2,400 – 13,800
2,400 – 13,800
2,400 – 13,800
21,900
22,600
25,100
27,900
40,100
43,300
45,000
61,200
Central-station-size turbine generators, direct-connected type, 85% pf, 3 phase, 60 Hz, 11,500 to 14,400 V
Efficiency, %
kVA
Poles
Speed,
r/min
kW
V
1⁄2
load
3⁄4
load
Full
load
Volume
of air,
ft 3/min
13,529
17,647
23,529
35,294
47,058
70,588
2
2
2
2
2
2
3,600
3,600
3,600
3,600
3,600
3,600
70
100
115
145
155
200
250
250
250
250
250
250
96.3
97.7
98.0
98.1
98.3
98.4
97.1
97.9
98.2
98.3
98.5
98.7
97.3
97.9
98.2
98.3
98.5
98.7
22,000
22,000
25,000
34,000
42,000
50,000
SOURCE: Westinghouse Electric Corp.
Excitation
Ventilation
Approx.
weight,
including
exciter, lb
Air-cooled
H 2-cooled
H 2-cooled
H 2-cooled
H 2-cooled
H 2-cooled
116,700
115,700
143,600
194,800
237,200
302,500
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15-32
ELECTRICAL ENGINEERING
Synchronous-Generator Design At the present time single-phase
generators are seldom built. For single-phase service two phases of a
standard three-phase Y-connected generator are used. A single-phase
load or unbalanced three-phase load produces flux pulsations in the
magnetic circuits of synchronous generators, which increase the iron
losses and introduce harmonics into the emf wave. Two-phase windings
consist of two similar single-phase windings displaced 90 electrical
space degrees on the armature and ordinarily occupying all the slots on
the armature. The most common type of winding is the three-phase
lap-wound two-layer type of winding. In three-phase windings three
windings are spaced 120 electrical space degrees apart, the individual
phase belts being spaced 60° apart. Usually, all the slots on the armature
are occupied. Standard voltages are 550, 1,100, 2,200, 6,600, 13,200,
and 20,000 V. It is much more difficult to insulate for 20,000 V than it is
for the lower voltages. However, if the power is to be transmitted at this
voltage, its use would be justified by the saving of transformers. In
machines of moderate and larger ratings it is common to generate at
6,600 and 13,200 V if transformers must be used. The higher voltage is
preferable, particularly for the higher ratings, because it reduces the
cross section of the connecting leads and bus bars.
The standard frequency in the United States for lighting and power
systems is 60 Hz; the few former 50-Hz systems have practically all
been converted to 60 Hz. The frequency of 25 Hz is commonly used in
street-railway and subway systems to supply power to the synchronous
converters and other ac-dc conversion apparatus; it is also commonly
used in railroad electrification, particularly for single-phase series-motor locomotives (see Sec. 11). At 25 Hz incandescent lamps have noticeable flicker. In European (and most other) countries 50 Hz is standard. The frequency of a synchronous machine
f ⫽ P ⫻ r/min/120
Hz
(15.1.98)
where P is the number of poles. Synchronous generators are rated in
kVA rather than in kW, since heating, which determines the rating,
is dependent only on the current and is independent of power factor. If
the kilowatt rating is specified, the power factor should also be specified.
Induced EMF The induced emf per phase in synchronous generator is
E ⫽ 2.22k b k p⌽f Z
V/phase
(15.1.99)
where k b ⫽ breadth factor or belt factor (usually 0.9 to 1.0), which
depends on the number of slots per pole per phase, 0.958 for threephase, four slots per pole per phase; k p ⫽ pitch factor ⫽ 1.0 for
full pitch, 0.966 for 5⁄6 pitch; ⌽ ⫽ total flux Wb, entering armature from
one north pole and is assumed to be sinusoidally distributed along
the air gap; f ⫽ frequency; and Z ⫽ number of series conductors per
phase.
Synchronous generators usually are Y-connected. The advantages are
that for a given line voltage the voltage per phase is 1/√3 that of the
delta-connected winding; third-harmonic currents and their multiples
cannot circulate in the winding as with a delta-connected winding;
third-harmonic emfs and their multiples cannot exist in the line emfs; a
neutral point is available for grounding.
Regulation The terminal voltage of synchronous generator at constant frequency and field excitation depends not only on the current load
but on the power factor as well. This is illustrated in Fig. 15.1.62, which
shows the voltage-current characteristics of a synchronous generator
Fig. 15.1.62 Synchronous generator characteristics.
with lagging current, leading current and in-phase current (pf ⫽ 1.00).
With leading current the voltage may actually rise with increase in load;
the rate of voltage decrease with load becomes greater as the lag of the
current increases. The regulation of a synchronous generator is defined
by the ANSI/IEEE Std. 100-1992 Standard Dictionary of Electrical and
Electronic Terms as follows:
The voltage regulation of a synchronous generator is the rise in voltage with
constant field current, when, with the synchronous generator operated at rated
voltage and rated speed, the specified load at the specified power factor is reduced
to zero, expressed as a percent of rated voltage.
For example, in Fig. 15.1.62 the regulation under each condition is
100(ac ⫺ bc)/bc
(15.1.100)
With leading current the regulation may be negative.
Three factors affect the regulation of synchronous generators; the
effective armature resistance, the armature leakage reactance, and the armature reaction. With alternating current the armature loss is greater
than the value obtained by multiplying the square of the armature
current by the ohmic resistance. This is due to hysteresis and eddycurrent losses in the iron adjacent to the conductor and to the alternating
flux-producing losses in the conductors themselves. Also the current is
not distributed uniformly over conductors in the slot, but the current
density tends to be greatest in the top of the slot. These factors all have
the effect of increasing the resistance. The ratio of effective to ohmic
resistance varies from 1.2 to 1.5. The armature leakage reactance is due
to the flux produced by the armature current linking the conductors in
the slots and also the end connections.
The armature mmf reacts on the field to change the value of the flux.
With a single-phase generator and with an unbalanced load on a polyphase generator, the armature mmf is pulsating and causes iron losses in
the field structure. With polyphase machines under a constant balanced
load, the armature mmf is practically constant in magnitude and fixed in
its relation to the field poles. Its direction with relation to the field-pole
axis is determined by the power factor of the load.
A component of current in phase with the no-load induced emf, or the
excitation emf, merely distorts the field by strengthening the trailing
pole tip and weakening the leading pole tip. A component of current
lagging the excitation emf by 90° weakens the field without distortion.
A component of current leading the excitation emf by 90° strengthens
the field without distortion. Ordinarily, both cross magnetization and
one of the other components are acting simultaneously.
The foregoing effects are called armature reaction. Frequently the
effects of armature reactance and armature reaction can be combined
into a single quantity.
It is difficult to determine the regulation of synchronous generator by
actual loading, even when in service, owing to the difficulty of obtaining, controlling, and absorbing the large balanced loads. Hence methods
of predetermining regulation without actually loading the machine are
used.
Fig. 15.1.63
Phasor diagram for synchronous impedance method.
Synchronous Impedance Method Both armature reactance and armature reaction have the same effect on the terminal voltage. In the
synchronous impedance method the generator is considered as having
no armature reaction, but the armature reactance is increased a sufficient
amount to account for the effect of armature reaction. The phasor
diagram for a current I lagging the terminal voltage V by an angle
␪ is shown in Fig. 15.1.63. In a polyphase generator the phasor diagram is applicable to one phase, a balanced load almost always being assumed.
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SYNCHRONOUS GENERATORS
The power factor of the load is cos ␪ ; IR is the effective armature
resistance drop and is parallel to I; IXs is the synchronous reactance drop
and is at right angles to I and leading it by 90°. IXs includes both the
reactance drop and the drop in voltage due to armature reaction. That
part of IXs which replaces armature reaction is in reality a fictitious
quantity. The synchronous impedance drop is given by IZs . The no-load
or open-circuit (excitation) voltage
E ⫽ √(V cos ␪ ⫹ IR)2 ⫹ (V sin ␪ ⫾ IXs)2
15-33
factor angle. The effective resistance drop IR and the leakage reactance
drop IX are drawn parallel and perpendicular to the current phasor. Ea ,
the phasor sum of V, IR, and IX, is the internal induced emf. Arcs are
swung with O as the center and V and E a as radii to intercept the axis of
ordinates at B and C. OK, tangent to the straight portion of the saturation
V (15.1.101)
All quantities are per phase. The negative sign is used with leading
current.
Regulation ⫽ 100(E ⫺ V )/V
(15.1.102)
With leading current E may be less than V and a negative regulation
results.
The synchronous impedance is determined from an open-circuit and a
short-circuit test, made with a weak field. The voltage E⬘ on open circuit
is divided by the current I⬘ on short circuit for the same value of field
current.
Xs ⫽ √Z 2s ⫺ R 2
⍀
(15.1.103)
Z s ⫽ E⬘/I⬘
R is so small compared with Xs that for all practical purposes Xs ⫽ Z s .
R may be determined by measuring the ohmic resistance per phase and
multiplying by 1.4 to 1.5 to obtain the effective resistance. This value of
R and the value of Xs obtained from Eq. (15.1.103) may then be substituted in Eq. (15.1.101) to obtain E at the specified load and power
factor.
Since the synchronous reactance is determined at low saturation of
the iron and used at high saturation, the method gives regulations that
are too large; hence it is called the pessimistic method.
MMF Method In the mmf method the generator is considered as
having no armature reactance but the armature reaction is increased by
an amount sufficient to include the effect of reactance. That part of
armature reaction which replaces the effect of armature reactance is in
reality a fictitious quantity. To obtain the data necessary for computing
the regulation, the generator is short-circuited and the field adjusted to
give rated current in the armature. The corresponding value of field
current Ia is read. The field is then adjusted to give voltage E⬘ equal to
rated terminal voltage ⫹ IR drop (⫽ V ⫹ IR, as phasors, Fig. 15.1.64) on
open circuit and the field current I⬘ read.
Ia is 180° from the current phasor I, and I⬘ leads E⬘ by 90° (Fig.
15.1.64). The angle between I⬘ and Ia is 90 ⫺ ␪ ⫹ ␾, but since ␾ is
small, it can usually be neglected. The phasor sum of Ia and I⬘ is Io . The
Fig. 15.1.64 Phasor diagram for the mmf method.
open-circuit voltage E corresponding to Io is the no-load voltage and can
be found on the saturation curve. The regulation is then found from Eq.
(15.1.102). This method gives a value of regulation less than the actual
value and hence is called the optimistic method. The actual regulation
lies somewhere between the values obtained by the two methods but is
more nearly equal to the value obtained by the mmf method.
ANSI Method The ANSI method (American Standard 50, Rotating
Electrical Machinery) which has become the accepted standard for the
predetermination of synchronous generator operation, eliminates in
large measure the errors due to saturation which are inherent in the
synchronous impedance and mmf methods. In Fig. 15.1.65a is shown
the saturation curve OAF of the generator. The axis OP is not only the
field-current axis but also the axis of the current phasor I as well. V the
terminal voltage is drawn ␪ deg from I or OP, where ␪ is the power
Fig. 15.1.65
ANSI method of synchronous generator regulation.
curve, is the air-gap line. If there is no saturation, Iv is the field current
necessary to produce V, and CK is the field current necessary to produce
Ea . The field current Is is the increase in field current necessary to take
into account the saturation corresponding to E a .
The corresponding phasor diagram to a larger scale is shown in Fig.
15.1.65b. I⬘f , the field current necessary to produce rated current at short
circuit, corresponding to Ia (Fig. 15.1.64), is drawn horizontally. The
field current Iv is drawn at an angle ␪ to the right of a perpendicular
erected at the right-hand end of I⬘f . Ir is the resultant of I⬘f and Iv . Is is
added to Ir giving If the resultant field current. The no-load emf E is
found on the saturation curve, Fig. 15.ž.65a, corresponding to I f ⫽ OD.
Excitation is commonly supplied by a small dc generator driven from
the generator shaft. On account of commutation, except in the smaller
sizes, the dc generator cannot be driven at 3,600 r/min, the usual speed
for turbine generators, and belt or gear drives are necessary. The use of
the silicon rectifier has made possible simpler means of excitation as
well as voltage regulation. In one system the exciter consists of a small
rotating-armature synchronous generator (which can run at high speed)
mounted directly on the main generator shaft. The three-phase armature
current is rectified by six silicon rectifiers and is conducted directly to
the main generator field without any sliding contacts. The main generator field current is controlled by the current to the stationary field of the
exciter generator. In another system there is no rotating exciter, the
generator excitation being supplied directly from the generator terminals, the 13,800 V, three-phase, being stepped down to 115 V, threephase, by small transformers and rectified by silicon rectifiers. Voltage
regulation is obtained by saturable reactors actuated by potential transformers connected across the generator terminals.
Most regulators such as the following operate through the field of the
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15-34
ELECTRICAL ENGINEERING
exciter. In the Tirrill regulator the field resistance of the exciter is shortcircuited temporarily by contacts when the bus-bar voltage drops. Actually, the contacts are vibrating continuously, the time that they are
closed depending on the value of the bus-bar voltage. The General
Electric Co. manufactures a direct-acting regulator in which the regulating rheostat is part of the regulator itself. The rheostat consists of stacks
of graphite plates, each plate being pivoted at the center. Tilting the
plates changes the path of the current through the rheostat and thus
changes the resistance. The plates are tilted by a sensitive torque armature which is actuated by variations of voltage from the normal value
(for regulators employing silicon rectifiers).
Parallel Operation of Synchronous Generators The kilowatt division of load between synchronous generators in parallel is determined
entirely by the speed-load characteristics of their prime movers and not
by the characteristics of the generators themselves. No appreciable adjustment of kilowatt load between synchronous generators in parallel
can be made by means of their field rheostats, as with dc generators.
Consider Fig. 15.1.66, which gives the speed-load characteristics in
terms of frequency, of two synchronous generators, no. 1 and no. 2,
these characteristics being the speed-load characteristics of their prime
movers. These speed-load characteristics are drooping, which is necessary for stable parallel operation. The total load on the two machines is
Fig. 15.1.66 Speed-load characteristics of synchronous generators in parallel.
P1 ⫹ P2 kW. Both machines must be operating at the same frequency
F1 . Hence generator 1 must be delivering P1 kW, and generator 2 must
be delivering P2 kW (the small generator losses being neglected). If,
under the foregoing conditions, the field of either machine is strengthened, it cannot deliver a greater kilowatt load, for its prime mover can
deliver more power only by dropping its speed. This is impossible, for
both generators must operate always at the same frequency f1 . For any
fixed total power load, the division of kilowatt load between synchronous generators can be changed only by modifying in some manner the
speed-load characteristics of their prime movers, such, for example, as
changing the tension in the governor spring. Synchronous generators in
parallel are of themselves in stable equilibrium. If the driving torque of
one machine is increased, the resulting electrical reactions between the
machines cause a circulating current to flow between machines. This
current puts more electrical load on the machine whose driving torque is
increased and tends to produce motor action in the other machines. In an
extreme case, the driving torque of one prime mover may be removed
entirely, and its generator will operate as a synchronous motor, driving
the prime mover mechanically.
Variations in driving torques cause currents to circulate between synchronous generators, transferring power which tends to keep the generators in synchronism. If the power transfer takes the form of recurring
pulsations, it is called hunting, which may be reduced by building heavy
copper grids called amortisseurs, or damper windings, into the pole faces.
Turbine- and waterwheel-driven synchronous generators are much better adapted to parallel operation than are synchronous generators which
are driven by reciprocating engines, because of their uniformity of
torque.
Increasing the field current of synchronous generators in parallel with
others causes it to deliver a greater lagging component of current. Since
the character of the load determines the total current delivered by the
system, the lagging components of current delivered by the other generators must decrease and may even become leading components. Likewise if the field of one generator is weakened, it delivers a greater
leading component of current and the other machines deliver compo-
nents of current which are more lagging. These leading and lagging
currents do not affect appreciably the division of kilowatt load between
the synchronous generators. They do, however, cause unnecessary heating in their armatures. The fields of all synchronous generators should
be so adjusted that the heating due to the quadrature components of
currents is a minimum. With two generators having equal armature
resistances, this occurs when both deliver equal quadrature currents.
Armature reactance in the armature of machines in parallel is desirable. If not too great, it stabilizes their operation by producing the
synchronizing action. Synchronous generators with too little reactance
are sensitive, and if connected in parallel with slight phase displacement
or inequality of voltage, considerable disturbance results. Armature
reactance also reduces the current on short circuit, particularly during
the first few cycles when the short-circuit current is a maximum. Frequently, external power-limiting reactances are connected in series to
protect the generators and equipment from injury that would result from
the tremendous short-circuit currents. For these reasons, poor regulation
in large synchronous generators is frequently considered to be an advantage rather than a disadvantage.
Ground Resistors Most power systems operate with a grounded
neutral. When the station generators deliver current directly to the system (without intervening transformers), it is customary to ground the
neutral (of the Y-connected windings) or one generator in a station; this
is usually done through a grounding resistor of from 2 to 6 ⍀. If the
neutral of more than one generator is grounded, third-harmonic (and
multiples thereof ) currents can circulate between the generators. The
ground resistor reduces the short-circuit currents when faults to ground
occur, and hence reduces the violence of the short circuit as well as the
duty of the circuit breakers. Grounding reactors are sometimes used but
have limited application owing to the danger of high voltages resulting
from resonant conditions.
INDUCTION GENERATORS
The induction generator is an induction motor driven above synchronous
speed. The rotor conductors cut the rotating field in a direction to convert shaft mechanical power to electrical power. Load increases as
speed increases, so the generator is self-regulating and can be used
without governor control. On short circuits the induction generator will
deliver current to the fault for only a few cycles because, unlike the
synchronous generator, it is not self-exciting. Since it is not self-excited,
an induction generator must always be used in parallel with an electrical
system where there are some synchronous machines, or capacitor banks,
to deliver lagging current (VARs) to the induction generator for excitation.
Induction generators have found favor in industrial cogeneration applications and as wind-driven generators where they provide a small part
of the total load.
CELLS
Fuel cells convert chemical energy of fuel and oxygen directly to electrical energy. Solar cells convert solar radiation to electrical energy. At
present, these conversion methods are not economically competitive
with historic generating techniques, but they have found applications in
isolated areas, such as microwave relaying stations, satellites, and residential lighting, where power requirements are small and costs for
transmitting electrical power from more conventional sources is prohibitive. As solar technology continues to improve, many other applications will be evaluated.
TRANSFORMERS
Transformer Theory The transformer is a device that transfers energy from one electric circuit to another without change of frequency
and usually, but not always, with a change in voltage. The energy is
transferred through the medium of a magnetic field: it is supplied to the
transformer through a primary winding and is delivered by means of a
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TRANSFORMERS
secondary winding. Both windings link the same magnetic circuit. With
no load on the secondary, a small current, called the exciting current,
flows in the primary and produces the alternating flux. This flux links
both primary and secondary windings and induces the same volts per
turn in each. With a sine wave the emf is
E ⫽ 4.44⌽m nf
V
(15.1.104)
where ⌽m ⫽ maximum instantaneous flux in webers, n ⫽ turns on
either winding, and f ⫽ frequency. Equation (15.1.104) may also be
written
E ⫽ 4.44Bm Anf
V
(15.1.105)
the short-circuit test one winding is short-circuited, and the current in the
other is adjusted to near its rated value. The voltage Vc , the current I1 ,
and the power input Pc are measured. When one winding of a transformer is short-circuited, the voltage across the other winding is 3 to 4
percent of rated value when rated current flows. Since a voltage range of
from 110 to 250 V is best adapted to measuring instruments, that winding whose rated voltage, multiplied by 0.03 or 0.04, is closest to this
voltage range should be used for making the short-circuit test, the other
winding being short-circuited. Practically all the power on short circuit
goes to supply the copper loss of primary and secondary. If the measurements are made on the primary,
R01 ⫽ Pc /I 21
Z 01 ⫽ Vc /I1
X01 ⫽ √Z 201 ⫺ R201
Bm ⫽ maximum instantaneous flux density in iron and A ⫽ net cross
section of iron. If Bm is in T, A is in m2; if Bm is in Mx/in2, A is in in2.
In English units, Eq. (15.1.104) becomes
E ⫽ 4.44⌽m nf 10⫺8
V
(15.1.104a)
where ⌽M is in maxwells; Eq. (15.1.105) becomes
E ⫽ 4.44Bm Anf 10⫺8
V
(15.1.105a)
where Bm is in Mx/in2 and A is in in2.
Bm is practically fixed. In large transformers with silicon steel it
varies between 60,000 and 75,000 Mx/in2 at 60 Hz and between 75,000
and 90,000 Mx/in2 at 25 Hz. It is desirable to operate the iron at as high
density as possible in order to minimize the weight of iron and copper.
On the other hand, with too high densities the eddy-current and hysteresis losses become too great, and with low frequency the exciting current
may become excessive. It follows from Eq. (15.1.104) that
E 1 /E 2 ⫽ n 1 /n 2
n 1 I1 ⫽ n 2 I2
I1 /I2 ⫽ n 2 /n 1
R01 ⫽ R1 ⫹ (n 1 /n 2)2R2
R02 ⫽ R2 ⫹ (n 2 /n 1)2R1
(15.1.112)
(15.1.113)
The ac or effective resistances, determined from Eq. (15.1.109), are
usually 10 to 15 percent greater than these values.
Regulation The regulation may be computed from the foregoing
data as follows:
V⬘1 ⫽ √(V1 cos ␪ ⫹ I1 R01)2 ⫹ (V1 sin ␪ ⫾ I1 X01)2 (15.1.114)
(15.1.115)
Regulation ⫽ 100(V ⬘1 ⫺ V1)/V1
V1 ⫽ rated primary terminal voltage; cos ␪ ⫽ load power factor; I1 ⫽
rated primary current; R01 ⫽ equivalent resistance referred to primary
[from Eq. (15.1.109)]; X01 ⫽ equivalent reactance referred to primary.
The ⫹ sign is used with lagging current and the ⫺ sign with leading
current. Equations (15.1.114) and (15.1.115) are equally applicable to
the secondary if the subscripts are changed.
Efficiency The only two losses in a constant-potential transformer
are the core loss in W, P0 , which is practically independent of load, and
Pc the copper loss in W, which varies as the load current squared. The
efficiency for any current I1 is
(15.1.107)
(15.1.108)
where I1 and I2 are the primary and secondary currents.
When load is applied to the secondary of a transformer, the secondary
ampere-turns reduce the flux slightly. This reduces the counter emf of
the primary, permitting more current to enter and thus supply the increased power demanded by the secondary.
Both primary and secondary windings must necessarily have resistance. All the flux produced by the primary does not link the secondary;
the counter ampere-turns of the secondary produce some flux which
does not link the primary. These leakage fluxes produce reactance in each
winding. The combined effect of the resistance and reactance produces
an impedance drop in each winding when current flows. These impedance drops produce a slight drop in the secondary terminal voltage with
load.
Transformer Testing Transformer regulation and losses are so
small that it is far more accurate to compute the regulation and efficiency than to determine them by actual measurement. The necessary
measurements and computations are comparatively simple, and little
power is involved in making the tests. In the open-circuit test, the power
input to either winding is measured at its rated voltage. Usually it is
more convenient to make this test on the low-voltage winding, particularly if it is rated at 110, 220, or 550 V. The open-circuit power practically all goes to supply the core losses, consisting of eddy-current and
hysteresis losses. Let this value of power be P0 . The eddy-current loss
varies as the square of the voltage and frequency; the hysteresis loss
varies as the 1.6 power of the voltage, and directly as the frequency. In
(15.1.109)
(15.1.110)
(15.1.111)
where R01 , Z 01 , and X01 are the equivalent resistance, impedance, and
reactance referred to the primary. Also R02 ⫽ R01(n 2 /n 1)2; Z 02 ⫽
Z 01(n 2 /n 1)2; X02 ⫽ X01(n 2 /n 1)2, these quantities being the equivalent
resistance, impedance, and reactance referred to the secondary. If the dc
resistances R1 and R2 of the primary and secondary are measured,
(15.1.106)
where E 1 and E 2 are the primary and secondary emfs and n 1 and n 2 are
the primary and secondary turns. Since the impedance drops in ordinary
transformers are small, the terminal voltages of primary and secondary
are also practically proportional to their number of turns. As the change
in secondary terminal voltage in the ordinary constant-potential transformer over its range of operation is small (1.5 to 3 percent), the flux
must remain substantially constant. Therefore, the added ampere-turns
produced by any secondary load must be balanced by opposite and
equal primary ampere-turns. Since the exciting current is small compared with the load current (1.5 to 5 percent) and the two are usually out
of phase, the exciting current may ordinarily be neglected. Hence,
15-35
␩ ⫽ V1 I1 cos ␪/(V1 I1 cos ␪ ⫹ P0 ⫹ I 21 R01)
(15.1.116)
Equation (15.1.116) applies equally well to the secondary if the subscripts are changed. The maximum efficiency occurs when the core and
copper losses are equal.
All-Day Efficiency Since transformers must usually be on the line
24 h/day, part of which time the load may be very light, the all-day
efficiency is important. This is equal to the total energy or watthour
output divided by the total energy or watthour input for the 24 h. That is,
␩⫽
(V1 I1 cos ␪1)t 1 ⫹ ⭈ ⭈ ⭈
(V1 I1 cos ␪1)t 1 ⫹ ⭈ ⭈ ⭈ ⫹ (I 21 R01)t 1 ⫹ ⭈ ⭈ ⭈ ⫹ 24 P0
(15.1.117)
where t 1 ⫽ time in hours that load V1 I1 cos ␪1 is being delivered, etc.
Polyphase Transformer Connections Three-phase transformer
banks may be connected ⌬-⌬, ⌬-Y, Y-Y, and Y-⌬. The ⌬-⌬ connection
is very common, particularly at the lower voltages, and has the important advantage that the bank will operate V-connected if one transformer is disabled. The ⌬-Y connection is advantageous for stepping up
to high voltages since the secondary of the transformers need be wound
only for 58 percent (1/√3) of the line voltage; it is also necessary when a
four-wire three-phase system is obtained from a three-wire three-phase
system since ‘‘a floating neutral’’ on the secondary cannot occur. This
connection has found increased application in step-down distribution at
600 V and lower because of the relative ease of applying sensitive fault
protection. The Y-Y system may be used for stepping up voltage. It
should not be used for obtaining a three-phase four-wire system from a
three-phase three-wire system, because of the ‘‘floating neutral’’ on the
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15-36
ELECTRICAL ENGINEERING
Fig. 15.1.67 Transformer connections for transforming moderate amounts of three-phase power. (a) Open delta
connection; (b) T connection.
secondary and the resulting high degree of unbalance of the secondary
voltages. The Y-⌬ system may be used to step down high voltages, the
reverse of the ⌬-Y connection. Y-connected windings also preclude
third harmonic, and their multiples, circulating currents in the transformer windings. In the ⌬-Y and Y-⌬ systems the ratio of line voltage is
obviously not that of the individual transformers. Because of different
phase displacement between primaries and secondaries, a ⌬-⌬ bank
cannot be connected in parallel (on both sides) with a ⌬-Y bank, etc.,
even if they both have the correct voltage ratios between lines.
Three-phase transformers combine the magnetic circuits of three single-phase transformers so that they have parts in common. A material
saving in cost, in weight, and in space results, the greatest saving occurring in the core and oil. The advantages of three-phase transformers are
often outweighed by their lack of flexibility. The failure of a single
phase shuts down the entire transformer. With three single units, one
unit may be readily replaced with a single spare. The primaries of single-phase transformers may be connected in Y or ⌬ at will and the
secondaries properly phased. The primaries, as well as the secondaries
of three-phase transformers, must be phased.
For the transformation of moderate amounts of power from threephase to three-phase, two transformers employing either the open delta
or T connection (Fig. 15.1.67) may be used. With each connection the
ratio of line voltages is the same as the transformer ratios. In the figure,
ratios 10 : 1 are shown. In the T connection the primary and the secondary of the main transformer must be provided with a center tap to which
one end of the teaser transformer is connected. The ratings of these
systems are only 58 percent of the rating of the system using three
similar transformers, one for each phase. Owing to dissymmetry, the
terminal voltages become somewhat unbalanced even with a balanced
load.
To transform from two- to three-phase or the reverse, the T connection
of Fig. 15.1.68 is used. To make the secondary voltages symmetrical a
tap (called a Scott tap) is brought out at 86.6 percent (√3/2) of the
primary winding of the teaser transformer as shown in Fig. 15.1.68.
With balanced no-load voltages the voltages become slightly unbalanced even under a symmetrical load, owing to unequal phase differences in the individual coils. The three-phase neutral O is one-third the
winding of the teaser transformer from the junction. In Fig. 15.1.68a the
transformation is from three-phase to a two-phase three-wire system. In
Fig. 15.1.68b the transformation is from three-phase to a four-phase,
five-wire system. The voltages are given on the basis of 100-V primaries with 1 : 1 transformer ratios.
An autotransformer, also called compensator, consists essentially of a
single winding linking a magnetic circuit. Part of the energy is transformed, and the remainder flows through conductively. Suitable taps are
provided so that, if the primary voltage is applied to two of the taps, a
voltage may be taken from any other two taps. The ratio of voltages is
equal practically to the ratio of the turns between their taps. An autotransformer should be installed only when the ratio of transformation is
not large. The ratio of power transformed to total power is 1 ⫺ n, where
n is the ratio of low-voltage to high-voltage emf. This gives the saving
over the ordinary transformer and is greatest when the ratio is not far
from unity. Figure 15.1.69a shows 100 kW being changed from 3,300
to 2,300 V; 30.3 kW only are being actually transformed, and the remainder of the power flows through conductively. Figure 15.1.69b
shows how an ordinary 10 : 1, 10-kW lighting transformer may be connected to boost 110 kW 10 percent in voltage. In Fig. 15.1.69b, however, the 230-V secondary must be insulated for 2,300 V to the core and
Fig. 15.1.69
Autotransformer.
ground. The voltage may likewise be reduced by reversing the 230-V
coil. An autotransformer should never be used when it is desired to keep
dangerous primary potentials from the secondary. It is used for starting
induction motors (Fig. 15.1.71) and for a number of similar purposes.
Data on Transformers Single-phase 55° self-cooled oil-insulated
transformers for 2,300-V primaries, 230/115-V secondaries, and in
sizes from 5 to 200 kVA for 60(25) Hz have efficiencies from one-half
to full load of about 98 (97 to 98.7) percent and regulation of 1.5 (1.1 to
2.1) percent with pf ⫽ 1, and 3.5 (2.7 to 4.1) percent with pf ⫽ 0.8.
Power transformers with 13,200-V primaries and 2,300-V secondaries
in sizes from 667 to 5,000 kVA and for both 60 and 25 Hz have efficiencies from one-half to full load of about 99.0 percent and regulation
of about 1.0 (4.2) percent with pf ⫽ 1 (0.8).
Fig. 15.1.68 Connections for transforming from three-phase to two- and four-phase power.
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
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AC MOTORS
AC MOTORS
The polyphase induction motor is the
most common type of motor used. It consists ordinarily of a stator
which is wound in the same manner as the synchronous-generator stator. If two-phase current is supplied to a two-phase winding or threephase current to a three-phase winding, a rotating magnetic field is
produced in the air gap. The number of poles which this field has is the
same as the number of poles that a synchronous generator employing
the same stator winding would have. The speed of the rotating field, or
the synchronous speed,
Polyphase Induction Motor
N ⫽ 120f/P
r/min
(15.1.119)
where N2 ⫽ the rotor speed, r/min. The rotor frequency
f2 ⫽ sf
value of slip at which it normally operates, the rotor frequency and
hence the rotor reactance become low and most of the rotor current now
flows in the low-resistance winding. Cage bars can be shaped so that
one winding gives comparable results. See Fig. 15.1.70c. The rotor
operates with a low value of slip. The high starting torque of the highresistance motor and the excellent constant-speed operating characteristics of the low-resistance squirrel-cage rotor are combined in one motor.
(15.1.118)
where f ⫽ frequency and P ⫽ number of poles.
There are two general types of rotors. The squirrel-cage type consists
of heavy copper or aluminum bars short-circuited by end rings, or the
bars and end rings may be an integral aluminum casting. The wound
rotor has a polyphase winding of the same number of poles as the stator,
and the terminals are brought out to slip rings so that external resistance
may be introduced. The rotor conductors must be cut by the rotating
field, hence the rotor cannot run at synchronous speed but must slip. The
per unit slip is
s ⫽ (N ⫺ N2)/N
15-37
(15.1.120)
The torque is proportional to the air-gap flux and the components of
rotor current in space phase with it. The rotor currents tend to lag the
emfs producing them, because of the rotor-leakage reactance. From Eq.
(15.1.120) the rotor frequency and hence the rotor reactance (X2 ⫽
2␲ f2 L2) are low when the motor is running near synchronous speed, so
that there is a large component of rotor current in space phase with the
flux. With large values of slip the increased rotor frequency increases
the rotor reactance and hence the lag of the rotor currents behind their
emfs, and therefore considerable space-phase difference between these
currents and the flux develops. Consequently even with large values of
current the torque may be small. The torque of the induction motor
increases with slip until it reaches a maximum value called the breakdown torque, after which the torque decreases (see Fig. 15.1.72). The
breakdown torque varies as the square of the voltage, inversely as the
stator impedance and rotor reactance, and is independent of the rotor
resistance.
The squirrel-cage motor develops moderate torque on starting (s ⫽
1.0) even though the current may be three to seven times rated current.
For any value of slip the torque of the induction motor varies as the
square of the voltage. The torque of the squirrel-cage motor which, on
starting, is only moderate may be reduced in the larger motors because
of starting voltage drop from inrush and possible necessity of applying
reduced-voltage starting.
Polyphase squirrel-cage motors are used mostly for constant-speed
work; however, recent developments in variable-frequency pulse-width
modulation (PWM) ac drives have seen these motors used in variablespeed applications where the ratio of maximum to minimum speed does
not exceed 4 : 1. They are used widely on account of their rugged construction and the absence of moving electrical contacts, which makes
them suitable for operation when exposed to flammable dust or gas.
General-purpose squirrel-cage motors have starting torques of 100 to
250 percent of full load torque at rated voltage. The highest torques
occur at the higher rated speeds. The locked rotor currents vary between
four and seven times full-load current. In the double-squirrel-cage type of
motor there is a high-resistance winding in the top of the rotor slots and
a low-resistance winding in the bottom of the slots. The low-resistance
winding is made to have a high leakage reactance, either by separating
the windings with a magnetic bridge, Fig. 15.1.70a, or by making the
slot very narrow in the area between the two windings, Fig. 15.1.70b.
On starting, because of the high reactance of the low-resistance winding, most of the rotor current will flow in the high-resistance winding,
giving the motor a large starting torque. As the rotor approaches the low
Fig. 15.1.70
Types of slots for squirrel-cage windings.
The single shaped cage bar is more economical, and almost any shape
for required characteristics can be extruded from aluminum. Single bars
also eliminate the problems of differential expansion with double cage
bars.
Nameplates of polyphase integral-hp squirrel-cage induction motors
carry a code letter and a design letter. These provide information about
motor characteristics, the former on locked rotor or starting inrush
current (see Table 15.1.26) and the latter on torque characteristics. National Electrical Manufacturers Association standards publication No.
MG1-1978 defines four design letters: A, B, C, and D. In all cases the
motors are designed for full voltage starting. Locked rotor current and
torque, pull-up torque and breakdown torque are tabulated according to
horsepower and speed. Designs A, B, and C have full load slips less than
5 percent and design D more than 5 percent. The nature of the various
designs can be understood by reference to the full voltage values for a
100-hp, 1800-r/min motor which follow:
Design
Locked rotor torque
Pull-up torque
Breakdown torque
Locked rotor current
Full load slip (%)
A
B
C
D
125*
100†
200†
...
5§
125*
100†
200†
600*
5§
200*
140†
190†
600*
5§
275*
...
...
600*
5‡
NOTE: All quantities, except slip, are percent of full load value.
* Upper limit.
† Not less than.
‡ Greater than.
§ Less than.
Starting It is desirable to start induction motors by direct connection across the line, since reduced voltage starters are expensive and
almost always reduce the starting torque. The capacity of the distribution system dictates when reduced voltage starting must be used to limit
voltage dips on the system. On stiff industrial systems 25,000-hp motors
have been successfully started across-the-line.
In Fig. 15.1.71a is shown an ‘‘across-the-line’’ starter which may be
operated from different push-button stations. The START push button
closes the solenoid circuit between phases C and A through three bimetallic strips in series. This energizes solenoid S, which attracts armature
D, which in turn closes the starting switch and the auxiliary blade G.
This blade keeps the solenoid circuit closed when the START push button is released. Pressing the STOP push button opens the solenoid circuit, permitting the starting switch to open. A prolonged heavy overload
raises the temperature of the heaters by an amount that will cause at
least one of the bimetallic strips to open the solenoid circuit, releasing
the starting switch.
A common method of applying reduced-voltage start is to use a compensator or autotransformer or autostarter (Fig. 15.1.71b). When the
switch is in the starting position, the three windings AB of the threephase autotransformer are connected in Y across the line and the motor
terminals are connected to the taps which supply reduced voltage. When
the switch is in the running position, the starter is entirely disconnected
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15-38
ELECTRICAL ENGINEERING
stop the motor in case of a failed SCR, which will normally fail shorted
and apply full voltage to the motor. The SCR starter has been combined
with a microprocessor to provide gradual motor terminal voltage increases during an adjustable acceleration period. This is commonly
called soft start. Other features include the ability to limit motor starting
current and save energy when the motor is lightly loaded.
By introducing resistance into the rotor circuit through slip rings, the
rotor currents may be brought nearly into phase with the air-gap flux
and, at the same time, any value of torque up to maximum torque
obtained. As the rotor develops speed, resistance may be cut out until
there is no external resistance in the rotor circuit. The speed may also be
controlled by inserting resistance in the rotor circuit. However, like the
armature-resistance method of speed control with shunt motors, this
method is also inefficient and gives poor speed regulation. Figure
15.1.72 shows graphically the effect on the torque of applying reduced
voltage (curves b, c) and of inserting resistance in the rotor circuit
(curve d). As shown by curves b and c, the torque for any given slip is
proportional to the square of the line voltage. The effect of introducing
resistance into the rotor circuit is shown by curve d. The point of maximum torque is shifted toward higher values of slip. The maximum
Fig. 15.1.72
motor.
Fig. 15.1.71 Starters for squirrel-cage induction motor. (a) Across-the-line
starter; (b) autostarter.
from the line. In modern practice, motors are protected by thermal or
magnetic overload relays (Fig. 15.1.71) which operate to trip the circuit
breaker. Since a time element is involved in the operation of such relays,
they do not respond to large starting currents, because of their short
duration. To limit the current to as low a value as possible, the lowest
taps that will give the motor sufficient voltage to supply the required
starting torque should be used. As the torque of an induction motor
varies as the square of the voltage, the compensator produces a very low
starting torque.
Resistors in series with the stator may also be used to start squirrel-cage
motors. They are inserted in each phase and are gradually cut out as the
motor comes up to speed. The resistors are generally made of wire-type
resistor units or of graphite disks enclosed within heat-resisting porcelain-lined iron tubes. The disadvantage of resistors is that if the motor is
started slowly the resistor becomes very hot and may burn out. Resistor
starters are less expensive than autotransformers. Their application is to
motors that start with light loads at infrequent intervals.
A phase-controlled, silicon-controlled rectifier (SCR) may be used to
limit the motor-starting current to any value that will provide sufficient
starting torque by reducing voltage to the motors. The SCR can also be
used to start and stop the motors. A positive opening device such as a
contactor or circuit breaker should be used in series with the SCR to
Speed-torque curve for 10-hp, 60-Hz, 1,140-r/min induction
torque at starting (slip ⫽ 1.0) occurs when the rotor resistance is equal
to the rotor reactance at standstill. The wound-rotor motor is used where
large starting torque is necessary as in railway work, hoists, and cranes.
It has better starting characteristics than the squirrel-cage motor, but,
because of the necessarily higher resistance of the rotor, it has greater
slip even with the rotor resistance all cut out. Obviously, the wound
rotor, controller, and external resistance make it more expensive than
the squirrel-cage type. See Table 15.1.13 for performance data.
One disadvantage of induction motors is that they take lagging current,
and the power factor at half load and less is low. The speed- and torqueload characteristics of induction motors are almost identical with those
of the shunt motor. The speed decreases slightly to full load, the slip
being from 10 percent in small motors to 2 percent in very large motors.
The torque is almost proportional to the load nearly up to the breakdown
torque. The power factor is 0.8 to 0.9 at full load. The direction of
rotation of any three-phase motor may be reversed by interchanging any
two stator wires.
Speed Control of Induction Motors The induction motor inherently
is a constant-speed motor. From Eqs. (15.1.118) and (15.1.119) the
rotor speed is
N2 ⫽ 120f(1 ⫺ s)/P
(15.1.121)
The speed can be changed only by changing the frequency, poles, or
slip. In some applications where the motors constitute the only load on
the generators, as with electric propulsion of ships, their speed may be
changed by changing the frequency. Even then the range is limited, for
both turbines and generators must operate near their rated speeds for
good efficiency. By employing two distinct windings or by reconnecting a single winding by switching it is possible to change the number of
poles. Complications prevent more than two speeds being readily obtained in this manner. Elevator motors frequently have two distinct
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AC MOTORS
Table 15.1.13
15-39
Drip-Proof Motors
3 phase, 230 V, 60 Hz, 1,750 r/min, squirrel cage
Weight,
lb
Current,
A
1
2
5
10
20
40
100
200
40
45
65
110
190
475
830
1,270
3.8
6.0
14.2
26.0
53.6
101.2
230.0
446.0
5
10
25
50
100
200
155
220
495
650
945
2,000
15.6
27.0
60.0
122.0
234.0
446.0
hp
Power factor, %
⁄ load
12
45.3
54.3
61.3
66.3
69.5
69.8
83.7
82.5
Efficiency, %*
⁄ load
Full load
64.8
67.6
73.7
77.7
78.4
79.4
88.2
87.8
66.2
76.5
80.7
83.1
80.9
83.6
89.0
89.2
34
⁄ load
⁄ load
Full load
63.8
75.2
77.0
83.5
85.0
86.8
92.2
93.5
71.4
79.9
80.9
86.0
87.0
88.3
92.4
94.2
75.5
81.5
81.5
86.5
86.5
88.5
91.7
94.1
77.6
84.3
88.1
86.7
90.0
90.5
81.0
85.4
88.6
87.9
90.4
92.1
81.4
84.8
87.8
87.7
89.8
92.5
90.8
91.7
92.1
92.5
93.4
93.8
92.8
93.7
94.1
90.8
91.7
92.1
92.5
93.4
93.8
92.8
93.7
94.1
12
34
3 phase, 230 V, 60 Hz, 1,750 r/min, wound rotor
50.7
66.8
79.2
72.4
78.4
79.8
64.1
77.5
86.6
82.3
86.2
87.6
74.0
82.5
89.4
86.8
89.4
90.8
3 phase, 2,300 V, 60 Hz, 1,750 r/min, squirrel cage
300
700
1,000
2,300
3,380
4,345
70.4
155.0
221.0
300
700
1,000
3,900
5,750
8,450
68
154
218
76.2
85.5
86.2
83.4
89.4
89.5
86.0
90.0
90.0
3 phase, 2,300 V, 60 Hz, 1,750 r/min, wound rotor
84.4
82.7
82.9
86.2
87.7
87.9
89.9
90.9
91.1
* High-efficiency motors are available at premium cost.
SOURCE: Westinghouse Electric Corp.
windings. Another objection to changing the number of poles is the fact
that the design is a compromise, and sacrifices of desirable characteristics usually are necessary at both speeds.
The change of slip by introducing resistance into the rotor circuit has
been discussed under the wound-rotor motor. It is also possible to introduce an inverter into the rotor circuit and convert the slip frequency
power to line frequency power and return it to the distribution system.
Inverters are also used to convert line frequencies to variable frequencies to operate squirrel-cage motors at almost any speed up to their
mechanical limitations. Inverters also reduce the starting stresses on a
motor.
Polyphase voltages should be evenly balanced to prevent phase
current unbalance. If voltages are not balanced, the motor must be derated in accordance with National Electrical Manufacturers Association
(NEMA), Publication No. MG-1-14.34.
Approximately 5 percent voltage unbalance would cause about 25
percent increase in temperature rise at full load. The input current unbalance at full load would probably be 6 to 10 times the input voltage
unbalance.
Single phasing, one phase open, is the ultimate unbalance and will
cause overheating and burnout if the motor is not disconnected from the
line.
Single-Phase Induction Motor Single-phase induction motors are
usually made in fractional horsepower ratings, but they are listed by
NEMA in integral ratings up to 10 hp. They have relatively high rotor
resistances and can operate in the single-phase mode without overheating. Single-phase induction motors are not self-starting.
However, the single-phase motor runs in the direction in which it is
started. There are several methods of starting single-phase induction
motors. Short-circuited turns, or shading coils, may be placed around the
pole tips which retard the time phase of the flux in the pole tip, and thus
a weak torque in the direction of rotation is produced. A high-resistance
starting winding, displaced 90 electrical degrees from the main winding,
produces poles between the main poles and so provides a rotating field
which is weak but is sufficient to start the motor. This is called the
split-phase method. In order to minimize overheating this winding is
ordinarily cut out by a centrifugal device when the armature reaches
speed. In the larger motors a repulsion-motor start is used. The rotor is
wound like an ordinary dc armature with a commutator, but with shortcircuited brushes pressing on it axially rather than radially. The motor
starts as a repulsion motor, developing high torque. When it nears its
synchronous speed, a centrifugal device pushes the brushes away from
the commutator, and at the same time causes the segments to be shortcircuited, thus converting the motor into a single-phase induction
motor.
Capacitor Motors Instead of splitting the phase by means of a highresistance winding, it has become almost universal practice to connect a
capacitor in series with the auxiliary winding (which is displaced 90
electrical degrees from the main winding). With capacitance, it is possible to make the flux produced by the auxiliary winding lead that produced by the main field winding by 90° so that a true two-phase rotating
field results and good starting torque develops. However, the 90° phase
relation between the two fields is obtainable at only one value of speed
(as at starting), and the phase relation changes as the motor comes up to
speed. Frequently the auxiliary winding is disconnected either by a
centrifugal switch or a relay as the motor approaches full speed, in
which case the motor is called a capacitor-start motor. With proper
design the auxiliary winding may be left in circuit permanently (frequently with additional capacitance introduced). This improves both the
power factor and torque characteristics. Such a motor is called permanent-split capacitor motor.
Phase Converter If a polyphase induction motor is operating single-phase, polyphase emfs are generated in its stator by the combination
of stator and rotor fluxes. Such a machine can be utilized, therefore, for
converting single-phase power into polyphase power and, when so used,
is called a phase converter. Unless corrective means are utilized, the
polyphase emfs at the machine terminals are somewhat unbalanced. The
power input, being single-phase and at a power factor less than unity,
not only fluctuates but is negative for two periods during each cycle.
The power output being polyphase is steady, or nearly so. The cyclic
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15-40
ELECTRICAL ENGINEERING
differences between the power output and the power input are accounted for in the kinetic energy stored in the rotating mass of the
armature. The armature accelerates and decelerates, but only slightly, in
accordance with the difference between output and input. The phase
converter is used principally on railway locomotives, since a single
trolley wire can be used to deliver single-phase power to the locomotive, and the converter can deliver three-phase power to the three-phase
wound-rotor driving motors.
AC Commutator Motors Inherently simple ac motors are not
adapted to high starting torques and variable speed. There are a number
of types of commutating motor that have been developed to meet the
requirement of high starting torque and adjustable speed, particularly
with single phase. These usually have been accompanied by compensating windings, centrifugal switches, etc., in order to overcome low
power factors and commutation difficulties. With proper compensation,
commutator motors may be designed to operate at a power factor of
nearly unity or even to take leading current.
One of the simplest of the single-phase commutator motors is the ac
series railway motor such as is used on the erstwhile New York, New
Haven, and Hartford Railroad. It is based on the principle that the torque
of the dc series motor is in the same direction irrespective of the polarity
of its line terminals. This type of motor must be used on low frequency,
not over 25 Hz, and is much heavier and more costly than an equivalent
dc motor. The torque and speed curves are almost identical with those of
the dc series motor. Unlike most ac apparatus the power factor is highest
at light load and decreased with increasing load. Such motors operate
with direct current even better than with alternating current. For example, the New Haven locomotives also operate from the 600-V dc thirdrail system (two motors in series) from the New York City line (238th
St.) into Grand Central Station. (See also Sec. 11.)
On account of difficulties inherent in ac operation such as commutation and high reactance drops in the windings, it is economical to construct and operate such motors only in sizes adaptable to locomotives,
the ratings being of the order of 300 to 400 hp. Universal motors are
small simple series motors, usually of fractional horsepower, and will
operate on either direct or alternating current, even at 60 Hz. They are
used for vacuum cleaners, electric drills, and small utility purposes.
Synchronous Motor Just as dc shunt generators operate as motors,
a synchronous generator, connected across a suitable ac power supply,
will operate as a motor and deliver mechanical power. Each conductor
on the stator must be passed by a pole of alternate polarity every half
cycle so that at constant frequency the rpm of the motor is constant and
is equal to
N ⫽ 120f/P
r/min
(15.1.122)
and the speed is independent of the load.
There are two types of synchronous motors in general use: the slipring type and the brushless type. The motor field current is transmitted to
the motor by brushes and slip rings on the slip-ring type. On the brushless type it is generated by a shaft-mounted exciter and rectified and
controlled by shaft-mounted static devices. Eliminating the slip rings is
advantageous in dirty or hazardous areas.
The synchronous motor has the desirable characteristic that its power
factor can be varied over a wide range merely by changing the field
excitation. With a weak field the motor takes a lagging current. If the
load is kept constant and the excitation increased, the current decreases
(Fig. 15.1.73) and the phase difference between voltage and current
becomes less until the current is in phase with the voltage and the power
factor is unity. The current is then at its minimum value such as I0 , and
the corresponding field current is called the normal excitation. Further
increase in field current causes the armature current to lead and the
power factor to decrease. Thus underexcitation causes the current to lag;
overexcitation causes the current to lead. The effect of varying the field
current at constant values of load is shown by the V curves (Fig.
15.1.73). Unity power factor occurs at the minimum value of armature
current, corresponding to normal excitation. The power factor for any
point such as P is I0 /I1 , leading current. Because of its adjustable power
factor, the motor is frequently run light merely to improve power factor
or to control the voltage at some part of a power system. When so used
the motor is called a synchronous condenser. The motor may, however,
deliver mechanical power and at the same time take either leading or
lagging current.
Fig. 15.1.73
V curves of a synchronous motor.
Synchronous motors are used to drive centrifugal and axial compressors, usually through speed increasers, pumps, fans, and other highhorsepower applications where constant speed, efficiency, and power factor correction are important. Low-speed synchronous motors, under 600
r/min, sometimes called engine type, are used in driving reciprocating
compressors and in ball mills and in other slow-speed applications. Their
low length-to-diameter ratio, because of the need for many poles, gives
them a high moment of inertia which is helpful in smoothing the pulsating torques of these loads.
If the motor field current is separately supported by a battery or constant voltage transformer, the synchronous motor will maintain speed on
a lower voltage dip than will an induction motor because torque is
proportional to voltage rather than voltage squared. However, if a synchronous motor drops out of step, it will normally not have the ability to
reaccelerate the load, unless the driven equipment is automatically unloaded.
The synchronous motor is usually not used in smaller sizes since both
the motors and its controls are more expensive than induction motors,
and the ability of a small motor to supply VARs to correct power factor
is limited.
If situated near an inductive load the motor may be overexcited, and
its leading current will neutralize entirely or in part the lagging quadrature current of the load. This reduces the I 2R loss in the transmission
lines and also increases the kilowatt ratings of the system apparatus. The
synchronous condenser and motor can also be used to control voltage and
to stabilize power lines. If the condenser or motor is overexcited, its
leading current flowing through the line reactance causes a rise in voltage at the motor; if it is underexcited, the lagging current flowing
through the line reactance causes a drop in voltage at the motor. Thus
within limits it becomes possible to control the voltage at the end of a
transmission line by regulating the fields of synchronous condensers or
motors. Long 220-kV lines and the 287-kV Hoover Dam – Los Angeles
line require several thousand kVa in synchronous condensers floating at
their load ends merely for voltage control. If the load becomes small, the
voltage would rise to very high values if the synchronous condensers
were not underexcited, thus maintaining nearly constant voltage. See
Table 15.1.14 for characteristics.
A salient-pole synchronous motor may be started as an induction motor.
In laminated-pole machines conducting bars of copper, copper alloy, or
aluminum, damper or amortisseur windings are inserted in the pole face
and short-circuited at the ends, exactly as a squirrel-cage winding in the
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AC-DC CONVERSION
Table 15.1.14
Power,
hp
15-41
Performance Data for Coupled Synchronous Motors
Poles
Speed,
r/min
Current,
A
Excitation,
kW
Efficiencies, %
⁄ load
12
⁄ load
Full load
Weight,
lb
95.2
97.1
97.3
97.9
93.9
94.6
95.6
95.3
97.2
97.5
98.0
94.3
95.0
95.6
5,000
15,000
27,000
45,000
7,150
15,650
54,000
94.0
96.1
96.3
97.3
93.4
94.2
95.3
94.1
96.2
96.5
97.4
93.6
94.4
95.5
6,500
24,000
37,000
70,000
9,500
17,500
11,500
34
Unity power factor, 3 phase, 60 Hz, 2,300 V
500
2,000
5,000
10,000
500
1,000
4,000
4
4
4
6
18
24
48
1,800
1,800
1,800
1,200
400
300
150
500
2,000
5,000
10,000
500
1,000
4,000
4
4
4
6
18
24
48
1,800
1,800
1,800
1,200
400
300
150
100
385
960
1,912
99.3
197
781
3
9
13
40
5
8.4
25
94.5
96.5
96.5
97.5
92.9
93.7
94.9
80% power factor, 3 phase, 60 Hz, 2,300 V
127
486
1,212
2,405
125
248
982
4.5
13
21
50
7.2
11.6
40
93.3
95.5
95.5
96.8
92.4
93.3
94.6
SOURCE: Westinghouse Electric Corp.
induction motor is connected. The bars can be designed only for starting
purposes since they carry no current at synchronous speed and have no
effect on efficiency. In solid-pole motors a block of steel is bolted to the
pole and performs the current-carrying function of the damper winding
in the laminated-pole motor. At times the pole faces are interconnected
to minimize starting-pulsating torques. When the synchronous motor
reaches 95 to 98 percent speed as an induction motor, the motor field is
applied by a timer or slip frequency control circuit, and the motor pulls
into step at 100 percent speed. While accelerating, the motor field is
connected to resistances to minimize induced voltages and currents.
Two-pole motors are built as turbine type or round-rotor motors for
mechanical strength and do not have the thermal capacity or space for
starting windings, so they must be started by supplementary means.
One such supplementary starting means is the use of a variable-frequency source, either a variable-speed generator or more commonly a
static converter-inverter. The motor is brought up to speed in synchronism with a slowly increasing frequency. One common application is
the starting of the large motor-generators used in pump-storage utility
systems.
Variable frequency may be used to start salient-pole machines also.
Requirements for a start without high torques and pulsations or high
voltage drops on small electrical systems may dictate the use of something other than full voltage starting.
The synchronous reluctance motor is similar to an induction machine
with salient poles machined in the rotors. Under light loads the motor
will synchronize on reluctance torque and lock in step with the rotating
field at synchronous speed. These motors are used in small sizes with
variable-frequency inverters for speed control in the paper and textile
industry.
The synchronous-induction motor is fundamentally a wound-rotor
slip-ring induction motor with an air gap greater than normal, and the
rotor slots are larger and fewer. On starting, resistance is inserted in the
rotor circuit to produce high torque, and this is cut out as the speed
increases. As synchronism is approached, the rotor windings are connected to a dc power source and the motor operates synchronously.
Timing or clock motors operate synchronously from ac power systems.
Figure 15.1.74a illustrates the Warren Telechron motor which operates
on the hysteresis principle. The stator consists of a laminated element
with an exciting coil, and each pole piece is divided, a short-circuited
shading turn being placed on each of the half poles so formed. The rotor
consists of two or more hard-steel disks of the shape shown, mounted on
a small shaft. The shaded poles produce a 3,600 r/min rotating magnetic
field (at 60 Hz), and because of hysteresis loss, the disk follows the field
just as the rotor of an induction motor does. When the rotor approaches
the synchronous speed of 3,600 r/min, the rotating magnetic field takes
a path along the two rotor bars and locks the rotor in with it. The rotor
and the necessary train of reducing gears rotate in oil sealed in a small
metal can. Figure 15.1.74b shows a subsynchronous motor. Six
squirrel-cage bars are inserted in six slots of a solid cylindrical iron
rotor, and the spaces between the slots form six salient poles. The
motor, because of the squirrel cage, starts as an induction motor, attempting to attain the speed of the rotating field, or 3,600 r/min (at 60
Hz). However, when the rotor reaches 1,200 r/min, one-third synchronous speed, the salient poles of the rotor lock in with the poles of the
stator and hold the rotor at 1,200 r/min.
Fig. 15.1.74 Synchronous motors for timing. (a) Warren Telechron motor;
(b) Holtz induction-reluctance subsynchronous motor.
AC-DC CONVERSION
Static Rectifiers Silicon devices, and to a lesser extent gas tubes,
are the primary means of ac to dc or dc to ac conversion in modern
installations. They are advantageous when compared to synchronous
converters or motor generators because of efficiency, cost, size, weight,
and reliability. Various bridge configurations for single-phase and
three-phase applications are shown in Fig. 15.1.75a. Table 15.1.15
shows the relative outputs of rectifier circuits. The use of two threephase bridges fed from an ac source consisting of a three-winding transformer with both a ⌬ and Y secondary winding so that output voltages
are 30° out of phase will reduce dc ripple to approximately 1 percent.
The use of silicon-controlled rectifiers (SCRs) to replace rectifiers in
the various bridge configurations allows the output voltages to be varied
from rated output voltage to zero. The output voltage wave will not be a
sine wave but a series of square waves, which may not be suitable for
some applications. Dc to ac conversions are shown in Fig. 15.1.75b.
A new technology of ac-to-dc conversion commonly called switch
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15-42
ELECTRICAL ENGINEERING
Table 15.1.15
Devices
Relationships for AC-DC Conversion Static
Voltages, %
Currents, %
Device
description
E ac
E dc
I ac
I dc
Ripple,
%
1 ␾, half wave
1 ␾, full wave
3 ␾, full wave
100
100
100
45
90
135
100
100
100
100
90
123
121
48
4.2
mode power supplies (SMPSs) is found in almost all microprocessorbased modern electronic equipment for dc supplies between 3 and 15 V.
The supply voltage (120 V ac) is rectified by a single-phase full-wave
bridge circuit. See Fig. 15.1.75. The output is stored in a capacitor. A
switcher will then switch the dc voltage from the capacitor on and off at
a high frequency, usually between 10 and 100 kHz. These high-frequency pulses are stepped down in voltage by a transformer and rectified by diodes. The diodes’ output is filtered to the dc supply required.
These SMPSs are small in size, have higher efficiency, and are lower in
cost. They do create harmonic and power quality problems that have to
be addressed.
Because of the materially increased rating, converters are nearly all
operated six-phase. The rating decreases rapidly with decrease in power
factor, and hence the converter should operate near unity power factor.
The diametrical ac voltage is the ac voltage between two slip-ring taps
180 electrical degrees apart. With a two-pole closed winding, i.e., a
winding that closes on itself when the winding is completed, the diametrical ac voltage is the voltage between any two slip-ring taps diametrically opposite each other.
With a sine-voltage wave, the dc voltage is the peak of the diametrical ac voltage wave. The voltage relations for sine waves are as follows: dc volts, 141; single phase, diametrical, 100; three-phase, 87;
four-phase, diametrical, 100; four phase, adjacent taps, 71; six phase,
diametrical, 100; six phase, adjacent taps, 50. These relations are obtained from the sides of polygons inscribed in a circle having a diameter
of 100 V, as shown in Fig. 15.1.76.
Selsyns The word selsyn is an abbreviation of self-synchronizing
and is applied to devices which are connected electrically, and in which
an angular displacement of the rotating member of one device produces
an equal angular displacement in the rotating member of the second
Fig. 15.1.76
Fig. 15.1.75 AC-DC conversion with static devices.
SYNCHRONOUS CONVERTERS
The synchronous converter is essentially a dc generator with slip rings
connected by taps to equidistant points in the armature winding. Alternating current may also be taken from and delivered to the armature.
The machine may be single-phase, in which case there are two slip rings
and two slip-ring taps per pair of poles; it may be three-phase, in which
case there are three slip rings and three slip-ring taps per pair of poles,
etc. Converters are usually used to convert alternating to direct current,
in which case they are said to be operating direct; they may equally well
convert direct to alternating current, in which case they are said to be
operating inverted. A converter will operate satisfactorily as a dc motor,
a synchronous motor, a dc generator, a synchronous generator, or it may
deliver direct and alternating current simultaneously, when it is called a
double-current generator.
The rating of a converter increases very rapidly with increase in
the number of phases owing, in part, to better utilization of the armature copper and also because of more uniform distribution of armature
heating.
EMF relations in a converter.
device. There are several types of selsyns and they may be dc or ac,
single-phase or polyphase. A simple and common type is shown in Fig.
15.1.77. The two stators S1 , S2 are phase-wound stators, identical electrically with synchronous-generator or induction-motor stators. For
simplicity Gramme-ring windings are shown in Fig. 15.1.77. The two
stators are connected three-phase and in parallel. There are also two
bobbin-type rotors R1 , R2 , with single-phase windings, each connected
to a single-phase supply such as 115 V, 60 Hz. When R1 and R2 are
in the same angular positions, the emfs induced in the two stators by the
ac flux of the rotors are equal and opposite, there are no interchange
currents between stators, and the system is in equilibrium. However, if
the angular displacement of R1 , for example, is changed, the magnitudes of the emfs induced in the stator winding of S1 are correspondingly changed. The emfs of the two stators then become unbalanced,
currents flow from S1 to S2 , producing torque on R2 . When R2 attains
the same angular position as R1 , the emfs in the two rotors again become
equal and opposite, and the system is again in equilibrium.
If there is torque load on either rotor, a resultant current is necessary
to sustain the torque, so that there must be an angular displacement
between rotors. However, by the use of an auxiliary selsyn a current
may be fed into the system which is proportional to the angular difference of the two rotors. This current will continue until the error is
corrected. This is called feedback. There may be a master selsyn, controlling several secondary units.
Selsyns are used for position indicators, e.g., in bridge – engine-room
signal systems. They are also widely used for fire control so that from
any desired position all the turrets and guns on battleships can be turned
and elevated simultaneously through any desired angle with a high degree of accuracy. The selsyn itself rarely has sufficient power to perform these operations, but it actuates control through power multipliers
such as amplidynes.
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RATING OF ELECTRICAL APPARATUS
15-43
Fig. 15.1.77 Selsyn system.
RATING OF ELECTRICAL APPARATUS
The rating of electrical apparatus is almost always determined by the
maximum temperature at which the materials in the machine, especially
the insulation and lubricant, may be operated for long periods without
deterioration. It is permissible, as far as temperature is concerned, to
overload the apparatus so long as the safe temperature is not exceeded.
The ANSI/IEEE Standard 100-1992 classifies insulating materials in
seven different classes:
1. Class 90 insulation. Materials or combinations of materials such
as cotton, silk, and paper without impregnation which will have suitable
thermal endurance if operated continually at 90°C.
2. Class 105 insulation. Materials or combinations of materials such
as cotton, silk, and paper when suitably impregnated or coated or when
immersed in a dielectric liquid. This class has sufficient thermal endurance at 105°C.
3. Class 130 insulation. Materials or combinations of materials such
as mica, glass fiber, asbestos, etc., with suitable bonding substances.
This class has sufficient thermal endurance at 130°C.
4. Class 155 insulation. Same materials as class 130 but with bonding substances suitable for continuous operation at 155°C.
5. Class 180 insulation. Materials or combinations of materials such
as silicone elastomer, mica, glass fiber, asbestos, etc., with suitable
bonding substances such as appropriate silicone resins. This class has
sufficient thermal life at 180°C.
6. Class 220 insulation. Materials suitable for continuous operation
at 220°C.
7. Class over-220 insulation. Materials consisting entirely of mica,
porcelain, glass, quartz, and similar inorganic materials which have
suitable thermal life at temperatures over 220°C.
NOTE: In all cases, other materials or combinations of materials other than
those mentioned above may be used in a given class if from experience or accepted tests they can be shown to have comparable thermal life. It is common
practice also to specify insulation systems in electrical machinery by letter. For example, integral horsepower ac motors may have a maximum temperature rise in the
winding (determined by winding resistance) or 60°C for class A insulation, 80°C
for class B, 105°C for class F, and 125°C for class H — all based on a 40°C ambient.
The recommended methods of measurement are: (1) the thermometer
method is preferred for uninsulated windings, exposed metal parts,
gases and liquids, or surface methods generally; thermocouples are preferred for rapidly changing surface temperatures; (2) the appliedthermocouple method is suitable for making surface temperature measurements when it is desired to measure the temperature of surfaces
that are accessible to thermocouples but not to liquid-in-glass thermometers; (3) the contact-thermocouple method is suitable for measuring
temperatures of bare metal surfaces such as those of commutator bars
and slip rings; (4) the resistance method is suitable for insulated windings, except for windings of such low resistance that measurements
cannot be accurately made due to uncontrollable resistance in contacts
or where it is impracticable to make connections to obtain measurements before an undesirable drop in temperature occurs; (5) the embedded-detector method is suitable for interior measurements at designated
locations as specified in the standards for certain kinds of equipment,
such as large rotating machines.
Efficiency of Electrical Motors
Methods of determining efficiency are by direct measurement or by segregated losses. Methods are outlined in Standard Test Procedure for Polyphase Induction Motors and Generators, ANSI/IEEE Std. 112-1991;
Standard Test Code for DC Machines, IEEE Std. 113-1985, Test Procedure for Single-Phase Induction Motors, ANSI/IEEE Std. 114-1982;
and Test Procedures for Synchronous Machines, IEEE Std. 115-1983.
Direct measurements can be made by using calibrated motors, generators, or dynamometers for input to generators and output from motors,
and precision electrical motors for input to motors and output from
generators.
Efficiencies ⫽
output
input
(15.1.123)
The segregated losses in motors are classified as follows: (1) Stator
I 2R (shunt and series field I 2R for dc); (2) rotor I 2R (armature I 2R for
dc); (3) core loss; (4) stray-load loss; (5) friction and windage loss;
(6) brush-contact loss (wound rotor and dc); (7) brush-friction loss
(wound rotor and dc); (8) exciter loss (synchronous and dc); and
(9) ventilating loss (dc). Losses are calculated separately and totaled.
Measure the electrical output of the generator; then
Efficiency ⫽
output
output ⫹ losses
(15.1.124a)
Measure the electrical input of the motors; then
Efficiency ⫽
input ⫺ losses
input
(15.1.124b)
When testing dc motors, compensation should be made for the harmonics associated with rectified ac used to provide the variable dc voltage to the motors. Instrumentation should be chosen to accurately reflect the rms value of currents.
Temperature rise under full-load conditions may be measured by tests
as outlined in the IEEE Standards referred to above. Methods of loading
are: (1) Load motor with dynamometer or generator of similar capacity
and run until temperatures stabilize; (2) load generator with motor-generator set or plant load and run until temperature stabilizes; (3) alternately apply dual frequences to motor until it reaches rated temperature;
(4) synchronous motor may be operated as synchronous condenser at no
load with zero power factor at rated current, voltage, and frequency until
temperatures stabilize.
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15-44
ELECTRICAL ENGINEERING
Industrial Applications of Motors
The induction motor, particularly the
squirrel-cage type, is preferable to the dc motor for constant-speed
work, for the initial cost is less and the absence of a commutator reduces
maintenance. Also there is less fire hazard in many industries, such as
sawmills, flour mills, textile mills, and powder mills. The use of the
induction motor in such places as cement mills is advantageous since
with dc motors the grit makes the maintenance of commutators difficult.
For variable-speed work like cranes, hoists, elevators, and for adjustable speeds, the dc motor characteristics are superior to induction-motor
characteristics. Even then, it may be desirable to use induction motors
since their less desirable characteristics are more than balanced by their
simplicity and the fact that ac power is available, and to obtain dc power
conversion apparatus is usually necessary. Where both lights and
motors are to be supplied from the same ac system, the 208/120-V
four-wire three-phase system is now in common use. This gives 208 V
three-phase for the motors, and 120 V to neutral for the lights.
Full-load speed, temperature rise, efficiency, and power factor as
well as breakdown torque and starting torque have long been parameters
of concern in the application and purchase of motors. Another qualification is service factor. The service factor of an alternating current
motor is a multiplier applicable to the horsepower rating. When so
applied, the result is a permissible horsepower loading under the conditions specified for the service factor. When operated at service factor
load with 1.15 or higher service factor, the permissible temperature rise
by resistance is as follows: class A insulation 70°C; class B, 90°C; and
class F, 115°C.
Special enclosures, fittings, seals, ventilation systems, electromagnetic design, etc., are required when the motor is to be operated under
unusual service conditions, such as exposure to (1) combustible, explosive, abrasive, or conducting dusts, (2) lint or very dirty conditions
where the accumulation of dirt might impede the ventilation, (3) chemical fumes or flammable or explosive gases, (4) nuclear radiation,
(5) steam, salt laden air, or oil vapor, (6) damp or very dry locations,
radiant heat, vermin infestation, or atmosphere conducive to the growth
of fungus, (7) abnormal shock, vibration, or external mechanical loading, (8) abnormal axial thrust or side forces on the motor shaft,
(9) excessive departure from rated voltage, (10) deviation factors of
the line voltage exceeding 10 percent, (11) line voltage unbalance exceeding 1 percent, (12) situations where low noise levels are required,
(13) speeds higher than the highest rated speed, (14) operation in a
poorly ventilated room, in a pit, or in an inclined attitude, (15) torsional impact loads, repeated abnormal overloads, reversing or electric
braking, (16) operation at standstill with any winding continuously
energized, and (17) operation with extremely low structureborne and
airborne noise. For dc machines, a further unusual service condition
occurs when the average load is less than 50 percent over a 24-h period
or the continuous load is less than 50 percent over a 4-h period.
The standard direction of rotation for all nonreversing dc motors, ac
single-phase motors, synchronous motors, and universal motors is
counterclockwise when facing the end of the machine opposite the drive
end. For dc and ac generators, the rotation is clockwise.
Further information may be found in Publication No. MG-1 of the
National Electrical Manufacturers Association.
It must be recognized that heat is conducted by electrical conductors.
Windings in motors operating in a 40°C ambient at class F temperature
rises are running at temperatures 90°C higher than the maximum allowable temperature (75°C) of cable ordinarily used in interior wiring. Heat
conducted by the motor leads in such a situation could cause a failure of
the branch circuit cable in the terminal box. See Tables 15.1.21 and
15.1.22.
Alternating or Direct Current
ELECTRIC DRIVES
Cranes and Hoists The dc series motor is best adapted to cranes
and hoists. When the load is heavy the motor slows down automatically
and develops increased torque thus reducing the peaks on the electrical
system. With light loads, the speed increases rapidly, thus giving a
lively crane. The series motor is also well adapted to moving the bridge
itself and also the trolley along the bridge. Where alternating current
only is available and it is not economical to convert it, the slip-ring type
of induction motor, with external-resistance speed control, is the best
type of ac motor. Squirrel-cage motors with high resistance end rings to
give high starting torque (design D) are used (design D motors; also see
Ilgner system).
Constant-Torque Applications Piston pumps, mills, extruders, and
agitators may require constant torque over their complete speed range.
They may require high starting torque design C or D squirrel-cage
motors to bring them up to speed. Where speed is to be varied while
running, a variable armature voltage dc motor or a variable-frequency
squirrel-cage induction-motor drive system may be used.
Centrifugal Pumps Low WK 2 and low starting torques make design
B general-purpose squirrel-cage motors the preference for this application. When variable flow is required, the use of a variable-frequency
power supply to vary motor speed will be energy efficient when compared to changing flow by control-valve closure to increase head.
Centrifugal Fans High WK 2 may require high starting torque design C or D squirrel-cage motors to bring the fan up to speed in a
reasonable period of time. When variable flow is required, the use of a
variable-frequency power supply or a multispeed motor to vary fan speed
will be energy efficient when compared to closing louvers. For large
fans, synchronous-motor drives may be considered for high efficiency
and improved power factor.
Axial or Centrifugal Compressors For smaller compressors, say, up
to 100 hp, the squirrel-cage induction motor is the drive of choice.
When the WK 2 is high, a design C or D high-torque motor may be
required. For larger compressors, the synchronous motor is more efficient and improves power factor. Where variable flow is required, the
variable-frequency power supply to vary motor speed is more efficient
than controlling by valve and in some applications may eliminate a gearbox by allowing the motor to run at compressor operating speed.
Pulsating-Torque Applications Reciprocating compressors, rock
crushers, and hammer mills experience widely varying torque pulsations
during each revolution. They usually have a flywheel to store energy, so
a high-torque, high-slip design D motor will accelerate the high WK 2
rapidly and allow energy recovery from the flywheel when high torque
is demanded. On larger drives a slow-speed, engine-type synchronous
motor can be directly connected. The motor itself supplies significant
WK 2 to smooth out the torque and current pulsations of the system.
SWITCHBOARDS
Switchboards may, in general, be divided into four classes: direct-control
panel type; remote mechanical-control panel type; direct-control truck
type; electrically operated. With direct-control panel-type boards the
switches, rheostats, bus bars, meters, and other apparatus are mounted
on or near the board and the switches and rheostats are operated directly, or by operating handles if they are mounted in back of the board.
The voltages, for both direct current and alternating current, are usually
limited to 600 V and less but may operate up to 2,500 V ac if oil circuit
breakers are used. Such panels are not recommended for capacities
greater than 3,000 kVA. Remote mechanical-control panel-type boards are
ac switchboards with the bus bars and connections removed from the
panels and mounted separately away from the load. The oil circuit
breakers are operated by levers and rods. This type of board is designed
for heavier duty than the direct-control type and is used up to 25,000
kVA. Direct-control truck-type switchboards for 15,000 V or less consist
of equipment enclosed in steel compartments completely assembled by
the manufacturers. The high-voltage parts are enclosed, and the equipment is interlocked to prevent mistakes in operation. This equipment is
designed for low- and medium-capacity plants and auxiliary power in
large generating stations. Electrically operated switchboards employ solenoid or motor-operated circuit breakers, rheostats, etc., controlled by
small switches mounted on the panels. This makes it possible to locate
the high-voltage and other equipment independently of the location of
switchboard.
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SWITCHBOARDS
15-45
Fig. 15.1.78 Current-carrying capacity of copper bus bars.
In all large stations the switching equipment and buses are always
mounted entirely either in separate buildings or in outdoor enclosures.
Such equipment is termed bus structures and is electrically operated
from the main control board.
Marble has high dielectric qualities and was formerly used exclusively for the panels. It is now used occasionally where its appearance is
desired for architectural purposes. Slate is used extensively and is finished in black enamel, marine, and natural black. Ebony asbestos is also
used frequently, is lighter than marble or slate, has high dielectric
strength and insulation resistivity, and can be readily cut, drilled, and
machined. Steel panels, usually 1⁄8 in thick, are light, economical in
construction and erection, and at the present time are favored over other
types.
Switchboards should be erected at least 3 to 4 ft from the wall.
Switchboard frames and structures should be grounded. The only exceptions are effectively insulated frames of single-polarity dc switchboards. For low-potential work, the conductors on the rear of the
switchboard are usually made up of flat copper strip, known as bus-bar
copper. The size required is based upon a current density of about 1,000
A/in2. Figure 15.1.78 gives the approximate continuous dc carrying
capacity of copper bus bars for different arrangements and spacings for
35°C temperature rise.
Switchboards must be individually adapted for each specific electrical system. Space permits the showing of the diagrams of only three
boards each for a typical electrical system (Fig. 15.1.79). Aluminum
bus bars are also frequently used.
Fig. 15.1.79 Switchboard wiring diagrams for generators. (a) 125-V or 250-V dc generator; (b) three-phase, synchronous generator and exciter for a small or isolated plant; (c) three-wire dc generator for a small or isolated plant. A,
ammeter; AS, three-way ammeter switch; CB, circuit breaker, CT, current transformer; DR, ground detector receptacle;
L, ground detector lamp; OC, overload coil; OCB, oil circuit breaker; PP, potential ring; PR, potential receptacle; PT,
potential transformer; Rheo, rheostat; RS, resistor; S, switch; Sh, shunt; V, voltmeter; WHM, watthour meter.
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15-46
ELECTRICAL ENGINEERING
Equipment of Standard Panels Following are enumerated the
various parts required in the equipment of standard panels for varying
services:
Generator or synchronous-converter panel, dc two-wire system: 1 circuit
breaker; 1 ammeter; 1 handwheel for rheostat; 1 voltmeter; 1 main
switch (three-pole single throw or double throw) or 2 single-pole
switches.
Generator or synchronous-converter panel, dc three-wire system: 2 circuit breakers; 2 ammeters; 2 handwheels for field rheostats; 2 field
switches; 2 potential receptacles for use with voltmeter; 3 switches;
1 four-point starting switch.
Generator or synchronous-motor panel, three-phase three-wire system:
3 ammeters; 1 three-phase wattmeter; 1 voltmeter; 1 field ammeter;
1 double-pole field switch; 1 handwheel for field rheostats; 1 synchronizing receptacle (four-point); 1 potential receptacle (eight-point);
1 field rheostat; 1 triple-pole oil switch; 1 power-factor indicator;
1 synchronizer; 2 series transformers; 1 governor control switch.
Synchronous-converter panel, three-phase: 1 ammeter; 1 power-factor
indicator; 1 synchronizing receptacle; 1 triple-pole oil circuit breaker;
2 current transformers; 1 potential transformer; 1 watthour meter (polyphase); 1 governor control switch.
Induction motor panel, three-phase: 1 ammeter; series transformers;
1 oil switch.
Feeder panel, dc, two-wire and three-wire: 1 single-pole circuit breaker;
1 ammeter; 2 single-pole main switches; potential receptacles (1 fourpoint for two-wire panel; 1 four-point and 1 eight-point for three-wire
panel).
Feeder panel, three-wire, three-phase and single-phase: 3 ammeters;
1 automatic oil switch (three-pole for three-phase, two-pole for singlephase); 2 series transformers; 1 shunt transformer; 1 wattmeter; 1 voltmeter; 1 watthour meter; 1 handwheel for control of potential regulator.
Exciter panel (for 1 or 2 exciters): 1 ammeter (2 for 2 exciters); 1 field
rheostat (2 for 2 exciters); 1 four-point receptacle (2 for 2 exciters);
1 equalizing rheostat for regulator.
Switches The current-carrying parts of switches are usually designed for a current density of 1,000 A/in2. At contact surfaces, the
current density should be kept down to about 50 A/in2.
Circuit Breakers Switches equipped with a tripping device constitutes an elementary load interrupter switch. The difference between a
load interrupter switch and a circuit breaker lies in the interrupting
capacity. A circuit breaker must open the circuit successfully under
short circuit conditions when the current through the contacts may be
several orders of magnitude greater than the rated current. As the circuit
is being opened, the device must withstand the accompanying mechanical forces and the heat of the ensuing arc until the current is permanently
reduced to zero.
The opening of a metallic circuit while carrying electric current
causes an electric arc to form between the parting contacts. If the action
takes place in air, the air is ionized (a plasma is formed) by the passage
of current. When ionized, air becomes an electric conductor. The space
between the parting contacts thus has relatively low voltage drop and
the region close to the surface of the contacts has relatively high voltage
drop. The thermal input to the contact surfaces (VI) is therefore relatively large and can be highly destructive. A major aim in circuit
breaker design is to quench the arc rapidly enough to keep the contacts
in a reusable state. This is done in several ways: (1) lengthening the arc
mechanically, (2) lengthening the arc magnetically by driving the
current-carrying plasma sideways with a magnetic field, (3) placing
barriers in the arc path to cool the plasma and increase its length,
(4) displacing and cooling the plasma by means of a jet of compressed
air or inert gas, and (5) separating the contacts in a vacuum chamber.
By a combination of shunt and series coils the circuit breaker can be
made to trip when the energy reverses. Circuit breakers may trip unnecessarily when the difficulty has been immediately cleared by a local
breaker or fuse. In order that service shall not be thus interrupted unnecessarily, automatically reclosing breakers are used. After tripping, an automatic mechanism operates to reclose the breaker. If the short circuit
still exists, the breaker cannot reclose. The breaker attempts to reclose
two or three times and then if the short circuit still exists it remains
permanently locked out.
Metal-clad switch gears are highly developed pieces of equipment that
combine buses, circuit breakers, disconnecting devices, controlling devices, current and potential transformers, instruments, meters, and interlocking devices, all assembled at the factory as a single unit in a
compact steel enclosing structure. Such equipment may comprise trucktype circuit breakers, assembled as a unit, each housed in a separate
steel compartment and mounted on a small truck to facilitate removal
for inspection and servicing. The equipment is interlocked to prevent
mistakes in operation and in the removal of the unit; the removal of the
unit breaks all electrical connections by suitable disconnecting switches
in the rear of the compartment, and all metal parts are grounded. This
design provides compactness, simplicity, ease of inspection, and safety
to the operator.
High-voltage circuit breakers can be oil type, in which the contacts
open under oil, air-blast type, in which the arc is extinguished by a
powerful blast of air directed through an orifice across the arc and into
an arc chute, H 2 S type, or vacuum contact type. The tripping of highvoltage circuit breakers is initiated by an abnormal current acting
through the secondary of a current transformer on an inverse-time relay
in which the time of closing the relay contacts is an inverse time function of the current; i.e., the greater the current the shorter the time of
closing. The breaker is tripped by a dc tripping coil, the dc circuit being
closed by the relay contacts. Modern circuit breakers should open the
circuit within 3 cycles from the time of the closing of the relay contacts.
Vacuum circuit breakers have received wide acceptance in all fields in
recent years, both for indoor work and for outdoor applications. Indoor
breakers are available up to 40 kV and interrupting capacities up to 2.5
GVA. Outdoor breakers are available in ratings up to that of the EHV
(extra-high voltage 765-kV three-pole breaker capable of interrupting 55 GVA, or 40,000-A symmetrical current. Its operating rating is
3,000 A, 765 kV. The arc is extinguished in a vacuum. Switching stations, gas-insulated and operating at 550 kV, are also in use.
POWER TRANSMISSION
Power for long-distance transmission is usually generated at 6,600,
13,200, and 18,000 V and is stepped up to the transmission voltage by
⌬-Y-connected transformers. The transmission voltage is roughly 1,000
V/mi. Preferred or standard transmission voltages are 22, 33, 44, 66,
110, 132, 154, 220, 287, 330, 500, and 765 kV. High-voltage lines
across country are located on private rights of way. When they reach
urban areas, the power must be carried underground to the substations
which must be located near the load centers in the thickly settled districts. In many cases it is possible to go directly to underground cables
since these are now practicable up to 345 kV between three-phase line
conductors (200 kV to ground). High-voltage cables are expensive in
both first cost and maintenance, and it may be more economical to step
down the voltage before transmitting the power by underground cables.
Within a city, alternating current may be distributed from a substation at
13,200, 6,600, or 2,300 V, being stepped down to 600, 480, and 240 V,
three-phase for power and 240 to 120 V single-phase three-wire for
lights, by transformers at the consumers’ premises. Direct current at
1,200 or 600 V for railways, 230 to 115 V for lighting and power, is
supplied by motor-generator sets, synchronous converters, and rectifiers. Constant current for series street-lighting systems is obtained
through constant-current transformers.
Transmission Systems
Power is almost always transmitted three-phase. The following fundamental relations apply to any transmission system. The weight of conductor required to transmit power by any given system with a given
percentage power loss varies directly with the power, directly as the
square of the distance, and inversely as the square of the voltage. The
cross-sectional area of the conductors with a given percentage power
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POWER TRANSMISSION
15-47
Fig. 15.1.80 Three equivalent symmetrical transmission, or distribution, systems. (a) Single-phase; (b) three-phase;
(c) four-phase.
loss varies directly with the power, directly with the distance, and inversely as the square of the voltage.
For two systems of the same length transmitting the same power at
different voltages and with the same power loss for both systems, the
cross-sectional area and weight of the conductors will vary inversely as
the square of the voltages. The foregoing relations between the cross
section or weight of the conductor and transmission distance and voltage hold for all systems, whether dc, single-phase, three-phase, or fourphase. With the power, distance, and power loss fixed, all symmetrical
systems having equal voltages to neutral require equal weights of conductor. Thus, the three symmetrical systems shown in Fig. 15.1.80 all
deliver the same power, have the same power loss and equal voltages to
neutral, and the transmission distances are all assumed to be equal. They
all require the same weight of conductor since the weights are inversely
proportional to all resistances. (No actual neutral conductor is used.)
The respective power losses are (1) 2I 2R W; (2) 3(2I/3)2(3R/2) ⫽ 2I 2R
W; (3) 4(I/2)2(2R) ⫽ 2I 2R W, which are all equal.
Size of Transmission Conductor Kelvin’s law states, ‘‘The most
economical area of conductor is that for which the annual cost of energy
wasted is equal to the interest on that portion of the capital outlay which
can be considered proportional to the weight of copper used.’’ In Fig.
15.1.81 are shown the annual interest cost, the annual cost of I 2R loss,
and the total cost as functions of circular mils cross section for both
typical overhead conductors and three-conductor cables. Note that the
total-cost curves have very flat minimums, and usually other factors
such as the character of the load and the voltage regulation, are taken
into consideration.
In addition to resistance, overhead power lines have inductive reactance to alternating currents. The inductive reactance
X ⫽ 2␲f{80 ⫹ 741.1 log [(D ⫺ r)/r]} 10⫺6
⍀/conductor mile (15.1.126)
where f ⫽ frequency, D ⫽ distance between centers of conductors (in),
and r their radius (in). Table 15.1.16 gives the inductive reactance per
mile at 60 Hz and the resistance of stranded and solid copper conductor.
(See Table 15.1.20.)
Any symmetrical system having n conductors can be divided into n
equal single-phase systems, each consisting of one wire and a return
circuit of zero impedance and each having as its voltage the system
voltage to neutral.
Fig. 15.1.82
Three-phase power system.
Figure 15.1.82 shows a symmetrical three-phase system, with one
phase detached. The load or received voltage between line conductors is
E⬘R so that the receiver voltage to neutral is E R ⫽ E⬘R /√3V. The current is
I A, the load power factor is cos ␪, and the line resistance and reactance
are R and X ⍀ per wire, and the sending-end voltage is E S . The phasor
diagram is shown in Fig. 15.1.83 (compare with Fig. 15.1.63). Its solution is
E S ⫽ √(E R cos ␪ ⫹ IR)2 ⫹ (E R sin ␪ ⫹ IX)2
(15.1.127)
[see Eq. (15.1.101)].
Fig. 15.1.83
Fig. 15.1.81 Most economical sizes of overhead and underground conductors.
Phasor diagram for a power line.
Figure 15.1.84 (Mershon diagram) shows the right-hand portion of
Fig. 15.1.83 plotted to large scale, the arc 00 corresponding to the arc ab
(Fig. 15.1.83). The abscissa 0 (Fig. 15.1.84) corresponds to point b (Fig.
15.1.83) and is the load voltage E R taken as 100 percent. The concentric
circular arcs 0 – 40 are given in percentage of E R . To find the sending-
15-48
Resistance and Inductive Reactance per Single Conductor
Hard-drawn copper, stranded
60 Hz
Size,
cir mils
or
AWG
No. of
strands
500,000
400,000
300,000
250,000
0000
000
00
0
37
19
19
19
19
7
7
7
Spacing, ft
OD, in
Resistance,
⍀ /mi
1
2
3
4
5
6
7
8
10
12
15
20
30
0.814
0.725
0.628
0.574
0.528
0.464
0.414
0.368
0.1130
0.1426
0.1900
0.2278
0.2690
0.339
0.428
0.538
0.443
0.458
0.476
0.487
0.497
0.518
0.532
0.546
0.527
0.542
0.560
0.571
0.581
0.602
0.616
0.630
0.576
0.591
0.609
0.620
0.630
0.651
0.665
0.679
0.611
0.626
0.644
0.655
0.665
0.686
0.700
0.714
0.638
0.653
0.671
0.682
0.692
0.713
0.727
0.741
0.660
0.675
0.693
0.704
0.714
0.735
0.749
0.763
0.679
0.694
0.712
0.723
0.733
0.754
0.768
0.782
0.695
0.710
0.728
0.739
0.749
0.770
0.784
0.798
0.722
0.737
0.755
0.766
0.776
0.797
0.811
0.825
0.745
0.760
0.778
0.789
0.799
0.820
0.834
0.848
0.772
0.787
0.805
0.816
0.826
0.847
0.861
0.875
0.807
0.822
0.840
0.851
0.861
0.882
0.896
0.910
0.856
0.871
0.889
0.900
0.917
0.931
0.945
0.959
0.727
0.741
0.755
0.769
0.783
0.746
0.760
0.774
0.788
0.802
0.762
0.776
0.790
0.804
0.818
0.789
0.803
0.817
0.831
0.845
0.812
0.826
0.840
0.854
0.868
0.839
0.853
0.867
0.881
0.895
0.874
0.888
0.902
0.916
0.930
0.923
0.937
0.951
0.965
0.979
Hard-drawn copper, solid
0000
000
00
0
1
—
—
—
—
—
0.4600
0.4096
0.3648
0.3249
0.2893
0.264
0.333
0.420
0.528
0.665
0.510
0.524
0.538
0.552
0.566
0.594
0.608
0.622
0.636
0.650
0.643
0.657
0.671
0.685
0.699
0.678
0.692
0.706
0.720
0.734
0.705
0.719
0.733
0.747
0.761
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
Table 15.1.16
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
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POWER TRANSMISSION
end voltage E S for any power factor cos ␪, compute first the resistance
drop IR and the reactance drop IX in percentage of E R . Then follow the
ordinate corresponding to the load power factor to the inner arc 00 (a,
Fig. 15.1.83). Lay off the percentage IR drop horizontally to the right,
and the percentage IX drop vertically upward. The arc at which the IX
drop terminates (c, Fig. 15.1.83) when added to 100 percent gives the
sending-end voltage E S in percent of the load voltage E R .
15-49
referring to Table 13.1.16, 250,000 cir mils copper having a resistance of 0.2278
⍀/mi may be used. The total resistance R ⫽ 60 ⫻ 0.2278 ⫽ 13.67 ⍀. The
reactance X ⫽ 60 ⫻ 0.723 ⫽ 43.38 ⍀. The volts to neutral at the load, E R ⫽
66,000/√3 ⫽ 38,100 V. cos ␪ ⫽ 0.80; sin ␪ ⫽ 0.60. Using Eq. (15.1.127), E S ⫽
{[(38,100 ⫻ 0.80) ⫹ (218.8 ⫻ 13.67)]2 ⫹ [(38,100 ⫻ 0.60) ⫹ (218.8 ⫻
43.38)]2}1/2 ⫽ 46,500 V to neutral or √3 ⫻ 46,500 ⫽ 80,500 between lines at the
sending end. The line loss is 3(218.8)2 ⫻ 13.67 ⫽ 1,963 kW. The efficiency
␩ ⫽ 20,000/(20,000 ⫹ 1,963) ⫽ 0.911, or 91.1 percent. This same line is solved
by means of the Mershon diagram as follows. Let E R ⫽ 38,100 V ⫽ 100 percent.
IR ⫽ 218.8 ⫻ 13.67 ⫽ 2,991 V ⫽ 7.85 percent. IX ⫽ 218.8 ⫻ 43.38 ⫽ 9,490
V ⫽ 24.9 percent. Follow the 0.80 power-factor ordinate (Fig. 15.1.84) to its intersection with the arc 00; from this point go 7.85 percent horizontally to the
right and then 24.9 percent vertically. (These percentages are measured on
the horizontal scale.) This last distance terminates on the 22.5 percent arc. The
sending-end voltage to neutral is then 1.225 ⫻ 38,100 ⫽ 46,500 V, so that
the sending-end voltage between line conductors is E⬘S ⫽ 46,500 √3 ⫽ 80,530 V.
In Table 15.1.16 the spacing is the distance between the centers of the
two conductors of a single-phase system or the distance between the
centers of each pair of conductors of a three-phase system if they are
equally spaced. If they are not equally spaced, the geometric mean
3
distance (GMD) is used, where GMD ⫽ √D1 D2 D3 (Fig. 15.1.85a). With
3
the flat horizontal spacing shown in Fig. 15.1.85b, GMD ⫽ √2D 3 ⫽
1.26D.
Fig. 15.1.85 Unequal spacing of three-phase conductors. (a) GMD ⫽
(D1 D2 D3 )1/3 ; (b) flat horizontal spacing; GMD ⫽ 1.26 D.
Fig. 15.1.84 Mershon diagram for determining voltage drop in power lines.
EXAMPLE. Let it be desired to transmit 20,000 kW three-phase 80 percent
power factor lagging current, a distance of 60 mi. The voltage at the receiving end
is 66,000 V, 60 Hz and the line loss must not exceed 10 percent of the power
delivered. The conductor spacing must be 7 ft (84 in). Determine the sendingend voltage and the actual efficiency. I ⫽ 20,000,000/(66,000 ⫻ 0.80 ⫻ √3) ⫽
218.8 A. 3 ⫻ 218.82 ⫻ R⬘ ⫽ 0.10 ⫻ 20,000,000. R⬘ ⫽ 13.9 ⍀ ⫽ 0.232 ⍀/mi. By
Table 15.1.17
In addition to copper, aluminum cable steel-reinforced (ACSR),
Table 15.1.17, is used for transmission conductor. For the same resistance it is lighter than copper, and with high voltages the larger diameter
reduces corona loss.
Until 1966, 345 kV was the highest operating voltage in the United
States. The first 500 kV system put into operation (1966) was a 350-mi
transmission loop of the Virginia Electric and Power Company; the
longest transmission distance was 170 mi. The towers, about 94 ft high,
Properties of Aluminum Cable Steel-Reinforced (ACSR)
⍀ /mi of single conductor at 25°C
Cir mils or AWG
Cross section, in2
No. of wires
200 A
600 A
Aluminum
Copper
equivalent
Aluminum
Steel
OD, in
Aluminum
Total
Total
lb /mi
0 A dc
25 Hz
60 Hz
25 Hz
60 Hz
1,590,000
1,431,000
1,272,000
1,192,500
1,113,000
1,000,000
900,000
800,000
750,000
700,000
54
54
54
54
54
19
19
19
19
19
1.545
1.465
1.382
1.338
1.293
1.249
1.124
0.9990
0.9366
0.8741
1.4071
1.2664
1.1256
1.0553
0.9850
10,777
9,699
8,621
8,082
7,544
0.0587
0.0652
0.0734
0.0783
0.0839
0.0589
0.0654
0.0736
0.0785
0.0841
0.0594
0.0659
0.0742
0.0791
0.0848
0.0592
0.0657
0.0738
0.0787
0.0843
0.0607
0.0671
0.0752
0.0801
0.0857
1,033,500
954,000
874,500
795,000
715,500
650,000
600,000
550,000
500,000
450,000
54
54
54
26
54
7
7
7
7
7
1.246
1.196
1.146
1.108
1.036
0.8117
0.7493
0.6868
0.6244
0.5620
0.9170
0.8464
0.7759
0.7261
0.6348
7,019
6,479
5,940
5,770
4,859
0.0903
0.0979
0.107
0.117
0.131
0.0906
0.0980
0.107
0.117
0.131
0.0913
0.0985
0.108
0.117
0.133
0.0908
0.0983
0.107
0.117
0.131
0.0922
0.0997
0.109
0.117
0.133
636,000
556,500
477,000
397,500
336,400
400,000
350,000
300,000
250,000
0000
54
26
26
26
26
7
7
7
7
7
0.977
0.927
0.858
0.783
0.721
0.4995
0.4371
0.3746
0.3122
0.2642
0.5642
0.5083
0.4357
0.3630
0.3073
4,319
4,039
3,462
2,885
2,442
0.147
0.168
0.196
0.235
0.278
0.147
0.168
0.196
0.235
0.278
0.149
0.168
0.196
0.235
0.278
0.147
0.168
0.196
0.235
0.278
0.149
0.168
0.196
0.235
0.278
266,800
0000
000
00
0
000
00
0
1
2
26
6
6
6
6
7
1
1
1
1
0.642
0.563
0.502
0.447
0.398
0.2095
0.1662
0.1318
0.1045
0.0829
0.2367
0.1939
0.1537
0.1219
0.0967
1,936
1,542
1,223
970
769
0.350
0.441
0.556
0.702
0.885
0.350
0.443
0.557
0.703
0.885
0.350
0.446
0.561
0.707
0.889
0.350
0.447
0.562
0.706
0.887
0.350
0.464
0.579
0.718
0.893
SOURCE: Aluminum Co. of America.
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15-50
ELECTRICAL ENGINEERING
are of corrosion-resistant steel, and the conductors are 61-strand cables
of aluminum alloy, rather than the usual aluminum cable with a steel
core (ACSR). The conductor diameter is 1.65 in with two ‘‘bundled’’
conductors per phase and 18-in spacing. The standard span is 1,600 ft,
the conductor spacing is flat with 30-ft spacing between phase-conductor centers, and the minimum clearance to ground is 34 to 39 ft. To
maintain a minimum clearance of 11 ft to the towers and 30 ft spacing
between phases, vee insulator strings, each consisting of twenty-four
10-in disks, are used with each phase. The highest EHV system in North
American is the 765-kV system in the midwest region of the United
States. A dc transmission line on the west coast of the United States
operating at ⫾ 450 kV is transmitting power in bulk more than 800
miles.
High-voltage dc transmission has a greater potential for savings and a
greater ability to transmit large blocks of power longer distances than
has three-phase transmission. For the same crest voltage there is a saving of 50 percent in the weight of the conductor. Because of the power
stability limit due to inductive and capacitive effects (inherent with ac
transmission), the ability to transmit large blocks of power long distances has not kept pace with power developments, even at the present
highest ac transmission voltage of 765 kV. With direct current there is
no such power stability limit.
Where cables are necessary, as under water, the capacitive charging
current may, with alternating current become so large that it absorbs a
large proportion, if not all, of the cable-carrying capability. For example, at 132 kV, three-phase (76 kV to ground), with a 500 MCM cable,
at 36 mi, the charging current at 60 Hz is equal to the entire cable
capability so that no capability remains for the load current. With direct
current there is no charging current, only the negligible leakage current,
and there are no ac dielectric losses. Furthermore, the dc voltage at
which a given cable can operate is twice the ac voltage.
The high dc transmission voltage is obtained by converting the ac
power voltage to direct current by means of mercury-arc rectifiers; at
the receiving end of the line the dc voltage is inverted back to a powerfrequency voltage by means of mercury-arc inverters. The maximum dc
transmission voltage in use in the United States, as of 1994, is 500 kV.
Alternating to direct to alternating current is nonsynchronous transmission of electric power. It can be overhead or under the surface. In the
early 1970s the problem of bulk electric-power transmission over highvoltage transmission lines above the surface developed the insistent
discussion of land use and environmental cost.
While the maximum ac transmission voltage in use in the United
States (1994) is 765 kV, ultrahigh voltage (UHV) is under consideration. Transmission voltages of 1,200 to 2,550 kV are being tested presently. The right-of-way requirements for power transmission are significantly reduced at higher voltages. For example, in one study the
transmission of 7,500 MVA at 345 kV ac was found to require 14
circuits on a corridor 725 ft (221.5 m) wide, whereas a single 1,200-kV
ac circuit of 7,500-MVA capacity would need a corridor 310 ft (91.5 m)
wide.
Continuing research and development efforts may result in the development of more economic high-voltage underground transmission
links. Sufficient bulk power transmission capability would permit power
wheeling, i.e., the use of generating capacity to the east and west to serve
a given locality as the earth revolves and the area of peak demand glides
across the countryside.
Corona is a reddish-blue electrical discharge which occurs when the
voltage-gradient in air exceeds 30 kV peak, 21.1 kV rms, at 76 cm
pressure. This electrical discharge is caused by ionization of the air and
becomes more or less concentrated at irregularities on the conductor
surface and on the outer strands of stranded conductors. Corona is accompanied by a hissing sound; it produces ozone and, in the presence of
moisture, nitrous acid. On high-voltage lines corona produces a substantial power loss, corrosion of the conductors, and radio and television
interference. The fair-weather loss increases as the square of the voltage
above a critical value e 0 and is greatly increased by fog, smoke, rainstorms, sleet, and snow (see Fig. 15.1.86). To reduce corona, the diameter of high-voltage conductors is increased to values much greater than
would be required for the necessary conductance cross section. This is
accomplished by the use of hollow, segmented conductors and by the
use of aluminum cable, steel-reinforced (ACSR), which often has inner
layers of jute to increase the diameter. In extra-high-voltage lines (400
kV and greater), corona is reduced by the use of bundled conductors in
which each phase consists of two or three conductors spaced about 16 in
(0.41 m) from one another.
Fig. 15.1.86
Corona loss with snowstorm.
Underground Power Cables
Insulations for power cables include heat-resisting, low-water-absorptive synthetic rubber compounds, varnished cloth, impregnated paper,
cross-linked polyethylene thermosetting compounds, and thermoplastics such as polyvinyl chloride (PVC) and polyethylene (PE) compounds (see Sec. 6).
Properly chosen rubber-insulated cables may be used in wet locations
with a nonmetallic jacket for protective covering instead of a metallic
sheath. Commonly used jackets are flame-resisting, such as neoprene
and PVC. Such cables are relatively light in weight, easy to train in
ducts and manholes, and easily spliced. When distribution voltages exceed 2,000 V phase to phase, an ozone-resisting type of compound is
required. Such rubber insulation may be used in cables carrying up to
28,000 V between lines in three-phase grounded systems. The insulation wall will be thicker than with varnished cloth, polyethylene, ethylene propylene rubber, or paper.
Varnished-cloth cables are made by applying varnish-treated closely
woven cloth in the form of tapes, helically, to the metallic conductor.
Simultaneously a viscous compound is applied between layers which
fills in any voids at laps in the taping and imparts flexibility when the
cable is bent by permitting movement of one tape upon another. This
type of insulation has higher dielectric loss than impregnated paper but
is suitable for the transmission of power up to 28,000 V between phases
over short distances. Such insulated cables may be used in dry locations
with flame-resisting fibrous braid, reinforced neoprene tape, or PVC
jacket and are often further protected with an interlocked metallic tape
armor; but in wet locations these cables should be protected by a continuous metallic sheath such as lead or aluminum. Since varnish-cloth-insulated cable has high ozone resistance, heat resistance, and impulse
strength, it is well adapted for station or powerhouse wiring or for any
service where the temperature is high or where there are sudden increases in voltage for short periods. Since the varnish is not affected by
mineral oils, such cables make excellent leads for transformers and oil
switches.
PVC is readily available in several fast, bright colors and is often
chosen for color-coded multiconductor control cables. It has inherent
flame and oil resistance, and as single conductor wire and cable with the
proper wall thickness for a particular application, it usually does not
need any outside protective covering. On account of its high dielectric
constant and high power factor, its use is limited to low voltages, i.e.,
under 1,000 V, except for series lighting circuits.
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POWER DISTRIBUTION
15-51
Table 15.1.18 Ampacities of Insulated Cables in Underground Raceways
[ Based on conductor temperature of 90°C, ambient earth temperature of 20°C, 100 percent load factor, thermal resistance (RHO) of 90, and three circuits in group]
Copper
Aluminum
Conductor
size,
AWG or
MCM
2,001 – 5,000-V
ampacity
5,001 – 35,000-V
ampacity
2,001 – 5,000-V
ampacity
5,001 – 35,000-V
ampacity
2,001 – 5,000-V
ampacity
5,001 – 35,000-V
ampacity
2,001 – 5,000-V
ampacity
5,001 – 35,000-V
ampacity
8
6
4
2
1
1/0
2/0
3/0
4/0
250
350
500
750
1,000
56
73
95
125
140
160
185
210
235
260
315
375
460
525
77
99
130
145
165
185
210
240
260
310
370
440
495
53
69
89
115
135
150
170
195
225
245
295
355
430
485
75
97
125
140
160
185
205
230
255
305
360
430
485
44
57
74
96
110
125
145
160
185
205
245
295
370
425
60
77
100
110
125
145
165
185
200
245
290
355
405
41
54
70
90
105
120
135
155
175
190
230
280
345
400
59
75
100
110
125
140
160
180
200
240
285
350
400
3-1/C cables per raceway
1-3/C cable per raceway
3-1/C cables per raceway
1-3/C cable per raceway
NOTE: This is a general table. For other temperatures and installation conditions, see NFPA 70, 1993, National Electrical Code, Tables 77 through 80 and associated notes.
Polyethylene, because of its excellent electrical characteristics, first
found use when it was adapted especially for high-frequency cables
used in radio and radar circuits; for certain telephone, communication,
and signal cables; and for submarine cables. Submarine telephone
cables with built-in repeaters laid first in the Atlantic Ocean and then in
the Pacific are insulated with polyethylene. Because of polyethylene’s
thermal characteristics, the standard maximum conductor operating
temperature is 75°C. It is commonly used for power cables (including
large use for underground residential distribution), with transmissions
up to 15,000 V. Successful installations have been in service at 46 kV
and some at 69 kV. The upper limit has not been reached, inasmuch as
work is in progress on higher-voltage polyethylene power cables as a
result of advancements in the art of compounding.
Cross-linked polyethylene is another insulation which is gaining in
favor in the process field. For power cable insulations, the cross-linking
process is most commonly obtained chemically. It converts polyethylene from a thermoplastic into a thermosetting material; the result is a
compound with a unique combination of properties, including resistance to heat and oxidation, thus permitting an increase in maximum
conductor operating temperature to 90°C. The service record with this
compound has been good at voltages which have been gradually increased to 35 kV. Another thermosetting material, ethylene propylene
rubber (EPR), has found wide acceptance in the 5- to 35-kV range.
Impregnated-paper insulation is used for very-high-voltage cables
whose range has been extended to 345 kV. To eliminate the detrimental
effects of moisture and to maintain proper impregnation of the paper,
such cables must have a continuous metallic sheath such as lead or
aluminum or be enclosed within a steel pipe; the operation of the cable
depends absolutely on the integrity of that enclosure. In three-conductor
belted-type cables the individual insulated conductors are surrounded
by a belt or wall of impregnated paper over which the lead sheath is
applied. When all three conductors are within one sheath, their inductive effects practically neutralize one another and eddy-current loss in
the sheath is negligible. In the type-H cable, each of the individual
conductors is surrounded with a perforated metallic covering, either
aluminum foil backed with a paper tape or thin perforated metal tapes
wound over the paper. All three conductors are then enclosed within the
metal sheath. The metallic coverings being grounded electrically, each
conductor acts as a single-conductor cable. This construction eliminates
‘‘tangential’’ stresses within the insulation and reduces pockets or
voids. When paper tapes are wound on the conductor, impregnated with
an oil or a petrolatum compound, and covered with a lead sheath, they
are called solid type.
Three-conductor cables are now operating at 33,000 V, and singleconductor cables at 66,000 V between phases (38,000 V to ground). In
New York and Chicago, special hollow-conductor oil-filled single-conductor cables are operating successfully at 132,000 V (76,000 V to
ground). In France, cables are operating at 345 kV between conductors.
Other methods of installing underground cables are to draw them into
steel pipes, usually without the sheaths, and to fill the pipes with oil
under pressure (oilstatic) or nitrogen under 200 lb pressure. The ordinary
medium-high-voltage underground cables are usually drawn into duct
lines. With a straight run and ample clearance the length of cable between manholes may reach 600 to 1,000 ft. Ordinarily, the distance is
more nearly 400 to 500 ft. With bends of small radius the distance must
be further reduced.
Cable ratings are based on the permissible operating temperatures of
the insulation and environmental installation conditions. See Table
15.1.18 for ampacities.
POWER DISTRIBUTION
Distribution Systems The choice of the system of power distribution is determined by the type of power that is available and by the
nature of the load. To transmit a given power over a given distance with
a given power loss (I 2R), the weight of conductor varies inversely as the
square of the voltage. Incandescent lamps will not operate economically
at voltages much higher than 120 V; the most suitable voltages for dc
motors are 230, 500, and 550 V; for ac motors, standard voltages are
230, 460, and 575 V, three-phase. When power for lighting is to be
distributed in a district where the consumers are relatively far apart,
alternating current is used, being distributed at high voltage (2,400,
4,160, 4,800, 6,900, and 13,800 V) and transformed at the consumer’s
premises, or by transformers on poles or located in manholes or vaults
under the street or sidewalks, to 240/120 V three-wire for lighting and
domestic customers, and to 208, 240, 480, and 600 volts, three-phase,
for power.
The first central station power systems were built with dc generation
and distribution. The economical transmission distance was short.
Densely populated, downtown areas of cities were therefore the first
sections to be served. Growth of electric service in the United States
was phenomenal in the last two decades of the nineteenth century. After
1895 when ac generation was selected for the development of power
from Niagara Falls, the expansion of dc distribution diminished. The
economics overwhelmingly favored the new ac system.
Direct current service is still available in small pockets in some cities.
In those cases, ac power is generated, transmitted, and distributed. The
conversion to dc takes place in rectifiers installed in manholes near the
load or in the building to be served. Some dc customers resist the change
to ac service because of their need for motor speed control. Elevator and
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printing press drives and some cloth-cutting knives are examples of
such needs. (See Low-Voltage AC Network.)
Series Circuits These constant-current circuits were widely used for
street lighting. The voltage was automatically adjusted to match the
number of lamps in series and maintain a constant current. With the
advent of HID lamps and individual photocells on street lighting fixtures
which require parallel circuitry, this system fell into disuse and is rarely
seen.
Parallel Circuits Power is usually distributed at constant potential,
and all the devices or receivers in the circuit are connected in parallel,
giving a constant-potential system, Fig. 15.1.87a. If conductors of constant cross section are used and all the loads, L 1 , L 2 , etc., are operating,
there will be a greater voltage IR drop per unit length of wire in the
portion of the circuit AB and CD than in the other portions; also the
voltage will not be the same for the different lamps but will decrease
along the mains with distance from the generating end.
Series-Parallel Circuit For incandescent lamps the power must be
at low voltage (115 V) and the voltage variations must be small. If the
transmission distance is considerable or the loads are large, a large or
perhaps prohibitive investment in conductor material would be necessary. In some special cases, lamps may be operated in groups of two in
series as shown in Fig. 15.1.88. The transmitting voltage is thus doubled, and, for a given number of lamps, the current is halved, the permissible voltage drop (IR) in conductors doubled, the conductor resistance quadrupled, the weight of conductor material thus being reduced
to 25 percent of that necessary for simple parallel operation.
Fig. 15.1.88
Series-parallel system.
Three-Wire System In the series-parallel system the loads must be
used in pairs and both units of the pair must have the same power rating.
To overcome these objections and at the same time to obtain the economy in conductor material of operating at higher voltage, the three-wire
system is used. It consists merely of adding a third wire or neutral to the
system of Fig. 15.1.88 as shown in Fig. 15.1.89.
Fig. 15.1.89
Three-wire system.
Fig. 15.1.87 (a) Parallel circuit; (b) loop circuit.
If the neutral wire is of the same cross section as the two outer wires,
this system requires only 37.5 percent of the copper required by an
equivalent two-wire system. Since the neutral ordinarily carries less
current than the outers, it is usually smaller and the ratio of copper to
that of the two-wire system is even less than 37.5 percent (see Table
15.1.19).
When the loads on each half of the system are equal, there will be no
Loop Circuits A more nearly equal voltage for each load is obtained
in the loop system, Fig. 15.1.87b. The electrical distance from one
generator terminal to the other through any receiver is the same as that
through any other receiver, and the voltage at the receivers may be
maintained more nearly equal, but at the expense of additional conductor material.
Table 15.1.19 Resistance and 60-Hz Reactance for Wires with Small Spacings, ⍀, at 20°C
(See also Table 15.1.16.)
AWG and
size of
wire, cir
mils
Resistance
in 1,000 ft of
line (2,000 ft
of wire), copper
12
⁄
1
2
3
4
5
6
9
12
18
24
14 – 4,107
12 – 6,530
10 – 10,380
8 – 16,510
6 – 26,250
5.06
3.18
2.00
1.26
0.790
0.138
0.127
0.116
0.106
0.095
0.178
0.159
0.148
0.138
0.127
0.218
0.190
0.180
0.169
0.158
0.220
0.210
0.199
0.188
0.178
0.233
0.223
0.212
0.201
0.190
0.244
0.233
0.223
0.212
0.201
0.252
0.241
0.231
0.220
0.209
0.271
0.260
0.249
0.238
0.228
0.284
0.273
0.262
0.252
0.241
0.302
0.292
0.281
0.270
0.260
0.284
0.272
4 – 41,740
2 – 66,370
1 – 83,690
0 – 105,500
00 – 133,100
0.498
0.312
0.248
0.196
0.156
0.085
0.074
0.068
0.063
0.057
0.117
0.106
0.101
0.095
0.090
0.149
0.138
0.132
0.127
0.121
0.167
0.156
0.151
0.145
0.140
0.180
0.169
0.164
0.159
0.153
0.190
0.180
0.174
0.169
0.164
0.199
0.188
0.183
0.177
0.172
0.217
0.206
0.201
0.196
0.190
0.230
0.220
0.214
0.209
0.204
0.249
0.238
0.233
0.228
0.222
0.262
0.252
0.246
0.241
0.236
000 – 167,800
0000 – 211,600
250,000
300,000
350,000
400,000
0.122
0.098
0.085
0.075
0.061
0.052
0.052
0.046
—
—
—
—
0.085
0.079
0.075
0.071
0.067
0.064
0.116
0.111
0.106
0.103
0.099
0.096
0.135
0.130
0.125
0.120
0.188
0.114
0.148
0.143
0.139
0.134
0.128
0.127
0.158
0.153
0.148
0.144
0.141
0.138
0.167
0.161
0.157
0.153
0.149
0.146
0.185
0.180
0.175
0.171
0.168
0.165
0.199
0.193
0.189
0.185
0.182
0.178
0.217
0.212
0.207
0.203
0.200
0.197
0.230
0.225
0.220
0.217
0.213
0.209
500,000
600,000
700,000
800,000
900,000
1,000,000
0.042
0.035
0.030
0.026
0.024
0.022
—
—
—
—
—
—
—
—
—
—
—
—
0.090
0.087
0.083
0.080
0.077
0.075
0.109
0.106
0.102
0.099
0.096
0.094
0.122
0.118
0.114
0.112
0.109
0.106
0.133
0.128
0.125
0.122
0.119
0.117
0.141
0.137
0.133
0.130
0.127
0.125
0.160
0.155
0.152
0.148
0.146
0.144
0.172
0.169
0.165
0.162
0.159
0.158
0.192
0.187
0.184
0.181
0.178
0.176
0.202
0.200
0.197
0.194
0.191
0.188
Reactance in 1,000 ft of line (2,000 ft of wire) at 60 Hz
for the distance given in inches between centers of conductors
NOTE: For other frequencies the reactance will be in direct proportion to the frequency.
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WIRING CALCULATIONS
current in the middle or neutral wire, and the condition is the same as
that shown in Fig. 15.1.88. When the loads on the two sides are unequal,
there will be a current in the neutral wire equal to the difference of the
currents in the outside wires.
AC Three-Wire Distribution Practically all energy for lighting and
small motor work is distributed at 1,150, 2,300, or 4,160 V ac to transformers which step down the voltage to 240 and 120 V for three-wire
domestic and lighting systems as well as 208, 240, 480, and 600 volts,
three-phase, for power. For the three-wire systems the transformers are
so designed that the secondary or low-voltage winding will deliver
power at 240 V, and the middle or neutral wire is obtained by connecting to the center or midpoint of this winding (see Figs. 15.1.90 and
15.1.91).
Fig. 15.1.90 Three-wire generator.
Disturbances of utility power have been present since the inception of
the electric utility industry; however, in the past equipment was much
more forgiving and less affected. Present electronic data processing
equipment is microprocessor-based and consequently requires a higher
quality of ac power. Although the quality of utility power in the United
States is very good, sensitive electronic equipment still needs to be
protected against the adverse and damaging effects of transients, surges,
and other power-system aberrations.
distribution by large cables known as feeders. Power is distributed from
the distribution centers to the consumers through the mains and transformed to a usable voltage at the user’s site. As there are no loads
connected to the feeders between the generating station and centers of
distribution, the voltage at the latter points may be maintained constant.
Pilot wires from the centers of distribution often run back to the station,
allowing the operator or automatic controls to maintain constant voltage
at the centers of distribution. This system provides a means of maintaining very close voltage regulation at the consumer’s premises.
A common and economical method of supplying business and thickly
settled districts with high load densities is to employ a 208/120-V,
three-phase, four-wire low-voltage ac network. The network operates
with 208 V between outer wires giving 120 V to neutral (Fig. 15.1.92).
Motors are connected across the three outer wires operating at 208 V,
three-phase. Lamp loads are connected between outer wires and the
grounded neutral. The network is supplied directly from 13,800-V feeders by 13,800/208-V three-phase transformer units, usually located in
manholes, vaults, or outdoor enclosures. This system thus eliminates the
necessity for transformation in the substation. A large number of such
units feed the network, so that the secondaries are all in parallel. Each
transformer is provided with an overload reverse-energy circuit breaker
(network protector), so that a feeder and its transformer are isolated if
trouble develops in either. This system is flexible since units can be
easily added or removed in accordance with the rapid changes in local
loads that occur particularly in downtown business districts.
Fig. 15.1.92
Fig. 15.1.91 Three-wire 230/115-V ac system.
This has led to the development of technologies which condition
power for use by this sensitive equipment. Power-enhancing devices
(surge suppressors, isolation transformers, and line voltage regulators)
modify the incoming waveform to mitigate some aberrations. Powersynthesizing devices (motor-generator sets, magnetic synthesizers, and
uninterruptible power supplies) use the incoming power as a source of
energy to generate a new, completely isolated waveform. Each type has
advantages and limitations.
Grounding The neutral wire of the secondary circuit of the transformer should be grounded on the pole (or in the manhole) and at the
service switch in the building supplied. If, as a result of a lightning
stroke or a fault in the transformer insulation, the transformer primary
circuit becomes grounded at a (Fig. 15.1.91) and the transformer insulation between primary and secondary windings is broken down at b and
if there were no permanent ground connection in the secondary neutral
wire, the potential of wire 1 would be raised 2,300 V above ground
potential. This constitutes a very serious hazard to life for persons coming in contact with the 120 V system. The National Electrical Code
requires the use of a ground wire not smaller than 8 AWG copper. With
the neutral grounded (Fig. 15.1.91), voltages to ground on the secondary system cannot exceed 120 V. (See National Electrical Code 1991,
Art. 250.)
Feeders and Mains Where power is supplied to a large district,
improved voltage regulation is obtained by having centers of distribution.
Power is supplied from the station bus at high voltage to the centers of
15-53
208/120-V secondary network (single unit) showing voltages.
Voltage Drops In ac distribution systems the voltage drop from
transformer to consumer in lighting mains should not exceed 2 percent
in first-class systems, so that the lamps along the mains can all operate
at nearly the same voltage and the annoying flicker of lamps may not
occur with the switching of appliances. This may require a much larger
conductor than the most economical size. In transmission lines and in
feeders where there are no intermediate loads and where means of regulating the voltage are provided, the drop is not limited to the low values
that are necessary with mains and the matter of economy may be given
consideration.
WIRING CALCULATIONS
These calculations can be used for dc, and for ac if the reactance can be
neglected. The determination of the proper size of conductor is influenced by a number of factors. Except for short distances, the minimum size of conductor shown in Table 15.1.21, which is based on the
maximum permissible current for each type of insulation, cannot be
used; the size of conductor must be larger so that the voltage drop IR
shall not be too great. With branch circuits supplying an incandescentlamp load, this drop should not be more than a small percentage of the
voltage between wires. The National Electrical Code 1993 requires that
conductors for feeders, i.e., from the service equipment to the final
branch circuit overcurrent device, be sized to prevent (1) a voltage drop
of more than 3 percent at the farthest outlet of power, heating, and
lighting loads or combinations thereof, and (2) a maximum voltage drop
on combined feeders and branch circuits to the farthest outlet of more
than 5 percent.
The resistance of 1 cir mil ⭈ ft of commercial copper may be taken as
10.8 ⍀. The resistance of a copper conductor may be expressed as R ⫽
10.8l/A, where l ⫽ length, ft and A ⫽ area, cir mils. If the length is
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ELECTRICAL ENGINEERING
expressed in terms of the transmission distance d (since the two wires
are usually run parallel), the voltage drop IR to the end of the circuit is
e ⫽ 21.6Id/A
I ⫽ (P ⫻ 1,000)/(E ⫻ pf )
(15.1.128)
and the size of conductor in circular mils necessary to give the permissible voltage drop e is
A ⫽ 21.6Id/e
(15.1.131) gives the value of current in a single-phase circuit. See also
Table 15.1.25.
(15.1.129)
(15.1.131)
where I ⫽ current, A; P ⫽ kW; E ⫽ load voltage; and pf ⫽ power factor
of the load. The size of conductor is then determined by substituting this
value of I in Eq. (15.1.129) or (15.1.130).
For three-phase three-wire ac circuits the current per wire
If e is expressed as a percentage x of the voltage E between conductors,
then
I ⫽ 1,000P/√3Epf ⫽ 580P/Epf
A ⫽ 2,160Id/xE
Computations are usually made of voltage drop per wire (see Fig.
15.1.94). Hence, if reactance can be neglected, the conductor cross
section in cir mils is one-half that given by Eq. (15.1.129). That is,
(15.1.130)
EXAMPLE. Determine the size of conductor to supply power to a 10-hp, 220V dc motor 500 ft from the switchboard with 5 V drop. Assume a motor efficiency
of 86 percent. The motor will then require a current of (10 ⫻ 746)/(0.86 ⫻ 220) ⫽
39.4 A. From Eq. (15.1.129), A ⫽ 21.6 ⫻ 39.4 ⫻ 500/5 ⫽ 85,100 cir mils. The
next largest wire is no. 0 AWG.
The calculation of the size conductor for three-wire circuits is made
in practically the same manner. With a balanced circuit there is no
current in the neutral wire, and the current in each outside wire will be
equal to one-half the sum of the currents taken by all the receiving
devices connected between neutral and outside wires plus the sum of the
currents taken by the receivers connected between the outside wires.
Using this total current and neglecting the neutral wire, make calculations for the size of the outside wires by means of Eq. (15.1.129). The
neutral wire should have the same cross section as the outside wires in
interior wiring.
EXAMPLE. Determine the size wire which should be used for the three-wire
main of Fig. 15.1.93. Allowable drop is 3 V and the distance to the load center 40
ft; circuit loaded with two groups of receivers each taking 60 A connected between the neutral and the outside wires, and one group of receivers taking 20 A
connected across the outside wires.
Fig. 15.1.93 Three-wire 230/115-V main.
Solution: load ⫽ (60 ⫹ 60)/2 ⫹ 20 ⫽ 80 A. Substituting in Eq. (15.1.129), cir
mils ⫽ 21.6Id/e ⫽ 21.6 ⫻ 80 ⫻ 40/3 ⫽ 23,030 cir mils.
From Tables 15.1.19 and 15.1.21, no. 6 wire, which has a cross section of
26,250 cir mils, is the next size larger. This size of wire would satisfy the voltagedrop requirements, but rubber-insulated no. 6 has a safe carrying capacity of but
55 A. The current in the circuit is 80 A. Therefore, rubber-insulated wire no. 3,
which has a carrying capacity of 80 A, should be used. The neutral wire should be
the same size as the outside wires.
See also examples in the National Electrical Code 1993.
Wiring calculations for ac circuits require some consideration of power
factor, reactance, and skin effect. Skin effect becomes pronounced only
when very large conductors are used for alternating current. For interior
wiring, conductors larger than 700,000 cir mils should not be used, and
many prefer not to use conductors larger than 300,000 cir mils. Should
the required copper cross section exceed these values, a number of
conductors may be operated in parallel.
For voltages under 5,000 the effect of line capacitance may be neglected. With ordinary single-phase interior wiring, where the effect of
the line reactance may be neglected and where the power factor of the
load (incandescent lamps) is nearly 100 percent, the calculations are
made the same as for dc circuits. Three-wire ac circuits of ordinary
length with incandescent lamp loads are also determined in the same
manner. When the load is other than incandescent lamps, it is necessary
to know the power factor of the load in order to make calculations.
When the exact power factor cannot be accurately determined, the following approximate values may be used: incandescent lamps, 0.95 to
1.00; lamps and motors, 0.75 to 0.85; motors 0.5 to 0.80. Equation
A ⫽ 10.8Id/e
cir mils
(15.1.132)
(15.1.133)
where e in Eq. (15.1.133) is the voltage drop per wire. The voltage drop
between any two wires is √3e. The percent voltage drop should be in
terms of the voltage to neutral. That is, percent drop ⫽ [e/(E/√3)]100 ⫽
[√3e/E]100 (see Fig. 15.1.82).
Fig. 15.1.94
Three-phase lamp and induction motor load.
EXAMPLE. In Fig. 15.1.94, load 10 kW; voltage of circuit 230; power factor
0.85; distance 360 ft; allowable drop per wire 4 V. Substituting in Eq. (15.1.132)
I ⫽ (580 ⫻ 10)/(230 ⫻ 0.85) ⫽ 29.7 A. Substituting in Eq. (15.1.133), A ⫽
10.8 ⫻ 29.7 ⫻ 360/4 ⫽ 28,900 cir mils.
The next larger commercially available standard-size wire (see Table 15.1.19)
is 41,700 cir mils corresponding to AWG no. 4. From Table 15.1.21 this will carry
70 A with rubber insulation, and is therefore ample in section for 29.7 A. Three no.
4 wires would be used for this circuit.
From Table 15.1.19 the resistance of 1,000 ft of no. 4 copper wire is 0.249 ⍀.
Hence, the voltage drop per conductor, e ⫽ 29.7 ⫻ (360/1,000)0.249 ⫽ 2.66 V.
Percent voltage drop ⫽ √3 ⫻ 2.66/230 ⫽ 2.00 percent.
Where all the wires of a circuit, two wires for a single-phase circuit,
four wires for a four-phase circuit (see Fig. 15.1.32 and Fig. 15.1.80c),
and three wires for a three-phase circuit, are carried in the same conduit
or where the wires are separated less than 1 in between centers, the
effect of line (inductive) reactance may ordinarily be neglected. Where
circuit conductors are large and widely separated from one another and
the circuits are long, the inductive reactance may increase the voltage
drop by a considerable amount over that due to resistance alone. Such
problems are treated using IR and IX phasors. Line reactance decreases
somewhat as the size of wire increases and decreases as the distance
between wires decreases.
Fig. 15.1.95
Single-phase induction motor load on branch circuit.
EXAMPLE. Determine the size of wire necessary for the branch to the 50-hp,
60-Hz, 250-V single-phase induction motor of Fig. 15.1.95. The name-plate rating
of the motor is 195 A, and its full-load power factor is 0.85. The wires are run open
and separated 4 in; length of circuit, 600 ft. Assume the line drop must not exceed
7 percent, or 0.07 ⫻ 250 ⫽ 17.5 V. The point made by this example is emphasized
by the assumption of an outsize motor.
Solution. To ascertain approximately the size of conductor, substitute in Eq.
(15.1.129) giving cir mils ⫽ 21.6 ⫻ 195 ⫻ 600/17.5 ⫽ 144,400. Referring to
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INTERIOR WIRING
Table 15.1.19, the next larger size wire is no. 000 or 167,800 cir mils. This size
would be ample if there were no line reactance. In order to allow for reactance
drop, a larger conductor is selected and the corresponding voltage drop determined. Inasmuch as this is a motor branch, the code rules require that the carrying
capacity be sufficient for a 25 percent overload. Therefore the conductor should be
capable of carrying 195 ⫻ 1.25 ⫽ 244 A. From Table 15.1.21, a 350,000-cir mil
conductor rubber-insulated cable would be required to carry 244 A. Resistance
drop (see Table 15.1.19), IR ⫽ 195 ⫻ 0.061 ⫻ 0.6 ⫽ 7.14 V. 7.14/250 ⫽ 2.86
percent. From Table 15.1.19, X ⫽ 0.128 ⫻ 0.6 ⫽ 0.0768 ⍀. IX ⫽ 195 ⫻ 0.768 ⫽
14.98 V. 14.98/250 ⫽ 5.99 percent. Using the Mershon diagram (Fig. 15.1.84),
follow the ordinate corresponding to power factor, 0.85, until it intersects the
smallest circle. From this point, lay off horizontally the percentage resistance
drop, 2.86. From this last point, lay off vertically the percentage reactance drop
5.99. This last point lies about on the 6.0 percent circle, showing that with 195
amp the difference between the sending-end and receiving-end voltages is 0.06 ⫻
250 ⫽ 15.0 V, which is within the specified limits.
Also Eq. (15.1.127) may be used. cos ␪ ⫽ 0.85; sin ␪ ⫽ 0.527.
Es ⫽ {[(250 ⫻ 0.85) ⫹ 7.14]2 ⫹ [(250
⫻ 0.527) ⫹ 14.98]2}1/2 ⫽ 264.3 V
264.3/250 ⫽ 105.7 percent
In the calculation of three-phase three-wire circuits where line reactance must be considered, the method found above under Power Transmission may be used. The system is considered as being three singlephase systems having a ground return the resistance and inductance of
which are zero, and the voltages are equal to the line voltages divided by
√3. When the three conductors are spaced unequally, the value of GMD
given in Fig. 15.1.85 should be used in Tables 15.1.16 and 15.1.19.
(When the value of resistance or reactance per 1,000 ft of conductor is
desired, the values in Table 15.1.19 should be divided by 2.)
The National Electrical Code of 1993 specifies that the size of conductors for branch circuits should be such that the voltage drop will not
exceed 3 percent to the farthest outlet for power, heating, lighting, or
combination thereof, requiring further that the total voltage drop for
feeders and branch circuits should not exceed 5 percent overall. For
examples of calculations for interior wiring, see National Electrical
Code of 1993 (Chap. 9).
INTERIOR WIRING
Interior wiring requirements are based, for the most part, on the National
Electrical Code (NEC), which has been adopted by the National Fire
Protection Association, American National Standards Institute (ANSI),
and the Occupational Safety and Health Act (OSHA).
The Occupational Safety and Health Act of 1970 (OSHA) made the
National Electrical Code a national standard. Conformance with the
NEC became a requirement in most commercial, industrial, agricultural,
etc., establishments in the United States. Some localities may not accept
NEC standards. In those cases, local rules must be followed.
NEC authority starts at the point where the connections are made to
the conductor of the service drop (overhead) or lateral (underground)
from the electricity supply system. The service equipment must have a
rating not less than the load to be carried (computed according to NEC
methods). Service equipment is defined as the necessary equipment,
such as circuit breakers or fused switches and accompanying accessories. This equipment must be located near the point of entrance of supply
conductors to a building or other structure or an otherwise defined area.
Service equipment is intended to be the main control and means of
cutoff of the supply.
Service-entrance conductors connect the electricity supply to the service equipment. Service-entrance conductors running along the exterior
or entering a building or other structure may be installed (1) as separate
conductors, (2) in approved cables, (3) as cable bus, or (4) enclosed in
rigid conduit. Also, for voltages less than 600 V, the conductors may be
installed in electrical metallic tubing, wireways, auxiliary gutters, or
busways. Service-entrance cables which are exposed to physical damage from awnings, swinging signs, coal chutes, etc., must be of the
protected type or be protected by conduit, electrical metallic tubing, etc.
Service heads must be raintight. Thermoplastic or rubber insulation is
15-55
required in overhead services. A grounded conductor may be bare. If
exposed to the weather or embedded in masonry, raceways must be
raintight and arranged to drain. Underground service raceway or duct
entering from an underground distribution system must be sealed with a
suitable compound (spare ducts, also).
NEC rules permit multiple service to a building for various reasons,
such as: (1) fire pumps, (2) emergency light and power, (3) multiple
occupancy, (4) when the calculated load is greater than 3,000 A, (5)
when the building extends over a large area, and (6) where different
voltages, frequencies, number of phases, or classes of use are required.
Ordinary service drops (overhead) and lateral (underground) must be
large enough to carry the load but not smaller than no. 8 copper or no. 6
aluminum. As an exception, for installations to supply only limited
loads of a single branch circuit, such as small polyphase power, etc.,
service drops must not be smaller than no. 12 hard-drawn copper or
equivalent, and service laterals must be not smaller than no. 12 copper
or no. 10 aluminum.
The phrase large enough to carry the load requires elaboration. The
various conductors of public-utility electric-supply systems are sized
according to the calculations and decisions of the personnel of the specific public utility supplying the service drop or lateral. At the load end
of the drop or lateral, the NEC rules apply, and from that point on into
the consumer’s premises, NEC rules are the governing authority. There
is a discontinuity at this point in the calculation of combined load demand for electricity and allowable current (ampacity) of conductors,
cables, etc. This discontinuity in calculations results from the fact that
the utility company operates locally, whereas the NEC is a set of national standards and therefore cannot readily allow for regional differences in electrical coincident demand, ambient temperature, etc. The
NEC’s aim is the assurance of an electrically ‘‘safe’’ human environment. This will be fostered by following the NEC rules.
Service-entrance cables are conductor assemblies which bear the type
codes SE (for overhead services) and USE (for underground services).
Under specified conditions, these cables may also be used for interior
feeder and branch-circuit wiring.
The service-entrance equipment must have the capability of safely interrupting the current resulting from a short circuit at its terminals.
Available short-circuit current is the term given to the maximum current
that the power system can deliver through a given circuit to any negligible-impedance short circuit applied at a given point. (This value can be
in terms of symmetrical or asymmetrical, momentary or clearing
current, as specified.)
In most instances, the available short-circuit current is limited by the
impedance of the last transformer in the supply system. Large power
users, however, must become aware of changes in the electricity supply
system which, because of growth of system capacity or any other reason, would increase the short-circuit current available to their serviceentrance equipment. If this current is too great, explosive failure can
result.
Fig. 15.1.96
Motor and wiring protection.
Kilowatthour and sometimes demand-metering equipment are connected to the service-entrance conductors. Proceeding toward the utilization equipment, the power-supply system fans out into feeders and
branch circuits (see Fig. 15.1.96). Each of the feeders, i.e., a run of
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15-56
ELECTRICAL ENGINEERING
untapped conductor or cable, is connected to the supply through a
switch and fuses or a circuit breaker. At a point, usually near that portion of the electrical loads which are to be supplied, a panel box or
perhaps a load-center assembly of switching and/or control equipment is
installed. From this panel box or load-center assembly, circuits radiate;
i.e., circuits are installed to extend into the area being served to connect
electrical machinery or devices or make available electric receptacles
connected to the source of electric power.
Each feeder and each branch circuit will have its own over-current
protection and disconnect means in the form of a fuse and switch combination or a circuit breaker.
There is a provision in the NEC 1993 rules for the following types of
feeder and branch circuit wiring:
1. Open Wiring on Insulators (NEC 1993, Art. 320). This wiring
method uses approved cleats, knobs, tubes, and flexible tubing for the
protection and support of insulated conductors run on or in buildings
and not concealed by the building structure. It is permitted only in
industrial or agricultural establishments.
2. Concealed Knob-and-Tube Work (NEC 1993, Art. 324). Concealed knob-and-tube work may be used in the hollow spaces of walls
and ceilings. It may be used only for the extension of existing facilities.
3. Flat Conductor Cable, Type FCC (NEC 1993, Art. 328). Type
FCC cable may be installed under carpet squares. It may not be used
outdoors or in wet locations, in corrosive or hazardous areas, or in
residential, school, or hospital buildings.
4. Mineral-Insulated Metal-Sheathed Cable, Type MI (NEC 1993,
Art. 330). Type MI cable contains one or more electrical conductors
insulated with a highly compressed refractory mineral insulation and
enclosed in a liquid- and gastight metallic tube sheath. Appropriate
approved fittings must be used with it. It may be used for services,
feeders, and branch circuits either exposed or concealed and dry or wet.
It may be used in Class I, II, or III hazardous locations. It may be used
for under-plaster extensions and embedded in plaster finish or brick or
other masonry. It may be used where exposed to weather or continuous
moisture, for underground runs and embedded in masonry, concrete or
fill, in buildings in the course of construction or where exposed to oil,
gasoline, or other conditions. If the environment would cause destruction of the sheath, it must be protected by suitable materials.
5. Power and Control Tray Cable (NEC 1993, Art. 340). Type TC
cable is a factory assembly of two or more insulated conductors with or
without associated bare or covered-grounding conductors under a nonmetallic sheath, approved for installation in cable trays, in raceways, or
where supported by messenger wire.
6. Metal-Clad Cable, Type MC and AC Series (NEC 1993, Art. 333
and 334). These are metal-clad cables, i.e., an assembly of insulated
conductors in a flexible metal enclosure. Type MC are power cables and
in the range up to 600 V are made in conductor sizes of no. 4 and larger
for copper and no. 2 and larger for aluminum. Type AC are branch and
feeder cables with armor of flexible metal tape. All AC types except
ACL have an internal bonding strip of copper or aluminum in intimate
contact with the armor for its entire length. Metal-clad branch circuit
cable was formerly called BX. Metal-clad cables may generally be installed where not subject to physical damage, for feeders and branch
circuits in exposed or concealed work, with qualifications for wet locations, direct burial in concrete, etc. The use of Type AC cable is prohibited (1) in motion-picture studios, (2) in theaters and assembly halls,
(3) in hazardous locations, (4) where exposed to corrosive fumes or vapors, (5) on cranes or hoists except where flexible connections to
motors, etc., are required, (6) in storage-battery rooms, (7) in hoistways
or on elevators except (i) between risers and limit switches, interlocks,
operating buttons, and similar devices in hoistways and in escalators
and moving walkways and (ii) short runs on elevator cars, where free
from oil, and if securely fastened in place, or (8) in commercial garages
in hazardous locations. Type ACL (lead-covered) shall not be used for
direct burial in the earth.
7. Nonmetallic-Sheathed Cable, Types NM and NMC (NEC 1993,
Art. 336). These are assemblies of two more insulated conductors
(nos. 14 through 2 for copper, nos. 12 through 2 for aluminum) having
an outer sheath of moisture-resistant, flame-retardant, nonmetallic ma-
terial. In addition to the insulated conductors, the cable may have an
approved size of uninsulated or bare conductor for grounding purposes
only. The outer covering of NMC cable is flame retardant and corrosion
resistant. The use of this type of cable, commonly called Nomex, is
permitted in one- or two-family dwellings, multifamily dwellings, and
other structures provided that such structures do not exceed three floors
above grade.
8. Shielded Nonmetallic-Sheathed Cable, Type SNM (NEC 1993,
Art. 337). Type SNM is a factory-assembled cable consisting of two
or more insulated conductors (nos. 14 through 2 copper and nos. 12
through 2 aluminum) in an extruded core of moisture-resistant material,
covered with an overlapping spiral metal tape and wire shield and jacketed with an extruded moisture-, flame-, corrosion-, fungus-, and sunlight-resistant material. This cable is to be used (1) under appropriate
ambient-temperature conditions and (2) in continuous rigid-cable support or in raceways. It can be used in some hazardous locations as
defined by the NEC.
9. Service Entrance Cable, Types SE and USE (NEC 1993, Art.
338). These cables, containing one or more individually insulated
conductors, are primarily used for electric services. Type SE has a
flame-retardant, moisture-resistant covering and is not required to have
built-in protection against mechanical abuse. Type USE is recognized
for use underground. It has a moisture-resistant covering, but not necessarily a flame-retardant one. Like the SE cable, USE cable is not required to have inherent protection against mechanical abuse. Under
specified conditions, SE and USE cables can be used for feeders and
branch circuits.
10. Underground Feeder and Branch-Circuit Cable, Type UF (NEC
1993, Art. 339). This cable is made in sizes 14 through 4/0, and the
insulated conductors are Types TW, RHW, and others approved for the
purpose. As in the NM cable, the UF type may contain an approved size
of uninsulated or bare conductor for grounding purposes only. The outer
jacket of this cable shall be flame-retardant, moisture-resistant, fungusresistant, corrosive-resistant, and suitable for direct burial in the ground.
11. Other Installation Practices. The NEC details rules for nonmetallic circuit extensions and underplaster extensions. It also provides
detailed rules for installation of electrical wiring in (a) rigid metal conduit (which may be used for all atmospheric conditions and locations
with due regard to corrosion protection and choice of fittings), (b) rigid
nonmetallic conduit (which is essentially corrosion-proof ), in electrical
metallic tubing (which is lighter-weight than rigid metal conduit),
(c) flexible metal conduit, (d) liquidtight flexible metal conduit, (e) surface raceways, ( f ) underfloor raceways, (g) multioutlet assemblies,
(h) cellular metal floor raceways, (i) structural raceways, ( j) cellular
concrete floor raceways, (k) wireways (sheet-metal troughs with hinged
or removable covers), (l) flat, Type FC, cable assemblies installed in a
surface metal raceway (Type FC cable contains three or four no. 10
special stranded copper wires), (m) busways, and (n) cable-bus. Busways and cable-bus installations are permitted for exposed work only.
In all installation work, only approved outlets, switch and junction
boxes, fittings, terminal strips, and dead-end caps shall be used, and
they are to be used in an approved fashion (see NEC 1993, Art. 370).
Table 15.1.20 Wire Table for Standard Annealed Copper
at 20°C in SI Units
AWG
size
Diameter,
mm
kgf / km
m/⍀
Area, mm2
14
12
10
8
6
4
2
1
0
00
000
0000
1.628
2.053
2.588
3.264
4.115
5.189
6.544
7.348
8.252
9.266
10.40
11.68
18.50
29.42
46.77
74.37
118.2
188.0
299.0
377.0
475.4
599.5
755.9
935.2
120.7
191.9
305.1
485.2
771.5
1227
1951
2460
3102
3911
4932
6219
2.08
3.31
5.261
8.367
13.30
21.15
33.62
42.41
53.49
67.43
85.01
107.2
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
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INTERIOR WIRING
Table 15.1.20 relates AWG wire sizes to metric units. Table 15.1.21
lists the allowable ampacities of copper and aluminum conductors.
Table 15.1.22 lists various conductor insulation systems approved by
the 1993 NEC for conductors used in interior wiring. Dimensions and
allowable fill of conduit and tubing are listed in Table 15.1.23. Table
15.1.24 lists the cross-sectional area of various insulated conductors.
For installations not covered by the tables presented here, review the
1993 NEC. Estimated full-load currents of motors can be taken from
Table 15.1.25.
Switching Arrangements Small quick-break switches must be set
in or on a metal box or fitting and may be of the push, tumbler, or rotary
type. The following types of switches are used to control lighting circuits: (1) single-pole, (2) double-pole, (3) three-point or three-way, (4)
four-way, in combination with three-way switches to control lights from
three or more stations, (5) electrolier.
In all metallic protecting systems, such as conduit, armored cable, or
metal raceways, joints and splices in conductors must be made only in
junction boxes or other proper fittings; therefore, these fittings can be
15-57
located only in accessible places and never concealed in partitions.
Splices or joints in the wire must never be in the conduit piping, raceway, or metallic tubing itself, for the splices may become a source of
trouble as a result of corrosion or grounding if water should enter the
conduit.
All conductors of an ac system must be placed in the same metallic
casing so that their resultant magnetic field is nearly zero. If this is not
done, eddy currents are set up causing heating and excessive loss. With
single conductors in a casing, an excessive reactance drop may result.
Service wires are the conductors that bring the electric power into a
building and should enter the building as near as possible to the service
switch, so that when the switch is open all the electrical conductors and
equipment inside the building will be dead. The service wires must be
rubber- or thermoplastic-covered from the point of support on the outside of the building to the service switch or cutout and must be no. 6
wire or larger except for installations consisting of two-wire branch
circuits where no. 8 wire may be permitted. A minimum of 100-A
three-wire service is recommended for all single-family residences.
Table 15.1.21 Allowable Ampacities of Insulated Conductors
(Not more than three conductors in conduit. Based on ambient air temperature of 40°C.)
Temperature rating of insulation, °C
Copper
Conductor
size:
AWG or
MCM
14
12
10
8
Aluminum
60
75
90
60
75
90
Types TW, UF
Types RH, RHW,
THW, THWN,
XHHW, USE, ZW
Types SA,
AVB, FEP,
FEPB, THHN,
RHH, XHHW *
Types TW, UF
Types RH, RHW
THW, THWN,
XHHW, USE
Types SA, AVB,
THHN, RHH,
XHHW *
18†
23†
29†
36
22†
28†
37†
48
25†
32†
42†
55
—
18†
23†
28
—
22†
29†
37
—
26†
34†
43
6
4
3
2
1
50
65
76
87
104
64
83
98
112
134
75
97
114
130
156
37
50
59
68
81
50
65
76
87
104
58
76
89
102
122
0
00
000
0000
119
135
160
184
153
175
207
238
179
204
242
278
93
106
125
144
119
137
162
186
139
159
189
217
250
300
350
400
500
210
232
254
274
314
271
300
328
354
407
317
351
384
415
477
165
183
201
218
252
213
236
259
281
326
249
276
303
329
381
600
700
750
800
900
1,000
345
376
392
403
426
499
448
489
509
524
555
585
525
574
598
616
653
689
280
308
322
334
357
380
362
399
417
432
463
493
424
467
488
506
542
578
Ambient
temp., °C
21 – 35
26 – 30
31 – 35
36 – 40
41 – 45
46 – 50
51 – 55
56 – 60
61 – 70
71 – 80
For ambient temperatures other than 40°C, multiply the ampacities shown above by the appropriate factors shown below.
1.32
1.22
1.12
1.00
0.87
0.71
0.50
—
—
—
1.20
1.13
1.07
1.00
0.93
0.85
0.76
0.65
0.38
—
1.14
1.10
1.05
1.00
0.95
0.89
0.84
0.77
0.63
0.45
1.32
1.22
1.12
1.00
0.87
0.71
0.50
—
—
—
1.20
1.13
1.07
1.00
0.93
0.85
0.76
0.65
0.38
—
1.14
1.10
1.05
1.00
0.95
0.89
0.84
0.77
0.63
0.45
* For dry locations only, rated 75°C for wet locations.
† Overcurrent protection shall not exceed 15 A for no. 14 copper and no. 12 aluminum, 20 A for no. 12 copper, 25 A for no. 10 aluminum, and 30 A for no. 10 copper.
NOTE: This is a general table. For other installation conditions, see NFPA 70-1994 National Electric Code ®, Article 310.
SOURCE: NEC ® 1993, Table 310-16. Reprinted with permission from NFPA 70-1984, National Electrical Code ®, Copyright © 1992, National Fire Protection Association, Quincy, Massachusetts
02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.
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15-58
ELECTRICAL ENGINEERING
Table 15.1.22
Type letter
Conductor Type and Application
Insulation, trade name
(see NEC 1984, Art. 310 for complete information)
Environment
Outer
covering a
Max operating temperature ⫽ 60°C (140°F )
TW
TF
TFF
MTW
UF
Moisture-resistant thermoplastic
Thermoplastic-covered, solid or 7-strand
Thermoplastic-covered, flexible stranding
Moisture-, heat-, and oil-resistant thermoplastic machinetool wiring ( NFPA Stand. 79, NEC 1975, Art. 670)
Moisture-resistant, underground feeder
RH
RHW
THW
THWN
XHHW
RFH-1 and 2
FFH-1 and 2
UF
USE
ZW
Heat-resistant rubber
Moisture- and heat-resistant rubber e
Moisture- and heat-resistant thermoplastic
Moisture- and heat-resistant thermoplastic
Moisture- and heat-resistant cross-linked polymer
Heat-resistant rubber-covered solid or 7-strand
Heat-resistant rubber-covered flexible stranding
Moisture-resistant and heat-resistant
Heat- and moisture-resistant
Modified ethylene tetrafluoroethylene
MI
Mineral-insulated (metal-sheathed )
Dry and wet
Wet
None
None
None
None or nylon
Dry and wet
None
Dry
Dry and wet
Dry and wet
Dry and wet
Wet
Dry and wet
Dry and wet
Wet
1,2
1,2
None
Nylon
None
None
None
None
4
None
Dry and wet
Copper
Dry
Dry
f
Dry
Dry
Dry
Dry
1,2
Nylon
None
None
None
3
Nylon
Nylon
None or nylon
Dry
Asbestos or glass
Dry
None
Dry
None
3
None or glass braid
None
Nonmetallic
c,d
c,d
Max operating temperature ⫽ 75°C (167°F )
b–d
b–d
Max operating temperature ⫽ 85°C (185°F )
Max operating temperature ⫽ 90°C (194°F )
RHH
THHN
THW
XHHW
FEP
FEPB
TFN
TFFN
MTW
SA
Heat-resistant rubber
Heat-resistant thermoplastic
Moisture- and heat-resistant thermoplastic
Moisture- and heat-resistant cross-linked synthetic polymer
Fluorinated ethylene propylene
Fluorinated ethylene propylene
Heat-resistant thermoplastic covered, solid or 7-strand
Heat-resistant thermplastic flexible stranding
Moisture-, heat-, and oil-resistant thermoplastic machinetool wiring (NFPA Stand. 79, NEC 1975, Art. 670)
Silicone asbestos
c,d
Max operating temperature ⫽ 150°C (302°F )
Z, ZW
Modified ethylene tetrafluoroethylene
Max operating temperature ⫽ 200°C (392°F )
FEP, FEPB
PF, PGF
PFA
SF-2
Fluorinated ethylene propylene
Special applications
Fluorinated ethylene propylene
Perfluoroalkoxy
Silicone rubber, solid or 7-strand
c,d
Dry
c,d
Max operating temperature ⫽ 250°C (482°F )
MI
TFE
PFAH
PTF
Mineral-insulated (metal-sheathed), for special applications
Extruded polytetrafluoroethylene, only for leads within apparatus or within raceways connected to apparatus, or as
open wiring (silver or nickel-coated copper only)
Perfluoroalkoxy (special application)
Extruded polytetrafluoroethylene, solid or 7-strand (silver
or nickel-coated copper only)
Dry and wet
Dry
Copper
None
Dry
None
None
c,d
a 1: Moisture-resistant, flame-retardant nonmetallic; 2: outer covering not required when rubber insulation has been specifically approved for the
purpose; 3: no. 14-8 glass braid, no. 6-2 asbestos braid; 4: moisture-resistant nonmetallic.
b Limited to 300 V.
c No. 18 and no. 16 conductor for remote controls, low-energy power, low-voltage power, and signal circuits; NEC 1975 Sec. 725-16, Sec. 760-16.
d Fixture wire no. 18-16.
e For over 2,000 V, the insulation shall be ozone-resistant.
f Special applications within electric discharge lighting equipment. Limited to 1,000 V open-circuit volts or less.
SOURCE: NEC 1993 Tables 310-13 and 402-3. Reprinted with permission from NFPA 70-1993, National Electrical Code®, Copyright © 1992,
National Fire Protection Association, Quincy, Massachusetts 02269. This reprinted material is not the complete and official position of the NFPA on
the referenced subject, which is represented only by the standard in its entirety.
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
INTERIOR WIRING
Table 15.1.23
Tubing
Dimensions and Allowable Fill of Conduit and
Allowable fill, in2 of conductors
(not lead-covered)
Trade
size, in
⁄
3⁄4
12
1
11⁄4
11⁄2
2
21⁄2
3
31⁄2
4
41⁄2
5
6
ID, in
Area, in2
One
conductor,
53% fill
0.622
0.824
1.049
1.380
1.610
2.067
2.469
3.068
3.548
4.026
4.506
5.047
6.065
0.30
0.53
0.86
1.50
2.04
3.36
4.79
7.38
9.90
12.72
15.94
20.00
28.89
0.16
0.28
0.46
0.80
1.08
1.78
2.54
3.91
5.25
6.74
8.45
10.60
15.31
Two
conductors,
31% fill
Over two
conductors,*
40% fill
0.09
0.16
0.27
0.47
0.63
1.04
1.43
2.29
3.07
3.94
4.94
6.20
8.96
0.12
0.21
0.34
0.60
0.82
1.34
1.92
2.95
3.96
5.09
6.38
8.00
11.56
* For conductor derating with more than three conductors see NEC 1993, Art. 310.
SOURCE: NEC 1993, p. 70-824. Reprinted with permission from NFPA 70-1993, National
Electrical Code®, Copyright © 1992, National Fire Protection Association, Quincy, Massachusetts 02269. This reprinted material is not the complete and official position of the NFPA on the
referenced subject, which is represented only by the standard in its entirety.
Table 15.1.24
15-59
Generally, when the conductors from overhead lines enter a building,
the wires are encased in rigid conduit equipped with a weather cap or a
service entrance cable (type ASE armored or SE type, unarmored) may
be attached directly to the building wall. The inner end of the service
enters a metal service cabinet in which the service fuses and switch are
located. Service conductors may also terminate at an air-break or oilimmersed switch in a metal case or on a panel board which is accessible
to qualified persons only.
All underground service wires must be connected to the interior wiring through a blade of the service switch or circuit breaker and be fused
or automatically interrupted at the service switch. A service switch
controlling a three-wire dc, a single-phase or four-wire three-phase
system having a grounded neutral wire does not need to open that conductor.
The single-line diagram, Fig. 15.1.96, indicates a simplified interior
arrangement of circuits and the necessary protection of the conductors
and terminal load. Where a reduction is made in the wire size a protective device shall be installed to limit the conductor current to a safe
value. Large motors and other terminal loads should also have overcurrent protection.
The maximum permissible fuse ratings and the setting of the protective devices for starting and for running protection of motors are given
in Table 15.1.26.
Grounding of direct- and alternating-current systems of 300 V and
Nominal Cross-Sectional Area of Rubber-Covered and Plastic-Covered Conductors
Size, AWG
18
16
12
10
8
Cross-sectional area, in2
Type
RFH-2
SF-2
RH
RHH, RHW
RHH, RHW (without outer covering)
THW
TW, RUH
TF
THWN, THHN
TFN
FEPB, Z, ZF, ZFF
FEP, TFE
PTF
XHHW
14
0.0167
0.0167
—
—
—
—
—
0.0088
—
0.0064
—
—
0.0052
—
0.0196
0.0196
—
—
—
—
—
0.0109
—
0.0079
—
—
0.0066
—
6
4
(fixture wire)
0.0230
(fixture wire)
0.0230
0.0278
0.0460
0.0327
0.0384
0.0460
0.0206
0.0251
0.0311
0.0206
0.0251
0.0311
0.0135
0.0172
0.0224
—
—
—
0.0087
0.0117
0.0184
—
—
—
0.0087
0.0115
0.0159
0.0087
0.0115
0.0159
0.0087
—
—
0.0131
0.0167
0.0216
0.0854
0.0854
0.0598
0.0598
0.0471
—
0.0373
—
0.0272
0.0333
—
0.0456
Size, AWG
3
2
1
1/0
2/0
3/0
4/0
0.2715
—
0.2027
0.1385
—
—
0.1590
0.1590
0.1385
0.3107
—
0.2367
0.1676
—
—
0.1893
0.1893
0.1676
0.3578
—
0.2781
0.1974
—
—
0.2265
0.2265
0.1948
0.4151
—
0.3288
0.2436
—
—
0.2715
0.2715
0.2463
0.4840
—
0.3904
0.2999
—
—
0.3278
0.3278
0.3000
1500
2000
2.5475
2.2748
—
2.0612
3.2079
2.9013
—
2.6590
Cross-sectional area,
RH, RHH,* RHW*
RUH
TW, THW
TFE
FEPB, ZF, ZFF
FEP
THHN, THWN
XHHW
Z
0.1238
0.0819
0.0819
0.0467
0.0716
0.0467
0.0519
0.0625
0.0716
0.1605
0.1087
0.1087
0.0669
0.0962
0.0669
0.0845
0.0845
0.0962
0.1817
0.1263
0.1263
0.0803
0.1122
0.0803
0.0995
0.0995
0.1122
250
500
750
0.2067
0.1473
0.1473
0.0973
0.1316
0.0973
0.1182
0.1182
0.1320
in2
Size, MCM
1000
Cross-sectional area,
RH, RHH,* RHW*
TW, T, THW
THHN, THWN
XHHW
0.5917
0.4877
0.4026
0.4026
0.9834
0.8316
0.7163
0.7163
1.4082
1.2252
1.0623
1.0936
in2
1.7531
1.5482
1.3623
1.3893
* RHH and RHW without covering have the same dimension as THW.
SOURCE: NEC 1993, pp. 70-825 and 826. Reprinted with permission from NFPA 70-1993, National Electrical Code®, Copyright © 1992 National Fire Protection Association, Quincy,
Massachusetts 02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.
NOTE: For general branch and feeder circuits the minimum conductor size is 14. Sizes 14 to 8 are solid wire. Sizes 6 and larger are stranded.
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
15-60
ELECTRICAL ENGINEERING
Table 15.1.25 Approximate Full-Load Currents of Motors,* A
(See NEC 1993 for more complete information.)
Three-phase ac motors
squirrel-cage and woundrotor induction types
hp
230 V
460 V
575 V
12
⁄
3⁄ 4
1
11⁄2
2
3
2
2.8
3.6
5.2
6.8
9.6
1
1.4
1.8
2.6
3.4
4.8
5
71⁄2
10
15
20
25
15.2
22
28
42
54
68
7.6
11
14
21
27
34
Synchronous type,
unity power factor
Single-phase
ac motors
DC motors†
2,300 V
230 V
460 V
575 V
2,300 V
115 V ‡
230 V ‡
120 V
240 V
0.8
1.1
1.4
2.1
2.7
3.9
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
9.8
13.8
16
20
24
34
4.9
6.9
8
10
12
17
5.4
7.6
9.5
13.2
17
25
2.7
3.8
4.7
6.6
8.5
12.2
6.1
9
11
17
22
27
—
—
—
—
—
—
—
—
—
—
—
53
—
—
—
—
—
26
—
—
—
—
—
21
—
—
—
—
—
—
56
80
100
—
—
—
28
40
50
—
—
—
40
58
76
—
—
—
20
29
38
55
72
89
30
40
50
60
75
100
80
104
130
154
192
248
40
52
65
77
96
124
32
41
52
62
77
99
—
—
—
16
20
26
63
83
104
123
155
202
32
41
52
61
78
101
26
33
42
49
62
81
—
—
—
12
15
20
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
106
140
173
206
255
341
125
150
200
312
360
480
156
180
240
125
144
192
31
37
49
253
302
400
126
151
201
101
121
161
25
30
40
—
—
—
—
—
—
—
—
—
425
506
675
* The values of current are for motors running at speeds customary for belted motors and motors having normal torque characteristics. Use name-plate data for low-speed, high-torque, or multispeed
motors. For synchronous motors of 0.8 pf multiply the above amperes by 1.25, at 0.9 pf by 1.1. The motor voltages listed are rated voltages. Respective nominal system voltages would be 220 to 240,
440 to 480, and 550 to 600 V. For full-load currents of 208-V motors, multiply the above amperes by 1.10; for 200-V motors, multiply by 1.15.
† Ampere values are for motors running at base speed.
‡ Rated voltage. Nominal system voltages are 120 and 240.
Table 15.1.26
Maximum Rating or Setting of Motor Branch-Circuit Protective Devices and Starting-Inrush Code Letters
Percent of full-load current
Non-time
delay fuse
Dualelement
(time delay)
fuse
Instantaneous
breaker
Time-limit
breaker
Code
letter
kVA / hp with
locked rotor
300
175
700
250
300
300
250
150
175
175
175
150
700
700
700
700
250
250
200
150
250
175
700
200
200
250
200
150
175
175
175
150
700
700
700
700
200
200
200
150
High reactance squirrel-cage:
Not more than 30 A
No code letter
More than 30 A
No code letter
A
B
C
D
E
F
G
H
J
K
L
M
N
P
R
S
T
U
V
0 – 3.14
3.15 – 3.54
3.55 – 3.99
4.0 – 4.49
4.5 – 4.99
5.0 – 5.59
5.6 – 6.29
6.3 – 7.09
7.1 – 7.99
8.0 – 8.99
9.0 – 9.99
10.0 – 11.19
11.2 – 12.49
12.5 – 13.99
14.0 – 15.99
16.0 – 17.99
18.0 – 19.99
20.0 – 22.39
22.4 and up
250
175
700
250
200
175
700
200
Wound rotor, no code letter
150
150
700
150
DC motors:
No more than 50 hp
No code letter
More than 50 hp
No code letter
150
150
250
150
150
150
175
150
200
200
200
200
Type of motor
Single-phase, all types, no code letter
AC motors: single-phase, polyphase squirrel-cage, or
synchronous with full-voltage, resistor, or reactor
starting:
No code letter
F–V
B–E
A
AC squirrel-cage or synchronous motors with autotransformer starting:
Not more than 30 A
No code letter
More than 30 A
No code letter
F–V
B–E
A
Low-torque, low-speed (450 r /min or lower) synchronous motors which start unloaded
SOURCE: NEC 1993, Tables 430-152 and 430-7(b). Reprinted with permission from NFPA 70-1993, National Electrical Code®, Copyright © 1992, National Fire Protection Association, Quincy,
Massachusetts 02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
RESISTOR MATERIALS
less is usually required. Inside a building the grounded conductor (one of
the two conductors in two-wire system and the neutral conductor in a
three-wire or a four-wire system) should have a white or natural gray
covering throughout to distinguish it from the ungrounded conductors.
This identified grounded conductor must not be fused or be opened
unless the other conductors are opened simultaneously. Green wires only
shall be used for grounding electrical equipment, such as motors, as
well as conduits, armor, boxes, and such metallic enclosures. Four-wire
circuits have black, white, red, and blue conductors or alphanumeric
identification.
DC systems need be grounded only at the generating stations because
the grounded wire is electrically connected to one of the conductors in
all the circuits throughout the system. In ac systems, since one section
can be insulated from the other by a transformer, each section of 300 V
or under is grounded at the individual services. The conductor grounding the ac system should not be less than no. 8 copper wire and must be
without a joint or a splice and run from the supply side of the service
switch.
The service conduit that protects the service wires on the outside of
the building must be grounded by a wire at least as large as no. 8, run
directly to ground.
The entire metallic system surrounding the conductors must be at
ground potential. It is only necessary to ground the metallic system,
including motors and other equipment, at one point, provided that each
section makes a good electrical connection with the next.
Since January 1, 1973, ground-fault circuit interrupters have been required by the NEC in some areas for personnel protection from line-toground electrical shock. Such circuit interrupters are required by the
1984 NEC in branch circuits supplying certain areas in residences,
hotels and motels, health-care facilities, marinas and boat yards, mobile
homes and recreational vehicles, swimming pools, and construction
sites. The 1984 or subsequent code should be carefully reviewed for
exact requirements.
Ground-fault circuit protection may be used at other locations and, if
so used, will provide additional protection against line-to-ground shock
hazard.
Ground fault-circuit interrupters monitor the current in the two conductors of a circuit. These two currents should be equal. If they are not
equal, some current is leaking to ground, indicating a line to ground
fault. If the difference between the two currents is 5 mA or more, the
ground-fault circuit interrupter will automatically disconnect the faulted
circuit in about 0.025 s.
Overload Protection A fuse or circuit breaker must be provided in
all ungrounded conductors. Induction motors are usually protected by
overload or thermal relays operating as automatic circuit breakers. To
protect wiring properly, an automatic cutout must be installed at every
Table 15.1.27
15-61
point where a change is made in the size of the wire. Fuses or circuit
breakers should not be placed either in a grounded line or in a ground
wire. (See Fig. 15.1.96.)
Fuses, cutout bases, and switches are manufactured and change sizes, as
follows: Edison plug (125 V only), 0 to 30 A; spring-clip cartridge
(ferrule contact), 0 to 30, 31 to 60 A; knife-blade cartridge type, 61 to
100, 101 to 200, 201 to 400, 401 to 600 A; for 601 A and larger,
knife-blade cartridge type with equal-size fuses in parallel may be used
except for the protection of a branch motor circuit where a circuit
breaker can be installed.
Since the rating of a fuse is only about 90 percent of the current that it
will carry indefinitely, and since it may also take a few minutes before
the heat due to slightly excessive current would be sufficient to melt the
fuse wire and hence open the circuit, insulation may be permanently
damaged if the fuses are larger than the current-carrying capacities
given by Table 15.1.21.
Demand Calculations for Building Feeder Sizes The demand factor
or demand is the ratio of maximum demand to the total connected load.
This depends on the type of building, whether hotel, theater, factory,
etc. The demand factor for any particular class of installation decreases
as the floor area increases. Values of demand factors are found in the
1993 NEC Art. 220.
Load centers are panels or cabinets which act as distribution centers
and which are supplied by feeder or main conductors, and from which
the current to several branch circuits is taken. In each branch circuit
there is usually a small combined switch and circuit breaker. Load
centers for widely different types of circuits such as single-phase twowire, single-phase three-wire, and three-phase are readily obtainable
from several manufacturers.
RESISTOR MATERIALS
For use in rheostats, electric furnaces, ovens, heaters, and many electrical appliances, a resistor material with high melting point and high
resistivity which does not disintegrate or corrode at high temperatures is
necessary. These requirements are met by the nickel-chromium and
nickel-chromium-iron alloys. For electrical instruments and measuring
apparatus, the resistor material should have high resistivity, low temperature coefficient, and, for many uses, low thermoelectric power
against copper. The properties of resistor materials are given in Table
15.1.27. Most of these materials are available in ribbon as well as in
wire form. Cast-iron and steel wire are efficient and economical resistor
materials for many uses, such as power-absorbing rheostats and motor
starters and controllers. (See also Sec. 6.)
Advance has a low temperature coefficient and is useful in many types
of measuring instrument and precision equipment. Because of its high
Properties of Metals, Alloys, and Resistor Materials
Resistivity
Material
Composition
Sp gr
Advance
Comet
Bronze, commercial
Hytemco-Balco
Kanthal A
Cu 0.55; Ni 0.45
Ni 0.30; Cr 0.05; Fe 0.65
Cu; Zn
Ni 0.50; Fe 0.50
Al 0.055; Cr 0.22; Co
0.055; Fe 0.72
Ni 0.955; Mn 0.045
Cu 0.84; Mn 0.12; Ni 0.04
Ni 0.67; Cu 0.28
Ni 0.60; Fe 0.25; Cr 0.15
Ni 0.80; Cr 0.20
Ni 0.99
Pt
Ag
W
8.9
8.15
8.7
8.46
7.1
Magno
Manganin
Monel metal
Nichrome
Nichrome V
Nickel, pure
Platinum
Silver
Tungsten
* Tungsten subject to rapid oxidation in air above 150°C.
8.75
8.19
8.9
8.247
8.412
8.9
21.45
10.5
19.3
Ohms
cir-mil-ft
at 20°C
Temp coef
of resistance
per deg C
Temp range,
°C
Max safe
working temp,
°C
Approx
melting point,
°C
48.4
95
4.2
20
145
294
570
25
120
870
⫾ 0.00002
0.00088
0.0020
0.0045
0.00002
20 – 100
20 – 500
0 – 100
20 – 100
0 – 500
500
600
—
600
1330
1210
1480
1040
1425
1510
20
48.2
42.6
112
108
10
10.616
1.622
5.523
120
290
256
675
650
60
63.80
9.755
33.22
0.0036
⫾ 0.000015
0.0001
0.00017
0.00013
0.0050
0.003
0.00361
0.0045
20 – 100
15 – 35
0 – 100
20 – 100
20 – 100
0 – 100
0 – 100
0 – 100
—
400
100
425
930
1100
400
1200
650
*
1435
1020
1350
1350
1400
1450
1773
960
3410
Microhms-cm
at 20°C
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
this product is subject to the terms of its License Agreement. Click here to view.
15-62
ELECTRICAL ENGINEERING
thermoelectric power to copper, it is valuable for thermoelements and
pyrometers. It is noncorrosive and is used to a large extent in industrial
and radio rheostats. Hytemco is a nickel-iron alloy characterized by a
high temperature coefficient and is used advantageously where selfregulation is required as in immersion heaters and heater pads. Magno is
a manganese-nickel alloy used in the manufacture of incandescent
lamps and radio tubes. Manganin is a copper-manganese-nickel alloy
which, because of its very low temperature coefficient and its low thermal emf with respect to copper, is very valuable for high-precision
electrical measuring apparatus. It is used for the resistance units in
bridges, for shunts, multipliers, and similar measuring devices.
Nichrome V is a nickel-chromium alloy free from iron, is noncorrosive,
nonmagnetic, withstands high temperatures, and has high resistivity. It
is recommended as material for heating elements in electric furnaces,
hot-water heaters, ranges, radiant heaters, and high-grade electrical appliances. Kanthal is used for heating applications where higher operating temperatures are required than for Nichrome. Mechanically, it is
less workable than Nichrome. Platinum is used in specialized heating
applications where very high temperatures are required. Tungsten may
be used in very high temperature ovens with an inert atmosphere. Pure
nickel is used to satisfy the high requirements in the fabrication of radio
tubes, such as the elimination of all gases and impurities in the metal
parts. It also has other uses such as in incandescent lamps, for combustion boats, laboratory accessories, and resistance thermometers.
Carbon withstands high temperatures and has high resistance; its
temperature coefficient is negative; it will safely carry about 125 A/in2.
Amorphous carbon has a resistivity of 3,800 and 4,100 ␮ ⍀ ⭈ cm, retort
carbon about 720 ␮ ⍀ ⭈ cm, and graphite about 812 ␮ ⍀ ⭈ cm. The properties of any particular kind of carbon depend on the temperature at
which it was fired. Carbon for rheostats may best be used in the form of
compression rheostats. Silicon carbide is used to manufacture heating
rods that will safely operate at 1,650°C (3,000°F) surface temperature.
It has a negative coefficient of resistance up to 650°C, after which it is
positive. It must be mechanically protected because of inherent brittleness.
MAGNETS
A permanent magnet is one that retains a considerable amount of magnetism indefinitely. Permanent magnets are used in electrical instruments,
telephone receivers, loudspeakers, magnetos, tachometers, magnetic
chucks, motors, and for many purposes where a constant magnetic field
or a constant source of magnetism is desired. The magnetic material
should have high retentivity, a high remanence, and a high coercive
force (see Fig. 15.1.8). These properties are usually found with hardened steel and its alloys and also in ceramic permanent magnet materials.
Since permanent magnets must operate on the molecular mmf imparted to them when magnetized, they must necessarily operate on the
portion CDO of the hysteresis loop (see Fig. 15.1.8). The area CDO is
proportional to the stored energy within the magnet and is a criterion of
its usefulness as a permanent-magnetic material. In the left half of Fig.
15.1.97 are given the B-H characteristics of several permanent-magnetic materials; these include 5 to 6 percent tungsten steel (curve 1); 31⁄2
percent chrome magnet steel (curve 2); cobalt magnet steel, containing
16 to 36 percent cobalt and 5 to 9 percent chromium and in some alloys
tungsten (curve 3); and the carbon-free aluminum-nickel-cobalt-steel
alloys called Alnico. There are many grades of Alnico; the characteristics of three of them are shown by curves 4, 5, 6. Their composition is as
follows:
Composition, percent
Curve
Alnico
no.
Al
Ni
Co
Cu
Ti
Fe
4
5
6
5
6
12
8
8
6
14
14
18
24
24
35
3
3
. . .
. . .
1.25
8.0
51
49.75
33
All the Alnicos can be made by the sand or the precision-casting
(lost-wax) process, but the most satisfactory method is by the sintering
process.
Fig. 15.1.97 Characteristics of permanent magnet materials.
Copyright (C) 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Use of
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MAGNETS
If the alloys are held in a magnetic field during heat-treatment, a
magnet grain is established and the magnetic properties in the direction
of the field are greatly increased. The alloys are hard, can be formed
only by casting or sintering, and cannot be machined except by grinding.
The curves in the right half of Fig. 15.1.97 are ‘‘external energy’’
curves and give the product of B and H. The optimum point of operation
is at the point of maximum energy as is indicated at A1 on curve 5.
Considering curve 5, if the magnetic circuit remained closed, the
magnet would operate at point B. To utilize the flux, an air gap must be
introduced. The air gap acts as a demagnetizing force, H1 (⫽ B1 A1), and
the magnet operates at point A1 on the HB curve. The line OA1 is called
the air-gap line and its slope is given by tan ␪1 ⫽ B1 /H1 where H1 ⫽
Bg lg /l m and B1 /Bg ⫽ Ag /Am , where Bg ⫽ flux density in gap, T; l g ⫽
length of gap, m; l m ⫽ length of magnet, m; Ag , Am ⫽ areas of the gap
and magnet, m2.
If the air gap is lengthened, the magnet will operate at A2 corresponding to a lesser flux density B3 and the new air-gap line is OA2 . If the gap
is now closed to its original value, the magnet will not return to operation at point A1 but will operate at some point C on the line OA1 . If the
air gap is varied between the two foregoing values, the magnet will
operate along the minor hysteresis loop A2C. Return to point A1 can be
accomplished only by remagnetizing and coming back down the curve
from B to A1 .
Alnico magnets corresponding to curves 4 and 5 are best adapted to
operation with short air gaps, since the introduction of a long air gap
will demagnetize the magnet materially. On the other hand, a magnet
with a long air gap will operate most satisfactorily on curve 6 on account of the high coercive force H2 . With change in the length of the air
gap, the operation will be essentially along that curve and the magnet
will lose little of its original magnetization.
There are several other grades of Alnico with characteristics between
curves 4, 5, and 6. Ceramic PM materials have a very large coercive
force.
The steels for permanent magnets are cut in strips, heated to a red-hot
temperature, and forged into shape, usually in a ‘‘bulldozer.’’ If they are
to be machined, they are cooled in mica dust to prevent air hardening.
They are then ground, tumbled, and tempered. Alnico and ceramic types
are cast and then finish-ground.
Permanent magnets are magnetized either by placing them over a bus
bar carrying a large direct current, by placing them across the poles of a
powerful electromagnet, or by an ampere-turn pulse.
Unless permanent magnets are subjected to artificial aging, they
gradually weaken until after a long period they become stabilized usually at from 85 to 90 percent of their initial strength. With magnets for
electrical instruments, where a constant field strength is imperative,
artificial aging is accomplished by mechanical vibration or by immersion in oil at 250°F for a period of a few hours.
In an electromagnet the magnetic field is produced by an electric
current. The core is usually made of soft iron or mild steel because, the
permeability being higher, a stronger magnetic field may be obtained.
Also since the retentivity is low, there is little trouble due to the sticking
of armatures when the circuit is opened. Electromagnets may have the
form of simple solenoids, iron-clad solenoids, plunger electromagnets,
electromagnets with external armatures, and lifting magnets, which are
circular in form with a flat holding surface.
Table 15.1.28
15-63
A solenoid is a winding of insulated conductor and is wound helically;
the direction of winding may be either right or left. A portative electromagnet is one designed only for holding material brought in contact with
it. A tractive electromagnet is one designed to exert a force on the load
through some distance and thus do work. The range of an electromagnet
is the distance through which the plunger will perform work when the
winding is energized. For long range of operation, the plunger type of
tractive magnet is best suited, for the length of core is governed practically by the range of action desired, and the area of the core is determined by the pull. Solenoid and plunger is a solenoid provided with a
movable iron rod or bar called a plunger. When the coil is energized, the
iron rod becomes magnetized and the mutual action of the field in the
solenoid on the poles created on the plunger causes the plunger to move
within the solenoid. This force becomes zero only when the magnetic
centers of the plunger and solenoid coincide. If the load is attached to
the plunger, work will be done until the force to be overcome is equal to
the force that the solenoid exerts on the plunger. When the iron of the
plunger is not saturated, the strength of magnetic field in the solenoid
and the induced poles are both proportional to the exciting current, so
that the pull varies as the current squared. When the plunger becomes
highly saturated, the pull varies almost directly with the current.
The maximum uniform pull occurs when the end of the plunger is at
the center of the solenoid and is equal to
F ⫽ CAnI/l
lb
(15.1.134)
n ⫽ number of turns; I ⫽
where A ⫽ cross-sectional area of plunger,
current, A; l ⫽ length of the solenoid, in; and C ⫽ pull, (lb/in2)/(A-turn/
in). C depends on the proportions of the coil, the degree of saturation,
the length, and the physical and chemical purity of the plunger. Table
15.1.28 gives values of C for several different solenoids.
Curve 1, Fig. 15.1.98, shows the characteristic pull of an openmagnetic circuit solenoid, 12 in long, having 10,000 A-turns or 833
A-turns/in.
in2;
Fig. 15.1.98 Pull on solenoid with plunger. (1) Coil and plunger; (2) coil and
plunger with stop; (3) iron-clad coil and plunger; (4) and (5) same as (3) with
different lengths of stop.
When a strong pull is desired at the end of the stroke, a stop may be
used as shown in Fig. 15.1.99. Curve 2, Fig. 15.1.98, shows the pull
obtained by adding a stop to the plunger. It will be noted that, except
when the end of the plunger is near the stop, the stop adds little to the
Maximum Pull per Square Inch of Core for Solenoids with Open Magnetic Circuit
Length of
coil l, in
Length of
plunger, in
Core area
A, in2
Total
ampere-turns
I⫻n
Max pull
P, psi
6
9
9
10
10
10
Long
Long
Long
10
10
10
1.0
1.0
1.0
2.76
2.76
2.76
15,900
11,330
14,200
40,000
60,000
80,000
22.4
11.5
14.6
40.2
61.6
80.8
SOURCE: From data by Underhill, Elec. World, 45, 1906, pp. 796, 881.
1,000 ⫻ C
Length of
coil l, in
Length of
plunger, in
Core area
A, in2
Total
ampere-turns
I⫻n
Max pull
P, psi
1,000 ⫻ C
9.0
9.1
9.2
10.0
10.3
10.1
12
12
18
18
18
18
Long
Long
36
36
18
18
1.0
1.0
1.0
1.0
1.0
1.0
11,200
20,500
18,200
41,000
18,200
41,000
8.75
16.75
9.8
22.5
9.8
22.5
9.4
9.8
9.7
9.8
9.7
9.8
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15-64
ELECTRICAL ENGINEERING
solenoid pull. The pull is made up of two components: one due to the
attraction between plunger and winding, the other to the attraction between plunger and stop. The equation for the pull is
P ⫽ AIn[(In/l 2a C 21) ⫹ (C/l)]
(15.1.135)
n ⫽ number of turns; la ⫽ length of gap
where A ⫽ area of the core,
between core and stop; and C, C1 ⫽ constants. At the beginning of the
stroke the second member of the equation is predominant, and at the end
in2;
Fig. 15.1.99 Solenoid with stop.
of the stroke the first member represents practically the entire pull.
Approximate values of C and C1 are C1 ⫽ 2,660 (for l greater than 10d),
C ⫽ 0.0096, where d is the diameter of the plunger, in. In SI units
P ⫽ 1.7512AnI
冉
2.54nI
C
⫹
l 2a C 21
l
冊
N
(15.1.135a)
where A is in cm2, l and la in cm, and the pull P in N. All other quantities
are unchanged.
The range of uniform pull can be extended by the use of conical ends
of stop and plunger, as shown in Fig. 15.1.100. A stronger magnet
mechanically can be obtained by using an iron-clad solenoid, Fig.
15.1.101, in which an iron return path is provided for the flux. Except
for low flux densities and short air gaps the dimensions of the iron
Fig. 15.1.100 Conical plunger and stop.
return path are of no practical importance, and the fact that an iron
return path is used does not affect the pull curve except at short air gaps.
This is illustrated in Fig. 15.1.98 where curves 3, 4, and 5 are typical
pull curves for this same solenoid when it is made iron-clad, each curve
corresponding to a different position of the stop.
Fig. 15.1.101 Iron-clad solenoid.
Mechanical jar at the end of the stroke may be prevented by leaving
the end of the solenoid open. The plunger then comes to equilibrium
when its middle is at the middle of the winding, thus providing a magnetic cushion effect. Electromagnets with external armatures are best
adapted for short-range work, and the best type is the horseshoe magnet.
The pull for short-range magnets is expressed by the equation
F ⫽ B 2A/72,134,000
where B ⫽ flux density,
In SI units
Mx/in2,
lb
(15.1.136)
and A ⫽ area of the core, in2.
F ⫽ 397,840B 2A
N
(15.1.136a)
where the flux density B is in Wb/m2; A ⫽ area, m2; and F ⫽ force, N.
A greater holding power is obtained if the surfaces of the armature
and core are not machined to an absolutely smooth contact surface. If
the surface is slightly irregular, the area of contact A is reduced but the
flux density B is increased approximately in proportion (if the iron is
being operated below saturation) and the pull is increased since it varies
as the square of the density B. Nonmagnetic stops should be used if it is
desired that the armature may be released readily when the current is
interrupted.
Lifting magnets are of the portative type in that their function is merely
to hold the load. The actual lifting is performed by the hoisting apparatus. The magnet is almost toroidal in shape. The coil shield is of
manganese steel which is very hard and thus resists wear and is practically nonmagnetic. The holding power is given by Eq. (15.1.136),
where A ⫽ area of holding surface, in2. It is difficult to calculate accurately the holding force of a lifting magnet for it depends on the magnetic characteristics of the load, the area of contact, and the manner in
which the load is applied.
Rapid action in a magnet can be obtained by reducing the time constant of the winding and by subdividing the metal parts to reduce induced currents which have a demagnetizing effect when the circuit is
closed. The movement of the plunger through the winding causes the
winding and its bobbin to be cut by a magnetic field; if the bobbin is of
metal and not slotted longitudinally, it is a short-circuited turn linked by
a changing magnetic field and hence currents are induced in it. These
currents oppose the flux and hence reduce the pull during the transient
period. They also cause some heating. Where it is found impossible to
reduce the time constant sufficiently, an electromagnet designed for a
voltage much lower than normal is often used. A resistor is connected in
series which is short-circuited during the stroke of the plunger. At the
completion of the stroke the plunger automatically opens the short circuit, reducing the current to a value which will not overheat the magnet
under continuous operation. The extremely short time of overload produces very rapid action but does not injure the winding. The solenoids
on many automatic motor-starting panels are designed in this manner.
When slow action is desired, it can be obtained by using solid cores
and yoke and by using a heavy metallic spool or bobbin for the winding.
A separate winding short-circuited on itself is also used to some extent.
Sparking at switch terminals may be reduced or eliminated by neutralizing the inductance of the winding. This is accomplished by winding a separate short-circuited coil with its wires parallel to those of the
active winding. (This method can be used with dc magnets only.) This is
not economical, since one-half the winding space is wasted. By connecting a capacitor across the switch terminals, the energy of the inductive discharge on opening the circuit may be absorbed. For the purpose
of neutralizing the inductive discharge and causing a quick release, a
small reverse current may be sent through the coil winding automatically
on opening the circuit. Sleeves of tin, aluminum, or copper foil placed
over the various layers of the winding absorb energy when the circuit is
broken and reduce the energy dissipated at the switch terminals. This
scheme can be used for dc magnets only. Sticking of the parts of the
magnetic circuit due to residual magnetism may be prevented by the use
of nonmagnetic stops. In the case of lifting magnets subjected to rough
usage and hard blows (as in a steel works), these stops usually consist of
plates of manganese steel, which are extremely hard and nonmagnetic.
AC Tractive Magnets Because of the iron losses due to eddy
currents, the magnetic circuits of ac electromagnets should be composed of laminated iron or steel. The magnetic circuit of large magnets
is usually built up of thin sheets of sheet metal held together by means
of suitable clamps. Small cores of circular cross section usually consist
of a bundle of soft iron wires. Since the iron losses increase with the flux
density, it is not advisable to operate at as high a density as with direct
current. The current instead of being limited by the resistance of the
winding is now determined almost entirely by the inductive reactance as
the resistance is small. With the removal of the load the current rises to
high values. The pull of ac magnets is nearly constant irrespective of the
length of air gap.
In a single-phase magnet the pull varies from zero to a maximum and
back to zero twice every cycle, which may cause considerable chattering of the armature against the stop. This may be prevented by the use of
a spring or, in the case of a solenoid coil, by allowing the plunger to seek
its position of equilibrium in the coil. Chattering may also be prevented
by the use of a short-circuited winding or shading coil around one tip of
the pole piece or by the use of polyphase. In a two-phase magnet the pull
is constant and equal to the maximum instantaneous pull produced by
one phase so long as the voltage is a sine function. In a three-phase
magnet under the same conditions the pull is constant and equal to 1.5
times the maximum instantaneous pull of one phase. Should the load
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MAGNETS
become greater than the minimum instantaneous pull, there will be
chattering as in a single-phase magnet.
Heating of Magnets The lifting capacity of an electromagnet is
limited by the permissible current-carrying capacity of the winding,
which, in turn, is dependent on the amount of heat energy that the
winding can dissipate per unit time without exceeding a given temperature rise. Enamels, synthetic varnishes, and thermoplastic wire insulations
are available to allow a wide variety of temperature rises.
Design of Exciting Coil Let n ⫽ number of turns, l ⫽ mean length
of turn, in (l ⫽ 2␲ r, where r is the mean radius, in), A ⫽ cross section of
wire, cir mils. The resistance of 1 cir mil ⭈ ft of copper is practically 12 ⍀
at 60°C, or 1 ⍀/cir mil ⭈ in. Hence the resistance, R ⫽ nl/A ⍀; the
current, I ⫽ EA/nl; the ampere-turns, nI ⫽ EA/l; the power to be dissipated, P ⫽ E 2A/nl W. From the foregoing equations the cross section of
wire and the number of turns can be calculated.
Fig. 15.1.102 Winding space factor.
Space Factor of Winding Space factor of a coil is the ratio of the
space occupied by the conductor to the total volume of the coil or
winding. Only in the theoretical case of uninsulated square or rectangular conductor may the space factor be 100 percent. For wire of circular
Table 15.1.29
section with insulation of negligible thickness, wound as shown in Fig.
15.1.102a, the space factor will be 78.5 percent. When the turns of wire
are ‘‘bedded,’’ as shown in Fig. 15.1.102b (the case in most windings,
particularly with smaller wires), there is a theoretical gain of about 7
percent in space factor. Experiments have shown that in most cases this
gain is about neutralized in practice by the flattening out of the insulation of the wire due to the tension used in winding. When wound in a
haphazard manner, the space factors of magnet wires vary according to
size, substantially as follows:
Double cotton covered
Size, AWG
Space factor, %
0
60
5
53.8
10
45.5
15
35.1
Single cotton covered
20
32.2
25
32
30
25.7
35
16
Magnet wire is usually a soft insulated annealed copper wire of high
conductivity. It can be obtained in square, rectangular, and circular
section, but the round or cylindrical wire is used almost entirely in the
smaller sizes. Ribbons are frequently used in the larger sizes. Aluminum
has been used at times. A number of different varnish or enamel insulating compounds are available in different temperature classes, up to
220°C, the temperature rating and thickness for dielectric strength being
dependent on the usage. Where mechanical strength is important and
space is not at a premium, textile or paper insulation is used on the wires
and later varnish impregnated for high dielectric strength. Asbestoscovered magnet wire is used where the temperature is high. It combines
Magnet-Wire Dimensions, Sizes 14 to 44 AWG
Single
Heavy
AWG
Minimum
Nominal
Maximum
Minimum
increase in
diameter,
in
14
15
16
0.0635
0.0565
0.0503
0.0641
0.0571
0.0508
0.0644
0.0574
0.0511
0.0016
0.0015
0.0014
0.0666
0.0594
0.0531
0.0032
0.0030
0.0029
0.0682
0.0609
0.0545
17
18
19
0.0448
0.0399
0.0355
0.0453
0.0403
0.0359
0.0455
0.0405
0.0361
0.0014
0.0013
0.0012
0.0475
0.0424
0.0379
0.0028
0.0026
0.0025
0.0488
0.0437
0.0391
20
21
22
0.0317
0.0282
0.0250
0.0320
0.0285
0.0253
0.0322
0.0286
0.0254
0.0012
0.0011
0.0011
0.0339
0.0303
0.0270
0.0023
0.0022
0.0021
0.0351
0.0314
0.0281
23
24
25
0.0224
0.0199
0.0177
0.0226
0.0201
0.0179
0.0227
0.0202
0.0180
0.0010
0.0010
0.0009
0.0243
0.0217
0.0194
0.0020
0.0019
0.0018
0.0253
0.0227
0.0203
26
27
28
0.0157
0.0141
0.0125
0.0159
0.0142
0.0126
0.0160
0.0143
0.0127
0.0009
0.0008
0.0008
0.0173
0.0156
0.0140
0.0017
0.0016
0.0016
0.0182
0.0164
0.0147
29
30
0.0112
0.0099
0.0113
0.0100
0.0114
0.0101
0.0007
0.0007
0.0126
0.0112
0.0015
0.0014
0.0133
0.0119
31
32
33
0.0088
0.0079
0.0070
0.0089
0.0080
0.0071
0.0090
0.0081
0.0072
0.0006
0.0006
0.0005
0.0100
0.0091
0.0081
0.0013
0.0012
0.0011
0.0108
0.0098
0.0088
34
35
36
0.0062
0.0055
0.0049
0.0063
0.0056
0.0050
0.0064
0.0057
0.0051
0.0005
0.0004
0.0004
0.0072
0.0064
0.0058
0.0010
0.0009
0.0008
0.0078
0.0070
0.0063
37
38
39
0.0044
0.0039
0.0034
0.0045
0.0040
0.0035
0.0046
0.0041
0.0036
0.0003
0.0003
0.0002
0.0052
0.0047
0.0041
0.0008
0.0007
0.0006
0.0057
0.0051
0.0045
40
41
42
0.0030
0.0027
0.0024
0.0031
0.0028
0.0025
0.0032
0.0029
0.0026
0.0002
0.0002
0.0002
0.0037
0.0033
0.0030
0.0006
0.0005
0.0004
0.0040
0.0036
0.0032
43
44
0.0021
0.0019
0.0022
0.0020
0.0023
0.0021
0.0002
0.0001
0.0026
0.0024
0.0004
0.0004
0.0029
0.0027
Bare wire diameter, in
15-65
Maximum
overall
diameter,
in
Minimum
increase in
diameter,
in
Maximum
overall
diameter,
in
SOURCE: ‘‘Standard Handbook for Electrical Engineers,’’ Fink and Carrol, McGraw-Hill, New York, 1968.
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15-66
ELECTRICAL ENGINEERING
resistance to heat and abrasion, can resist mild acids, has good dielectric
strength, and is fireproof.
Table 15.1.29 gives the diameters of magnet wire with the different
types of insulation. For further data on electrical insulating materials,
see Sec. 6.
AUTOMOBILE SYSTEMS
Automobile Ignition Systems
The ignition system in an automobile produces the spark which ignites
the combustible mixture in the engine cylinders. This is accomplished
by a high-voltage, or high-tension, spark between metal points in a
spark plug. (A spark plug is an insulated bushing screwed into the
cylinder head.) Spark plugs usually have porcelain insulation, but for
some special uses, such as in airplane engines, mica may be used. There
are two general sources for the energy necessary for ignition; one is the
electrical system of the car which is maintained by the generator and the
battery (battery ignition), and the other is a magneto. Battery ignition
systems have traditionally operated electromechanically, using a spark
coil, a high-voltage distributor, and low-voltage breaker points. Electronic ignition systems, working from the battery, became standard on
U.S. cars in 1975. These vary in complexity from the use of a single
transistor to reduce the current through the points to pointless systems
triggered by magnetic pulses or interrupted light beams. Capacitor discharge into a pulse transformer is used in some systems to obtain the
high voltage needed to fire the spark plugs.
Battery ignition is most widely used since it is simple, reliable, and
low in cost, and the electrical system is a part of the car equipment. The
high voltage for the spark is obtained from an ignition coil which consists of a primary coil of relatively few turns and a secondary coil of a
large number of turns, both coils being wound on a common magnetic
core consisting of either thin strips of iron or small iron wires. In a 6-V
system the resistance of the primary coil is from 0.9 to 2 ⍀ and the
inductance is from 5 to 10 mH. The number of secondary turns varies
from 9,000 to 25,000, and the ratio of primary to secondary turns varies
from 1 : 40 to 1 : 100.
The coil operates on the following principle. It stores energy in a
magnetic field relatively slowly and then releases it suddenly. The
power developed ( p ⫽ dw/dt) is thus relatively large (w ⫽ stored energy). The high emf e2 which is required for the spark is induced by the
sudden change in the flux ␾ in the core of the coil when the primary
current is suddenly interrupted, e2 ⫽ ⫺ n 2(d␾/dt), where n 2 is the number of secondary turns. For satisfactory ignition, peak voltages from 10
to 20 kV are desirable. Figure 15.1.103 shows the relation between the
volts required to produce a spark and pressure with compressed air.
Fig. 15.1.103 Pressure-voltage curve for spark plug.
A battery ignition system for a four-cylinder engine is shown diagrammatically in Fig. 15.1.104. The primary circuit supplied by the
battery consists of the primary coil P and a set of contacts, or ‘‘points’’
operated by a four-lobe cam, in series. In order to reduce arcing and
burning of the contacts and to produce a sharp break in the current, a
capacitor C is connected across the contacts. The contacts, which are of
pure tungsten, are operated by a four-lobe cam which is driven at onehalf engine speed. A strong spring tends to keep the contacts closed.
In the Delco-Remy distributor (Fig. 15.1.105) two breaker arms are
connected in parallel; one coil and one capacitor are used. One set of
contacts is open when the other is just breaking but closes a few degrees
Fig. 15.1.104
Battery-ignition system.
after the break occurs. This closes the primary of the ignition coil immediately after the break and increases the time that the primary of the
ignition coil is closed and permits the flux in the iron to reach its full
value. The interrupter shown in Fig. 15.1.105 is designed for an eightcylinder engine.
Fig. 15.1.105
Delco-Remy eight-cylinder interrupter.
Electronic ignition systems have no problem operating at the required
speed.
The spark should advance with increase in engine speed so as to
allow for the time lag in the explosion. To take care of this automatically
most timers are now equipped with centrifugally operated weights
which advance the breaker cam with respect to the engine drive as the
speed increases.
Automobile Lighting and Starting Systems
Automobile lighting and starting systems initially operated at 6 V, but at
present nearly all cars, except the smaller ones, operate at 12 V because
larger engines, particularly V-8s, are now common and require more
starting power. With 12 V, for the same power, the starting current is
halved, and the effect of resistance in the leads, connections, and
brushes is materially reduced. In some systems the positive side of the
system is grounded, but more often the negative side is grounded.
A further development is the application of an ac generator, or alternator, combined with a rectifier as the generating unit rather than the
usual dc generator. One advantage is the elimination of the commutator,
made up of segments, which requires some maintenance due to the
sparking and wear of the carbon brushes. With the alternator the dc field
rotates, the brushes operating on smooth slip rings require almost no
maintenance. Also, the system is greatly simplified by the fact that
rectifiers are ‘‘one-way’’ devices, and the battery cannot deliver current
back to the generator when its voltage drops below that of the battery.
Thus, no cutout relay, such as is required with dc generators, is necessary. This ac development is the result of the development of reliable,
low-cost germanium and silicon semiconductor rectifiers.
Figure 15.1.106 shows a schematic diagram of the Ford system
(adapted initially to trucks). The generator stator is wound three-phase
Y-connected, and the field is bipolar supplied with direct current
through slip rings and brushes. The rectifier diodes are connected fullwave bridge circuit to supply the battery through the ammeter.
Regulator The function of the regulator is to control the generator
current so that its value is adapted to the battery voltage which is related
to the condition of charge of the battery (see Fig. 15.1.15). Thus, when
the battery voltage drops (indicating a lowered condition of charge), the
current should be increased, and, conversely, when the battery voltage
increases (indicating a high condition of charge), the current should be
decreased.
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AUTOMOBILE SYSTEMS
15-67
Fig. 15.1.106 Schematic diagram of Ford lighting and starting system.
Neglecting for the moment the starting procedure, when the ignition
switch is thrown to the normal ‘‘on’’ position at c, the coil actuating the
field relay is connected to the battery ⫹ terminal and causes the relay
contacts a to close. This energizes the regulator circuits, and, if the two
upper voltage regulator contacts b are closed as shown, the rotating field
of the alternator is connected directly to the battery ⫹ terminal, and the
field current is then at its maximum value and produces a high generator
voltage and large output current. At the same time the voltage regulator
coil in series with the 0.3- and 14-⍀ resistors is connected between the
battery ⫹ terminal and ground. If the voltage of the battery rises owing
to its higher condition of charge, the current to the voltage regulator coil
increases, causing it to open the two upper contacts at b. Current from
the battery now flows through the 0.3-⍀ resistor and divides, some
going through the 10-⍀ resistor and dividing between the field and the
50-⍀ resistor, and the remainder going to the voltage regulator coil. The
current to the rotating field is thus reduced, causing the alternator output
to be reduced. Because of the 0.3 ⍀ now in circuit, the current to the coil
of the voltage regulator is reduced to such a value that it holds the center
contact at b in the mid, or open, position. If the battery voltage rises to
an even higher value, the regulator coil becomes strong enough to close
the lower contacts at b; this short-circuits the field, reducing its current
almost to zero, and thus reducing the alternator output to zero. On the
other hand when the battery voltage drops, the foregoing sequence is
reversed, and the contacts at b operate to increase the current to the
alternator field.
As was mentioned earlier, the battery cannot supply current to the
alternator because of the ‘‘one-way’’ characteristic of the rectifier.
Thus, when the alternator voltage drops below that of the battery and
even when the alternator stops running, its current automatically becomes zero. The alternator has a normal rectifier open-circuit voltage of
about 14 V and a rating of 20 A.
Starting In most cases, for starting, the ignition key is turned far to
the right and held there until the motor starts. Then, when the key is
released, the ignition switch contacts assume a normal operating position. Thus, in Fig. 15.1.106, when the ignition-switch contact is in the
starting position S, the starter relay coil becomes connected by a lead to
the battery ⫹ terminal and thus becomes energized, closing the relay
contacts. The starter motor is then connected to the battery to crank the
engine. At the time that the contact closes it makes contact with a small
metal brush e which connects the battery ⫹ terminal to the primary
terminal of the ignition coil through the protective resistor R. After the
motor starts, the ignition switch contacts spring to the normal operating
position C, and the starter relay switch opens, thereby breaking contact
with the small brush e. However, when contact C is closed, the ignition
coil primary terminal is now connected through leads to the battery ⫹
terminal. The interrupter, the ignition coil, and the distributor now operate in the manner described earlier (see Fig. 15.1.104); the system
shown in Fig. 15.1.106 is that for a six-cylinder engine.
The connection of accessories to the electric system is illustrated in
Fig. 15.1.106 for the horn, head and other lights, and temperature and
fuel gages.
Magneto Ignition
Principle of Magneto A magneto is an electric generator in which
the magnetic flux is provided by one or more permanent magnets. It is a
self-contained unit and is used advantageously for ignition where a
generator and a battery are not needed to supply power to other accessories. The design of magnetos was radically changed when Alnico,
with its very high retentivity (Fig. 15.1.97), became available as a permanent-magnet material. One method of utilizing Alnico magnets is to
insert the bar magnets in the frame of the magneto (Fig. 15.1.107). The
rotor is a soft-iron bobbin. A primary winding of relatively few turns
and a secondary winding of a relatively large number of turns are wound
over the laminated yoke Y. The position of the rotor shown in (a) provides a low reluctance path for the magnetic flux of the left-hand mag-
Fig. 15.1.107
Magneto-ignition system.
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15-68
ELECTRONICS
net and a high reluctance path for the magnetic flux of the right-hand
magnet so that the flux goes through the yoke from left to right as
shown. When the rotor turns one-eighth of a revolution, it becomes
horizontal; obviously, each of the magnets acts in opposition relative to
the yoke, and the flux therein becomes zero. In (b), which also shows
the external electrical connections, the rotor is shown as having turned
one-fourth of a revolution, or 90°, from its position in (a).
The rotor now provides a low-reluctance path for the right-hand magnet and a high-reluctance path for the left-hand one. Thus the magnetic
flux now goes through the yoke from right to left. It follows that in each
90° interval the flux in the yoke undergoes a complete reversal. It will
be recognized that this is an inductor type of ac generator.
In the diagram in (b), one end each of the primary and of the secondary are grounded together. The other end of the primary is connected to
an insulated interrupter lever having a contact point P. This makes
intermittent contact with the grounded contact point P⬘. The contact
point P is actuated by a cam which is driven by the same shaft as the
magneto rotor and is in a definite relation to it. A switch S⬘ is provided
to ground and thus short-circuit the secondary when it is desired to stop
the engine. A capacitor C is connected between the point P and ground
to absorb the energy of the spark which occurs when the contacts open.
With the contacts closed, the primary is short-circuited, and the varying flux in the core produced by the rotation of the rotor induces an
alternating current in the winding which in turn produces an alternating
flux in the core. With the rotation of the rotor the current in the secondary rises cyclically to maximum values, and at these instants the cam
causes the points PP⬘ to open suddenly, interrupting the current in the
primary and thus causing a sudden collapse of the flux in the core. This
induces a high-impulse emf in the secondary which is transmitted to the
distributor and thence to the proper spark plug as shown.
On starting, the speed of the magneto may be so low that the emf is
15.2
not sufficient to produce a hot spark. This difficulty can be met by
impulse starting, in which the rotor is driven through a spring. During
cranking the rotor is restrained from turning until the engine comes to
the proper firing position, at which time the rotor is suddenly released.
The energy stored in the spring produces a high, instantaneous, angular
velocity to the rotor, resulting in a high emf and hot spark.
Inductor-type magnetos, having a large number of rotor poles and
arranged differently from those shown in Fig. 15.1.107, are used for
airplane-engine ignition. In another magneto design the rotor is a solid
cylindrical Alnico magnet, permanently magnetized with an N and an S
pole diametrically opposite. The frame is laminated, and there is a yoke
with primary and secondary windings. When the rotor rotates, its N and
S poles produce an alternating flux in the yoke which induces a shortcircuit current in the primary winding and the method of producing the
spark is then the same as with Fig. 15.1.107b.
Miscellaneous Automobile Electronics
Systems
The use of electronics on automobiles is expanding rapidly. Microprocessors are being installed to control many functions that were previously not controlled or poorly controlled.
Air-fuel ratio is controlled from sensing oxygen in the exhaust system.
Timing is based on crankshaft position, acceleration, and engine temperature. Air injection into exhaust and recirculation of exhaust reduces
emissions. Knock is prevented by stopping untimely detonations. Diagnosis of problems is performed. Car leveling and load matching are
made by shock absorber adjustments. Wheel spin is prevented. Transmission controls provide improved efficiency. Operator interfaces are by
cathode ray tube, fluid crystal, vacuum fluorescent displays, and speech synthesis. Navigation aids are in a rudimentary state.
ELECTRONICS
by Byron M. Jones
REFERENCES: ‘‘Reference Data for Radio Engineers,’’ Howard Sams & Co.
‘‘Transistor Manual,’’ General Electric Co. ‘‘SCR Manual,’’ General Electric Co.
‘‘The Semiconductor Data Book,’’ Motorola Inc. Fink, ‘‘Television Engineering,’’ McGraw-Hill. ‘‘Industrial Electronics Reference Book,’’ Wiley. Mano,
‘‘Digital and Logic Design,’’ Prentice-Hall. McNamara, ‘‘Technical Aspects of
Data Communication,’’ Digital Press. Fletcher, ‘‘An Engineering Approach to
Digital Design,’’ Prentice-Hall. ‘‘The TTL Data Book,’’ Texas Instruments, Inc.
‘‘1988 MOS Products Catalog,’’ American Microsystems, Inc. ‘‘1988 Linear
Data Book,’’ National Semiconductor Corp. ‘‘CMOS Standard Cell Data Book,’’
Texas Instruments, Inc. ‘‘Power MOSFET Transistor Data,’’ Motorola, Inc.
Franco, ‘‘Design with Operational Amplifiers and Analog Integrated Circuits,’’
McGraw-Hill. Soclof, ‘‘Applications of Analog Integrated Circuits,’’ PrenticeHall. Gibson, ‘‘Computer Systems Concepts and Design,’’ Prentice-Hall. Sedra
and Smith, ‘‘Microelectronic Circuits,’’ HRW Saunders. Millman and Grabel,
‘‘Microelectronics,’’ McGraw-Hill. Ghausi, ‘‘Electronic Devices and Circuits:
Discrete and Integrated,’’ HRW Saunders. Savant, Roden, and Carpenter, ‘‘Electronic Design Circuits and Systems,’’ Benjamin Cummings. Stearns and Hush,
‘‘Digital Signal Analysis,’’ Prentice-Hall. Kassakian, Schlecht, and Verghese,
‘‘Principles of Power Electronics,’’ Addison Wesley. Yariv, ‘‘Optical Electronics,’’ HRW Saunders.
The subject of electronics can be approached from the standpoint of
either the design of devices or the use of devices. The practicing mechanical engineer has little interest in designing devices, so the approach in this article will be to describe devices in terms of their external characteristics.
Components
Resistors, capacitors, reactors, and transformers are described earlier in
this section, along with basic circuit theory. These explanations are
equally applicable to electronic circuits and hence are not repeated here.
A description of additional components peculiar to electronic circuits
follows.
A rectifier, or diode, is an electronic device which offers unequal
resistance to forward and reverse current flow. Figure 15.2.1 shows the
schematic symbol for a diode. The arrow beside the diode shows the
direction of current flow. Current flow is taken to be the flow of positive
Fig. 15.2.1
Diode schematic symbol.
charges, i.e., the arrow is counter to electron flow. Figure 15.2.2 shows
typical forward and reverse volt-ampere characteristics. Notice that the
scales for voltage and current are not the same for the first and third
quadrants. This has been done so that both the forward and reverse
characteristics can be shown on a single plot even though they differ by
several orders of magnitude.
Diodes are rated for forward current capacity and reverse voltage
breakdown. They are manufactured with maximum current capabilities
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COMPONENTS
ranging from 0.05 A to more than 1,000 A. Reverse voltage breakdown
varies from 50 V to more than 2,500 V. At rated forward current, the
forward voltage drop varies between 0.7 and 1.5 V for silicon diodes.
Although other materials are used for special-purpose devices, by far
the most common semiconductor material is silicon. With a forward
flow is initiated by a gate pulse. The schematic symbol for an SCR is
shown in Fig. 15.2.4. The physical packaging of thyristors is similar to
that of rectifiers with similar ratings except, of course, that the thyristor
must have an additional gate connection.
Fig. 15.2.4
Fig. 15.2.2 Diode forward-reverse characteristic.
current of 1,000 A and a forward voltage drop of 1 V, there would be a
power loss in the diode of 1,000 W (more than 1 hp). The basic diode
package shown in Fig. 15.2.3 can dissipate about 20 W. To maintain an
acceptable temperature in the diode, it is necessary to mount the diode
on a heat sink. The manufacturer’s recommendation should be followed
very carefully to ensure good heat transfer and at the same time avoid
fracturing the silicon chip inside the diode package.
Fig. 15.2.3 Physical diode package.
The selection of fuses or circuit breakers for the protection of rectifiers
and rectifier circuitry requires more care than for other electronic devices. Diode failures as a result of circuit faults occur in a fraction of a
millisecond. Special semiconductor fuses have been developed specifically for semiconductor circuits. Proper protective circuits must be provided for the protection of not only semiconductors but also the rest of
the circuit and nearby personnel. Diodes and diode fuses have a shortcircuit rating in amperes-squared-seconds (I 2t). As long as the I 2t rating
of the diode exceeds the I 2t rating of its protective fuse, the diode and its
associated circuitry will be protected. Circuit breakers may be used to
protect diode circuits, but additional line impedance must be provided
to limit the current while the circuit breaker clears. Circuit breakers do
not interrupt the current when their contacts open. The fault is not
cleared until the line voltage reverses at the end of the cycle of the
applied voltage. This means that the clearing time for a circuit breaker is
about 1⁄2 cycle of the ac input voltage. Diodes have a 1-cycle overcurrent
rating which indicates the fault current the diode can carry for circuit
breaker protection schemes. Line inductance is normally provided to
limit fault currents for breaker protection. Often this inductance is in the
form of leakage reactance in the transformer which supplies power to
the diode circuit.
A thyristor, often called a silicon-controlled rectifier (SCR), is a rectifier
which blocks current in both the forward and reverse directions. Conduction of current in the forward direction will occur when the anode is
positive with respect to the cathode and when the gate is pulsed positive
with respect to the cathode. Once the thyristor has begun to conduct, the
gate pulse can return to 0 V or even go negative and the thyristor will
continue to pass current. To stop the cathode-to-anode current, it is
necessary to reverse the cathode-to-anode voltage. The thyristor will
again be able to block both forward and reverse voltages until current
15-69
Thyristor schematic symbol.
The gate pulse required to fire an SCR is quite small compared with
the anode voltage and current. Power gains in the range of 106 to 109 are
easily obtained. In addition, the power loss in the thyristor is very low,
compared with the power it controls, so that it is a very efficient powercontrolling device. Efficiency in a thyristor power supply is usually 97
to 99 percent. When the thyristor blocks either forward or reverse
current, the high voltage drop across the thyristor accompanies low
current. When the thyristor is conducting forward current after having
been fired by its gate pulse, the high anode current occurs with a forward voltage drop of about 1.5 V. Since high voltage and high current
never occur simultaneously, the power dissipation in both the on and off
states is low.
The thyristor is rated primarily on the basis of its forward-current
capacity and its voltage-blocking capability. Devices are manufactured
to have equal forward and reverse voltage-blocking capability. Like
diodes, thyristors have I 2t ratings and 1-cycle surge current ratings to
allow design of protective circuits. In addition to these ratings, which
the SCR shares in common with diodes, the SCR has many additional
specifications. Because the thyristor is limited in part by its average
current and in part by its rms current, forward-current capacity is a
function of the duty cycle to which the device is subjected. Since the
thyristor cannot regain its blocking ability until its anode voltage is
reversed and remains reversed for a short time, this time must be specified. The time to regain blocking ability after the anode voltage has been
reversed is called the turn-off time. Specifications are also given for
minimum and maximum gate drive. If forward blocking voltage is reapplied too quickly, the SCR may fire with no applied gate voltage pulse.
The maximum safe value of rate of reapplied voltage is called the dv/dt
rating of the SCR. When the gate pulse is applied, current begins to flow
in the area immediately adjacent to the gate junction. Rather quickly,
the current spreads across the entire cathode-junction area. In some
circuits associated with the thyristor an extremely fast rate of rise of
current may occur. In this event localized heating of the cathode may
occur with a resulting immediate failure or in less extreme cases a slow
degradation of the thyristor. The maximum rate of change of current for
a thyristor is given by its di/dt rating. Design for di/dt and dv/dt limits is
not normally a problem at power-line frequencies of 50 and 60 Hz.
These ratings become a design factor at frequencies of 500 Hz and
greater. Table 15.2.1 lists typical thyristor characteristics.
A triac is a bilateral SCR. It blocks current in either direction until it
receives a gate pulse. It can be used to control in ac circuits. Triacs are
widely used for light dimmers and for the control of small universal ac
motors. The triac must regain its blocking ability as the line voltage
crosses through zero. This fact limits the use of triacs to 60 Hz and
below.
A transistor is a semiconductor amplifier. The schematic symbol for a
transistor is shown in Fig. 15.2.5. There are two types of transistors,
p-n-p and n-p-n. Notice that the polarities of voltage applied to these
devices are opposite. In many sizes matched p-n-p and n-p-n devices are
available. The most common transistors have a collector dissipation
rating of 150 to 600 mW. Collector to base breakdown voltage is 20 to
50 V. The amplification or gain of a transistor occurs because of two
facts: (1) A small change in current in the base circuit causes a large
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15-70
ELECTRONICS
Table 15.2.1
Typical Thyristor Characteristics
Current, A
Voltage
rms
avg
I 2t, A2 ⭈ s
1-cycle surge,
A
di /dt,
A / ␮s
dv /dt,
V / ␮s
Turn-off
time, ␮s
400
1,200
400
1,200
400
1,200
400
1,200
35
35
110
110
235
235
470
470
20
20
70
70
160
160
300
300
165
75
4,000
4,000
32,000
32,000
120,000
120,000
180
150
1,000
1,000
3,500
3,500
5,500
5,500
100
100
100
100
100
75
50
50
200
200
200
200
200
200
100
100
10
10
40
40
80
80
150
150
change in current in the collector and emitter leads. This current amplification is designated hfe on most transistor specification sheets. (2) A
small change in base-to-emitter voltage can cause a large change in
either the collector-to-base voltage or the collector-to-emitter voltage.
Table 15.2.2 shows basic ratings for some typical transistors. There is a
great profusion of transistor types so that the choice of type depends
upon availability and cost as well as operating characteristics.
are thyristors. Circuits with multiple power MOSFETs are limited to
about 20 kW. Thyristors are limited to about 1500 kW. As a point of
interest, there are electric power applications in the hundreds of megawatts which incorporate massively series and parallel thyristors. An
Fig. 15.2.6 Field-effect transistor. (a) Bipolar junction type (JFET); (b) metaloxide-semiconductor type (MOSFET).
advantage of power MOSFETs is that they can be turned on and turned
off by means of the gate-source voltage. Thus low-power electric control can turn the device on and off. Thyristors can be turned on only by
their gate voltage. To turn a thyristor off it is necessary for its highpowered anode-to-cathode voltage to be reversed. This factor adds
Fig. 15.2.5 Transistor schematic symbol.
The gain of a transistor is independent of frequency over a wide
range. At high frequency, the gain falls off. This cutoff frequency may
be as low as 20 kHz for audio transistors or as high as 1 GHz for
radio-frequency (rf ) transistors.
The schematic symbols for the field effect transistor (FET) is shown
in Fig. 15.2.6. The flow of current from source to drain is controlled by
an electric field established in the device by the voltage applied between
the gate and the drain. The effect of this field is to change the resistance
of the transistor by altering its internal current path. The FET has an
extremely high gate resistance (1012 ⍀), and as a consequence, it is used
for applications requiring high input impedance. Some FETs have been
designed for high-frequency characteristics. The two basic constructions used for FETs are bipolar junctions and metal oxide semiconductors.
The schematic symbols for each of these are shown in Fig. 15.2.6a and
15.2.6b. These are called JFETs and MOSFETs to distinguish between
them. JFETs and MOSFETs are used as stand-alone devices and are
also widely used in integrated circuits. (See below, this section.)
A MOSFET with higher current capacity is called a power MOSFET.
Table 15.2.3 shows some typical characteristics for power MOSFETs.
Power MOSFETs are somewhat more limited in maximum power than
Table 15.2.2
JEDEC
number
2N3904
2N3906
2N3055
2N6275
2N5458
2N5486
Table 15.2.3
Drain
voltage
Typical Power MOSFET Characteristics
Device
Drain
amperes
Power
dissipation,
W
Case
type
600
MTH6N60
6.0
150
TO-218
400
MTH8N40
8.0
150
TO-218
100
MTH25N10
25
150
TO-218
50
MTH35N05
35
150
TO-218
600
MTP1N60
1.0
75
TO-220
400
MTP2N40
2.0
75
TO-220
100
MTP10N10
10
75
TO-220
200
MTE120N20
120
500
346-01
100
MTE150N10
150
500
346-01
50
MTE200N05
200
500
346-01
Typical Transistor Characteristics
Type
Collector-emitter
volts at breakdown,
BVCE
Collector dissipation,
Pc(25°C)
Collector current,
Ic
Current gain,
hfe
n-p-n
p-n-p
n-p-n
n-p-n
JFET
JFET
40
40
100
120
40
25
310 mW
310 mW
115 W
250 W
200 mW
200 mW
200 mA
200 mA
15 A
50 A
9 mA
200
200
20
30
*
†
* JFET, current gain is not applicable.
† High-frequency JFET — up to 400 MHz.
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DISCRETE-COMPONENT CIRCUITS
complication to many thyristor circuits. Another power device which is
similar to the power MOSFET is the insulated gate-bipolar transistor
(IGBT). This device is a Darlington combination of a MOSFET and a
bipolar transistor (see Fig. 15.2.13). A low-power MOSFET first transistor drives the base of a second high-power bipolar transistor. These
two transistors are integrated in a single case. The IGBT is applied in
high-power devices which use high-frequency switching. IGBTs are
available in ratings similar to thyristors (see Table 15.2.1), so they are
power devices. The advantage of the IGBT is that it can be switched on
and off by means of its gate. The advantage of an IGBT over a power
MOSFET is that it can be made with higher power ratings.
The unijunction is a special-purpose semiconductor device. It is a
pulse generator and widely used to fire thyristors and triacs as well as in
timing circuits and waveshaping circuits. The schematic symbol for a
unijunction is shown in Fig. 15.2.7. The device is essentially a silicon
resistor. This resistor is connected to base 1 and base 2. The emitter is
fastened to this resistor about halfway between bases 1 and 2. If a
positive voltage is applied to base 2, and if the emitter and base 1 are at
zero, the emitter junction is back-biased and no current flows in the
emitter. If the emitter voltage is made increasingly positive, the emitter junction
will become forward-biased. When this
occurs, the resistance between base 1 and
base 2 and between base 2 and the emitter
suddenly switches to a very low value.
This is a regenerative action, so that very
fast and very energetic pulses can be generated with this device.
Before the advent of semiconductors,
Fig. 15.2.7 Unijunction.
electronic rectifiers and amplifiers were
vacuum tubes or gas-filled tubes. Some use
of these devices still remains. If an electrode is heated in a vacuum, it
gives up surface electrons. If an electric field is established between this
heated electrode and another electrode so that the electrons are attracted
to the other electrode, a current will flow through the vacuum. Electrons
flow from the heated cathode to the cold anode. If the polarity is reversed, since there are no free electrons around the anode, no current
will flow. This, then, is a vacuum-tube rectifier. If a third electrode,
called a control grid, is placed between the cathode and the anode, the
flow of electrons from the cathode to the anode can be controlled. This
is a basic vacuum-tube amplifier. Additional grids have been placed
between the cathode and anode to further enhance certain characteristics
of the vacuum tube. In addition, multiple anodes and cathodes have
been enclosed in a single tube for special applications such as radio
signal converters.
If an inert gas, such as neon or argon, is introduced into the vacuum,
conduction can be initiated from a cold electrode. The breakdown voltage is relatively stable for given gas and gas pressure and is in the range
of 50 to 200 V. The nixie display tube is such a device. This tube contains 10 cathodes shaped in the form of the numerals from 0 to 9. If one
of these cathodes is made negative with respect to the anode in the tube,
the gas in the tube glows around that cathode. In this way each of the 10
numerals can be made to glow when the appropriate electrode is energized.
An ignitron is a vapor-filled tube. It has a pool of liquid mercury in the
bottom of the tube. Air is exhausted from the enclosure, leaving only
mercury vapor, which comes from the pool at the bottom. If no current
is flowing, this tube will block voltage whether the anode is plus or
minus with respect to the mercury-pool cathode. A small rod called an
ignitor can form a cathode spot on the pool of mercury when it is withdrawn from the pool. The ignitor is pulled out of the pool by an electromagnet. Once the cathode spot has been formed, electrons will continue
to flow from the mercury-pool cathode to the anode until the anode-tocathode voltage is reversed. The operation of an ignitron is very similar
to that of a thyristor. The anode and cathode of each device perform
similar functions. The ignitor and gate also perform similar functions.
The thyristor is capable of operating at much higher frequencies than
the ignitron and is much more efficient since the thyristor has 1.5 V
15-71
forward drop and the ignitron has 15 V forward drop. The ignitron has
an advantage over the thyristor in that it can carry extremely high overload currents without damage. For this reason ignitrons are often used as
electronic ‘‘crowbars’’ which discharge electrical energy when a fault
occurs in a circuit.
Discrete-Component Circuits
Several common rectifier circuits are shown in Fig. 15.2.8. The waveforms shown in this figure assume no line reactance. The presence of
line reactance will make a slight difference in the waveshapes and the
conversion factors shown in Fig. 15.2.8. These waveshapes are equally
applicable for loads which are pure resistive or resistive and inductive.
In a resistive load the current flowing in the load has the same waveshape as the voltage applied to it. For inductive loads, the current waveshape will be smoother than the voltage applied. If the inductance is
high enough, the ripple in the current may be indeterminantly small. An
approximation of the ripple current can be calculated as follows:
I⫽
E dc PCT
200␲ fNL
(15.2.1)
where I ⫽ rms ripple current, E dc ⫽ dc load voltage, PCT ⫽ percent
ripple from Fig. 15.2.8, f ⫽ line frequency, N ⫽ number of cycles of
ripple frequency per cycle of line frequency, L ⫽ equivalent series
inductance in load. Equation (15.2.1) will always give a value of ripple
higher than that calculated by more exact means, but this value is normally satisfactory for power-supply design.
Capacitance in the load leads to increased regulation. At light loads,
the capacitor will tend to charge up to the peak value of the line voltage
and remain there. This means that for either the single full-wave circuit
or the single-phase bridge the dc output voltage would be 1.414 times
the rms input voltage. As the size of the loading resistor is reduced, or as
the size of the parallel load capacitor is reduced, the load voltage will
more nearly follow the rectified line voltage and so the dc voltage will
approach 0.9 times the rms input voltage for very heavy loads or for
very small filter capacitors. One can see then that dc voltage may vary
between 1.414 and 0.9 times line voltage due only to waveform changes
when capacitor filtering is used.
Four different thyristor rectifier circuits are shown in Fig. 15.2.9.
These circuits are equally suitable for resistive or inductive loads. It will
be noted that the half-wave circuit for the thyristor has a rectifier across
the load, as in Fig. 15.2.8. This diode is called a freewheeling diode
because it freewheels and carries inductive load current when the thyristor is not conducting. Without this diode, it would not be possible to
build up current in an inductive load. The gate-control circuitry is not
shown in Fig. 15.2.9 in order to make the power circuit easier to see.
Notice the location of the thyristors and rectifiers in the single-phase
full-wave circuit. Constructed this way, the two diodes in series perform
the function of a free-wheeling diode. The circuit can be built with a
thyristor and rectifier interchanged. This would work for resistive loads
but not for inductive loads. For the full three-phase bridge, a freewheeling diode is not required since the carryover from the firing of one
SCR to the next does not carry through a large portion of the negative
half cycle and therefore current can be built up in an inductive load.
Capacitance must be used with care in thyristor circuits. A capacitor
directly across any of the circuits in Fig. 15.2.9 will immediately destroy the thyristors. When an SCR is fired directly into a capacitor with
no series resistance, the resulting di/dt in the thyristor causes extreme
local heating in the device and a resultant failure. A sufficiently high
series resistor prevents failure. An inductance in series with a capacitor
must also be used with caution. The series inductance may cause the
capacitor to ‘‘ring up.’’ Under this condition, the voltage across the
capacitor can approach twice peak line voltage or 2.828 times rms line
voltage.
The advantage of the thyristor circuits shown in Fig. 15.2.9 over the
rectifier circuits is, of course, that the thyristor circuits provide variable
output voltage. The output of the thyristor circuits depends upon the
magnitude of the incoming line voltage and the phase angle at which the
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ELECTRONICS
Fig. 15.2.8 Comparison of rectifier circuits.
thyristors are fired. The control characteristic for the thyristor power
supply is determined by the waveshape of the output voltage and also by
the phase-shifting scheme used in the firing-control means for the thyristor. Practical and economic power supplies usually have control
characteristics with some degree of nonlinearity. A representative characteristic is shown in Fig. 15.2.10. This control characteristic is usually
given for nominal line voltage with the tacit understanding that variations in line voltage will cause approximately proportional changes in
output voltage.
Fig. 15.2.10
Fig. 15.2.9 Basic thyristor circuits.
Thyristor control characteristic.
Transistor amplifiers can take many different forms. A complete discussion is beyond the scope of this handbook. The circuits described
here illustrate basic principles. A basic single-stage amplifier is shown in
Fig. 15.2.11. The transistor can be cut off by making the input terminal
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DISCRETE-COMPONENT CIRCUITS
sufficiently negative. It can be saturated by making the input terminal
sufficiently positive. In the linear range, the base of an n-p-n transistor
will be 0.5 to 0.7 V positive with respect to the emitter. The collector
voltage will vary from about 0.2 V to Vc (20 V, typically). Note that
there is a sign inversion of voltage between the base and the collector;
i.e., when the base is made more positive, the collector becomes less
positive. The resistors in this circuit serve the following functions. Resistor R1 limits the input current to the base of the transistor so that it is
not harmed when the input signal overdrives. Resistors R2 and R3 establish the transistor’s operating point with no input signal. Resistors R4
and R5 determine the voltage gain of the amplifier. Resistor R4 also
serves to stabilize the zero-signal operating point, as established by
resistors R2 and R3. Usual practice is to design single-stage gains of 10
to 20. Much higher gains are possible to achieve, but low gain levels
permit the use of less expensive transistors and increase circuit reliability.
Figure 15.2.12 illustrates a basic two-stage transistor amplifier using
complementary n-p-n and p-n-p transistors. Note that the first stage is
temperature drift and drift due to power supply voltage changes. The
differential amplifier minimizes drift because of the balanced nature of
the circuit. Whatever changes in one transistor tend to increase the
output are compensated by reverse trends in the second transistor. The
input signal does not affect both transistors in compensatory ways, of
course, and so it is amplified. One way to look at a differential amplifier
is that twice as many transistors are used for each stage of amplification
to achieve compensation. For very low drift requirements, matched
transistors are available. For the ultimate in differential amplifier performance, two matched transistors are encapsulated in a single unit.
Operational amplifiers made with discrete components frequently use
differential amplifiers to minimize drift and offset. The operational amplifier is a low-drift, high-gain amplifier designed for a wide range of
control and instrumentation uses.
Oscillators are circuits which provide a frequency output with no
signal input. A portion of the collector signal is fed back to the base of
the transistor. This feedback is amplified by the transistor and so maintains a sustained oscillation. The frequency of the oscillation is determined by parallel inductance and capacitance. The oscillatory circuit
consisting of an inductance and a capacitance in parallel is called an LC
tank circuit.
This frequency is approximately equal to
f ⫽ 1/2␲ √CL
Fig. 15.2.11 Single-stage
amplifier.
Fig. 15.2.12
Two-stage amplifier.
15-73
(15.2.2)
where f ⫽ frequency, Hz; C ⫽ capacitance, F; L ⫽ inductance, H. A
1-MHz oscillator might typically be designed with a 20-␮H inductance
in parallel with a 0.05-␮F capacitor. The exact frequency will vary from
the calculated value because of loading effects and stray inductance and
capacitance. The Colpitts oscillator shown in Fig. 15.2.15 differs from
the Hartley oscillator shown in Fig. 15.2.16 only in the way energy is fed
back to the emitter. The Colpitts oscillator has a capacitive voltage
identical to that shown in Fig. 15.2.11. This n-p-n stage drives the
following p-n-p stage. Additional alternate n-p-n and p-n-p stages can
be added until any desired overall amplifier gain is achieved.
Figure 15.2.13 shows the Darlington connection of transistors. The
amplifier is used to obtain maximum current gain from two transistors.
Fig. 15.2.15
Fig. 15.2.13 Darlington connection.
Colpitts oscillator.
Fig. 15.2.16
Hartley oscillator.
divider in the resonant tank. The Hartley oscillator has an inductive
voltage divider in the tank. The crystal oscillator shown in Fig. 15.2.17
has much greater frequency stability than the circuits in Figs. 15.2.15
and 15.2.16. Frequency stability of 1 part in 107 is easily achieved with
a crystal-controlled oscillator. If the oscillator is temperature-controlled
by mounting it in a small temperature-controlled oven, the frequency
Assuming a base-to-collector current gain of 50 times for each transistor, this circuit will give an input-to-output current gain of 2,500. This
high level of gain is not very stable if the ambient temperature changes,
but in many cases this drift is tolerable.
Figure 15.2.14 shows a circuit developed specifically to minimize
Fig. 15.2.17
Fig. 15.2.14 Differential amplifier.
Crystal-controlled oscillator.
stability can be increased to 1 part in 109. The resonant LC tank in the
collector circuit is tuned to approximately the crystal frequency. The
crystal offers a low impedance at its resonant frequency. This pulls the
collector-tank operating frequency to the crystal resonant frequency.
As the desired operating frequency becomes 500 MHz and greater,
resonant cavities are used as tank circuits instead of discrete capacitors
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15-74
ELECTRONICS
and inductors. A rough guide to the relationship between frequency and
resonant-cavity size is the wavelength of the frequency
␭ ⫽ 300 ⫻ 106/f
(15.2.3)
⫽ speed of light, m/s; f ⫽
where ␭ ⫽ wavelength, m; 300 ⫻
frequency, Hz. The resonant cavities will be smaller than indicated by
Eq. (15.2.3) because in general the cavity is either one-half or onefourth wavelength and also, in general, the electromagnetic wave velocity is less in a cavity than in free space.
The operating principles of these devices are beyond the scope of this
article. There are many different kinds of microwave tubes including
klystrons, magnetrons, and traveling-wave tubes. All these tubes employ
moving electrons to excite a resonant cavity. These devices serve as
either oscillators or amplifiers at microwave frequencies.
Lasers operate at a frequency of light of approximately 600 THz. This
corresponds to a wavelength of 0.5 ␮m, or, the more usual measure of
visual-light wavelength, 5 ⫻ 103 Å. Most laser oscillators are basically
a variation of the Fabry-Perot etalon, or interferometer. High-power
lasers may be gas, liquid, or ruby-based devices. High-power lasers are
used for machining and surveying. A more recent device, invented in
1961, is the semiconductor laser. The semiconductor laser is constructed using gallium arsenide (GaAs) as the semiconductor material.
These devices are quite small and can be controlled by means of fieldeffect transistors. The GaAs laser can be modulated (turned on and off )
at a 10 GHz rate, making it ideal for modern fiber-optic communications.
Most light is disorganized insofar as the axis of vibration and the
frequency of vibration are concerned. When radiation along different
axes is attenuated, as with a polarizing screen, the light is said to be
polarized. White light contains all visible frequencies. When white light
is filtered, the remaining light is colored, or frequency-limited. A single
color of light still contains a broad range of frequencies. Polarized,
colored light is still so disorganized that it is difficult to focus the light
energy into a narrow beam. Laser light is inherently a single-axis, single-frequency light. Lasers can be focused into an extremely narrow
beam, making them very accurate cutting tools and surveying devices.
Lasers are also widely used for high-speed, wide-frequency-band fiberoptic communications.
A radio wave consists of two parts, a carrier, and an information
signal. The carrier is a steady high frequency. The information signal
may be a voice signal, a video signal, or telemetry information. The
carrier wave can be modulated by varying its amplitude or by varying its
frequency. Modulators are circuits which impress the information signal
onto the carrier. A demodulator is a circuit in the receiving apparatus
which separates the information signal from the carrier. A simple amplitude modulator is shown in Fig. 15.2.18. The transistor is base-driven
with the carrier input and emitter driven with the information signal.
The modulated carrier wave appears at the collector of the transistor. An
the winding in the tank circuit and alters the operating frequency of the
oscillator.
The demodulator for an AM signal is shown in Fig. 15.2.20. The
diode rectifies the carrier plus information signal so that the filtered
106
Fig. 15.2.18 AM modulator.
FM modulator is shown in Fig. 15.2.19. The carrier must be changed in
frequency in response to the information signal input. This is accomplished by using a saturable ferrite core in the inductance of a Colpitts
oscillator which is tuned to the carrier frequency. As the collector
current in transistor T 1 varies with the information signal, the saturation
level in the ferrite core changes, which in turn varies the inductance of
Fig. 15.2.19
FM modulator.
voltage appearing across the capacitor is the information signal. Resistor R2 blocks the carrier signal so that the output contains only the
information signal. An FM demodulator is shown in Fig. 15.2.21. In this
circuit, the carrier plus information signal has a constant amplitude. The
information is in the form of varying frequency in the carrier wave. If
Fig. 15.2.20
AM demodulator.
inductor L1 and capacitor C1 are tuned to near the carrier frequency but
not exactly at resonance, the current through resistor R1 will vary as the
carrier frequency shifts up and down. This will create an AM signal
across resistor R1. The diode, resistors R2 and R3, and capacitor C2
demodulate this signal as in the circuit in Fig. 15.2.20.
Fig. 15.2.21
FM discriminator.
The waveform of the basic electronic timing circuit is shown in Fig.
15.2.22 along with a basic timing circuit. Switch S1 is closed from time
t 1 until time t 3. During this time, the transistor shorts the capacitor and
holds the capacitor at 0.2 V. When switch S1 is opened at time t 3, the
transistor ceases to conduct and the capacitor charges exponentially due
to the current flow through resistor R1. Delay time can be measured to
any point along this exponential charge. If the time is measured until
time t 6, the timing may vary due to small shifts in supply voltage or
slight changes in the voltage-level detecting circuit. If time is measured
until time t 4, the voltage level will be easy to detect, but the obtainable
time delay from time t 3 to time t 4 may not be large enough compared
with the reset time t 1 to t 2. Considerations like these usually dictate
detecting at time t 4. If this time is at a voltage level which is 63 percent
of Vc , the time from t 3 to t 4 is one time constant of R1 and C. This time
can be calculated by
t ⫽ RC
(15.2.4)
where t ⫽ time, s; R ⫽ resistance, ⍀; C ⫽ capacitance, F. A timing
circuit with a 0.1-s delay can be constructed using a 0.1-␮F capacitor
and a 1.0-M⍀ resistor.
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LINEAR INTEGRATED CIRCUITS
15-75
circuit devices are shown in Fig. 15.2.24. An IC costs far less than
circuits made with discrete components. Integrated circuits can be classified in several different ways. One way to classify them is by complexity. Small-scale integration (SSI), medium-scale integration (MSI),
large-scale integration (LSI), and very large scale integration (VLSI) refer
Fig. 15.2.22 Basic timing circuit.
An improved timing circuit is shown in Fig. 15.2.23. In this circuit,
the unijunction is used as a level detector, a pulse generator, and a reset
means for the capacitor. The transistor is used as a constant current
source for charging the timing capacitor. The current through the transistor is determined by resistors R1, R2, and R3. This current is adjustable by means of R1. When the charge on the capacitor reaches approximately 50 percent of Vc , the unijunction fires, discharging the capacitor
Fig. 15.2.24 Approximate physical dimensions of dual in-line pin (DIP) integrated circuits. All dimensions are in inches. Dual in-line packages are made in
three different constructions — molded plastic, cerdip, and ceramic.
Fig. 15.2.23 Improved timing circuit.
and generating a pulse at the output. The discharged capacitor is then
recharged by the transistor, and the cycle continues to repeat. The pulse
rate of this circuit can be varied from one pulse per minute to many
thousands of pulses per second.
Integrated Circuits
Table 15.2.4 lists some of the more common physical packages for
discrete component and integrated semiconductor devices. Although
discrete components are still used for electronic design, integrated circuits (ICs) are becoming predominant in almost all types of electronic
equipment. Dimensions of common dual in-line pin (DIP) integratedTable 15.2.4
Packaging
Semiconductor Physical
Signal devices
Plastic
Metal can
Power devices
Tab mount
Diamond case
Stud mount
Flat base
Flat pak (Hockey puck)
Integrated circuits
Dip (dual in-line pins)
Flat pack
Chip carrier (50-mil centers)
TO92
TO5, TO18, TO39
TO127, TO218, TO220
TO3, TO66
to this kind of classification. The cost and availability of a particular IC
are more dependent upon the size of the market for that device than on
the level of its internal complexity. For this reason, the classification by
circuit complexity is not as meaningful today as it once was. The literature still refers to these classifications, however. For the purpose of this
text, ICs will be separated into two broad classes: linear ICs and digital
ICs.
The trend in IC development has been toward greatly increased complexity at significantly reduced cost. Present-day ICs are manufactured
with internal spacings as low as 0.5 ␮m. The limitation of the contents
of a single device is more often controlled by external connections than
by internal space. For this reason, more and more complex combinations of circuits are being interconnected within a single device. There is
also a tendency to accomplish functions digitally that were formerly
done by analog means. Although these digital circuits are much more
complex than their analog counterparts, the cost and reliability of ICs
make the resulting digital circuit the preferred design. One can expect
these trends will continue based on current technology. One can also
anticipate further declines in price versus performance. It has been
demonstrated again and again that digital IC designs are much more
stable and reliable than analog designs.
For years, the complexity of large-scale integrated circuits doubled
each year. This meant that, over a 10-year period, the complexity of a
single device increased by 210 times, or over 1000 times, and, over the
period from 1960 to 1980 grew from one transistor on a chip to one
million transistors on a chip. More recently the complexity increase has
fallen off to only 1.7 times, or 1.710, or over 200 times in a 10-year
period. Manufacturers also have developed application-specific integrated circuits (ASICs). These devices allow a circuit designer to design
integrated circuits almost as easily as printed board circuits.
Linear Integrated Circuits
(See Fig. 15.2.24)
The basic building block for many linear ICs is the operational amplifier.
Table 15.2.5 lists the basic characteristics for a few representative IC
operational amplifiers. In most instances, an adequate design for an
operational amplifier circuit can be made assuming an ‘‘ideal’’ operational amplifier. For an ideal operational amplifier, one assumes that it
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15-76
ELECTRONICS
Table 15.2.5
Operational Amplifiers
Type
Purpose
Input bias
current, nA
Input
res., ⍀
Supply
voltage, V
Voltage gain
Unity-gain
bandwidth, MHz
LM741
LM224
LM255
LM444A
General purpose
Quad gen. purpose
FET input
Quad FET input
500
150
0.1
0.005
2 ⫻ 106
2 ⫻ 106
1012
1012
⫹ 20
3 to 32
⫹ 22
⫹ 22
25,000
50,000
50,000
50,000
1.0
1.0
2.5
1.0
has infinite gain and no voltage drop across its input terminals. In most
designs, feedback is used to limit the gain of each operational amplifier.
As long as the resulting closed-loop gain is much less than the openloop gain of the operational amplifier, this assumption yields results that
are within acceptable engineering accuracy. Operational amplifiers use
a balanced input circuit which minimizes input voltage offset. Furthermore, specially designed operational amplifiers are available which
have extremely low input offset voltage. The input voltage must be kept
low because of temperature drift considerations. For these reasons, the
assumption of zero input voltage, sometimes called a ‘‘virtual ground,’’
is justified. Figure 15.2.25 shows three operational amplifier circuits
and the equations which describe their behavior. In this figure S is the
Laplace transform variable. In these equations the input and output voltages are functions of S. The equations are written in the frequency
domain. A simple transformation to steady variable-frequency behavior
can be obtained by simply substituting cos ␻ t for the variable S wherever it appears in the equation. By varying ␻, the frequency in radians
per second, one can obtain the steady-state frequency response of the
circuit.
R1
R2
⫹
Output
⫺
⫹
Vdif
R1
R2
⫹
Vcm
Fig. 15.2.26
Difference amplifier.
The ratio of the common-mode gain to the difference gain is called the
common-mode rejection ratio (CMRR). In a well-designed difference amplifier, the CMRR is ⫺ 80 dB. Stated another way, the common mode
gain is 10,000 times smaller than the difference gain. The difference
amplifier is the most common input to instrumentation amplifiers and
analog-to-digital converter circuits. Bridge-connected transducers have
a common-mode voltage that is 100 times the difference voltage, or
more. Such transducers require a difference amplifier. The commonmode gain is highly dependent on the matching of the R1 resistors and
the R2 resistors. A 1 percent difference in these resistors causes the
CMRR to be degraded to ⫺ 30 or ⫺ 40 dB.
X1
⫹
⫺
R3
R2
R4
The operational amplifier circuits shown in Figs. 15.2.25, 15.2.26,
15.2.27, and 15.2.28, show only the signal wires. There are additional
connections to a dc power supply, and in some instances, to stabilizing
circuits and guard circuits. These connections have been omitted for
conceptual clarity.
One of the most useful analog integrated circuits is the difference
amplifier. This is a balanced input amplifier that is a fundamental component in instrumentation and control applications. The difference amplifier is shown in Fig. 15.2.26. This amplifier maximizes the voltage
gain for Voutput /Vdif , the difference gain, and minimizes the voltage gain
for Voutput /VCM , the common-mode gain. There are two resistors in the
circuit shown as R1 and two resistors shown as R2. The resistance
values of the two R1 resistors are equal, and similarly the resistance
values of the two R2 resistors are equal. The difference gain is given by
R2
Voutput
⫽⫺
Vdif
R1
⫺
⫹
Fig. 15.2.25 Operational amplifier circuits.
(15.2.5)
Vdif
X3
R1
R4
R2
⫺
⫹
X2
⫹
Vcm
Fig. 15.2.27
Instrumentation amplifier.
R3
Output
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LINEAR INTEGRATED CIRCUITS
An instrumentation amplifier is a high-grade difference amplifier. Although there are many implementations of the instrumentation amplifier, the three op-amp circuit is quite common, and will illustrate this
device. Figure 15.2.27 shows the three op-amp instrumentation amplifier. This is a two-stage amplifier. The first stage is composed of amplifiers X1 and X2 and resistors R1, R2, and R2. The second stage is
amplitude increase at the resonant frequency. For the circuit shown in
Fig. 15.2.28, ␻N is given by
⫺
R
Output
R
C
V1
Fig. 15.2.28 Modified Sallen-Key filter circuit.
composed of amplifier X3 and resistors R3, R3, R4, and R4. The
CMRR is usually much lower for the instrumentation amplifier, typically ⫺ 120 dB. The differential mode gain is much higher for the
instrumentation amplifier. For stability and frequency response considerations, the difference gain of a normal difference amplifier is usually
10 or less. For the instrumentation amplifier, the difference gain is
typically 1000. In the circuit shown in Fig. 15.2.27, the difference gain
of the first stage is given by
(2 ⫻ R2 ⫹ R1)
Vinternal
⫽
Vdif
R1
(15.2.9)
1
2 ⫺ (RB/RA)
(15.2.10)
and Q is given by
Q⫽
RB
1
RC
␻N ⫽
C
RA
15-77
The filter shown in Fig. 15.2.28 is a two-pole low-pass filter. Higherorder filters — four pole, six, etc. — can be constructed by cascading
sections of two-pole filters. Thus, a six-pole low-pass filter would consist of three circuits of the configuration shown in the figure. The components R, R, C, C, RA, and RB would vary in each filter section.
Continuing with the low-pass filter, there are three common variations in filter response: Butterworth response, Chebychev response, and
elliptical response. The Butterworth filter has no ripple in either the pass
band or the stop band. The Chebychev filter has equal ripple variations
in the pass band, but is flat in the stop band, and the elliptical filter has
equal ripple variations in both the pass band and the stop band. There is
a transition band of frequencies between the pass band and the stop
band. The Butterworth filter has the greatest transition band. The
Chebychev response has a sharper cutoff frequency characteristic
than the Butterworth response, and the elliptical response has the
sharpest transition from pass band to stop band. The circuits shown
in Figs. 15.2.27 and 15.2.28 can be used to realize Butterworth and
Chebychev filters. The elliptical filter requires a more complex circuit.
The high-pass filter can be formulated by substituting 1/S for S in Eq.
(15.2.8). The circuit configuration for a high-pass filter is shown in Fig.
15.2.29. Notice that only the position of the Rs and Cs have changed. In
general, the values of Rs, Cs, RA, and RB will change for the high-frequency filter, so it would be inappropriate to design a low-pass filter and
simply reverse the positions of the Rs and Cs.
R
(15.2.6)
RA
RB
The difference gain of the second is given by
R4
Voutput
⫽⫺
Vinternal
R3
(15.2.7)
The overall gain for the amplifier is the product of the individual stage
gains. In the typical case the gain of the first stage is 100, and for the
second stage is 10, giving an overall gain of 1,000. Integrated circuit
instrumentation amplifiers are available which include all of the circuitry in Fig. 15.2.27. The resistors are matched so that the circuit
designer does not have to contend with component matching.
A word about cost is in order. In 1994, high-grade operational amplifiers cost $0.50 for four op amps in a single IC chip, or about $0.125 for
each amplifier. The instrumentation amplifier is somewhat more expensive, but can be obtained for less than $5.00.
Filtering of electronic signals is often required. A filter passes some
frequencies and suppresses others. Filters may be classed as low pass,
high pass, band pass, or band stop. Figure 15.2.28 shows a low-pass
active filter circuit which is a modification of the Sallen-Key circuit. In
this circuit, the two resistors labeled R are matched and the two capacitors labeled C are matched. The frequency response of this circuit is
given by its transfer function:
Voutput
⫽
Vinput
1
␻
N
S ⫹ ␻ 2N
S2 ⫹
Q
(15.2.8)
In this equation, ␻N is the resonant frequency of this circuit, in radians per second, and Q is a quality factor that indicates the amount of
⫺
Output
C
C
R
V1
Fig. 15.2.29
High-pass filter section.
Band-pass and band-stop filters can be made by combinations of
low-pass and high-pass filter sections. Figure 15.2.30a shows the diagram configuration of filters to realize a band-pass filter. The low-pass
filter would be designed for the high-frequency transition, and the highpass filter would be designed for the low-frequency transition. Figure
15.2.30b shows the block diagram configuration for a band-stop filter.
The band-stop filter and the low-pass filter would be designed for the
low-frequency transition, and the high-pass filter would be designed for
the high-frequency transition. Band-pass and band-stop filters may also
be realized by means of special high-Q circuits. A more complete discussion of filter technology can be found in the references at the beginning of this section.
Table 15.2.6 lists some typical linear ICs, most of which contain
operational amplifiers with additional circuitry. The voltage comparator
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Input
Low-pass
filter
High-pass
filter
Output
Output
15-78
Frequency
(a)
Low-pass
filter
Summer
Output
Output
Input
High-pass
filter
Frequency
(b)
Fig. 15.2.30 (a) Band-pass and (b) band-stop filters.
is an operational amplifier that compares two input voltages, V1 and
V2. Its output voltage is positive when V1 is greater than V2, and
negative when V2 is greater than V1. The sample-and-hold circuit samples an analog input voltage at prescribed intervals, which are determined by an input clock pulse. Between clock pulses the circuit holds
the sample voltage level. This circuit is useful in converting from an
analog voltage to a digital number whose value is proportional to the
analog voltage. An analog-to-digital (A / D) converter is a signal-converting device which changes several analog signals into digital signals. It
consists of an input difference amplifier, an analog time multiplexer, a
sample-and-hold amplifier, a digital decoder, and the necessary logic to
interface with a digital computer. The A/D converter is programmable,
in that a computer can set up the device for the requisite number of input
channels and whether these input channels are differential inputs or
single-ended inputs. The A/D converter typically takes eight differential analog input signals and converts them to 10-, 12-, or 16-bit digital
signals. Typical A/D converters convert signals at a rate of 200,000
samples per second. Other types of A/D converters, such as flash converters, can convert over 10,000,000 samples per second.
A companion circuit to the A/D converter is the digital-to-analog
(D/A) converter, which is used to convert from digital signals to analog
signals. This is also a programmable device, but in general the D/A
converter is less complex than is the A/D converter. Voltage-regulators
and voltage references are electronic circuits which create precision dc
voltage sources. The voltage reference is more precise than the voltage
regulator. The voltage-controlled oscillator is a circuit which converts
from a dc signal to a proportional ac frequency. The output of a voltageTable 15.2.6
Devices
Linear Integrated-Circuit
Operational amplifier
Sample and hold
Analog-to-digital converter
Voltage regulator
Voltage-controlled oscillator
Voltage comparator
Digital-to-analog converter
Voltage reference
NE555 Timer /oscillator
controlled oscillator is usually a rectangular ac wave rather than a sine
wave. The NE555 timer/oscillator is a general-purpose timer/oscillator
which has been integrated into a single IC chip. It can function as a
monostable multivibrator, a free-running multivibrator, or as a synchronized multivibrator. It can also be used as a linear ramp generator, or for
time delay or sequential timing applications.
Table 15.2.7 lists linear ICs that are used in audio, radio, and television circuits. The degree of complexity that can be incorporated in a
single device is illustrated by the fact that a complete AM-FM radio
circuit is available in a single IC device. The phase-locked loop is a
device that is widely utilized for accurate frequency control. This device
produces an output frequency that is set by a digital input. It is a highly
accurate and stable circuit. This circuit is often used to demodulate FM
radio waves.
Table 15.2.8 lists linear IC circuits that are used in telecommunications. These circuits include digital circuits within them and/or are used
with digital devices. Whether these should be classed as linear ICs or
Table 15.2.7 Audio, Radio, and Television
Integrated-Circuit Devices
Audio amplifier
Dolby filter circuit
Intermediate frequency circuit
TV chroma demodulator
Video-IF amplifier-detector
Tone-volume-balance circuit
Phase-locked loop (PLL)
AM-FM radio
Digital tuner
Table 15.2.8 Telecommunication
Integrated-Circuit Devices
Radio-control transmitter-encoder
Radio-control receiver-decoder
Pulse-code modulator – coder-decoder (PCM CODEC)
Single-chip programmable signal processor
Touch-tone generators
Modulator-demodulator (modem)
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DIGITAL INTEGRATED CIRCUITS
digital ICs may be questioned. Several manufacturers include them in
their linear device listings and not with their digital devices, and for this
reason, they are listed here as linear devices. The radio-control transmitter-encoder and receiver-decoder provide a means of sending up to four
control signals on a single radio-control frequency link. Each of the four
channels can be either an on-off channel or a pulse-width-modulated
(PWM) proportional channel. The pulse-code modulator – coder-decoder
(PWM CODEC) is typical of a series of IC devices that have been designed to facilitate the design of digital-switched telephone circuits.
Integrated circuits are normally divided into two classes: linear or
analog ICs and digital ICs. There are a few hybrid circuits which have
both analog and digital characteristics. Some representative hybrid integrated circuits are listed in Table 15.2.9, and shown in Fig. 15.2.31. In
the superdiode, there are two modes of operation. When the input signal
is positive, the output of the op amp is negative, causing diode D2 to
conduct and diode D1 to block current flow. When the input is negative,
diode D2 blocks and diode D1 conducts. The output of the circuit is
zero when the input is negative, and the output is proportional to the
input when the input is positive. A normal diode has 0.5 to 1.0 V of
forward voltage drop. This circuit is called a super diode because its
switching point is at zero volts. There is a voltage inversion from input
to output, but this can be reversed by means of an additional op-amp
inverter following the super diode. The action of the circuit can be
reversed by reversing diodes D1 and D2. The limiter circuit, shown in
Fig. 15.2.31b, provides linear operation at low output voltages, either
positive or negative, and neither diode is conducting. When the input
voltage goes sufficiently negative, diode D1 begins to conduct, limiting
the negative output of the op-amp. Similarly when the input goes sufficiently positive, diode D2 begins to conduct, limiting the positive output of the op-amp. The Schmitt trigger circuit, shown in Fig. 15.2.31c,
has positive feedback to the op-amp, through resistors R1 and R2. If the
amplifier output is initially negative, the output will not change until the
input becomes as negative as the non-inverting terminal of the op-amp.
When the input becomes slightly more negative, the op-amp output will
suddenly switch from negative to positive due to the positive feedback.
The op-amp will remain in this condition until the input becomes sufficiently positive, causing the op-amp to switch to a negative output.
The dead-band circuit is similar to the limiter circuit except that for
input voltages around zero, there is no output voltage. Above a threshold input voltage, the output voltage is proportional to the input voltage.
The logarithmic amplifier relies upon the fact that the voltage drop
across a diode causes logarithmically varying current to flow through
the diode. This relationship is remarkably true for current changes of
106 to 1. The logarithmic amplifiers can have their outputs added together to effect a multiplication of the input signals.
until its ‘‘clock’’ input goes to a 1. At this time its output will stay in its
present state or change to a new state depending upon its input just prior
to the clock pulse. Its output will retain this information until the next
time the clock goes to a 1. The flip-flop has memory, because it retains
its output from one clock pulse to another. By connecting several flipflops together, several sequential states can be defined permitting the
design of a sequential logic circuit.
Table 15.2.11 shows three common flip-flops. The truth table, sometimes called a state table, shows the specification for the behavior of
each circuit. The present output state of the flip-flop is designated Q(t).
The next output state is designated Q(t ⫹ 1). In addition to the truth
R2
R1
D1
Input
D2
⫺
⫹
Output
(a)
V⫹
R3
D1
R4
R1
R2
Input
⫺
⫹
Output
R6
Digital Integrated Circuits
The basic circuit building block for digital ICs is the gate circuit. A gate
is a switching amplifier that is designed to be either on or off. (By
contrast, an operational amplifier is a proportional amplifier.) For 5-V
logic levels, the gate switches to a 0 whenever its input falls below
0.8 V and to a 1 whenever its input exceeds 2.8 V. This arrangement
ensures immunity to spurious noise impulses in both the 0 and the 1
state.
Several representative transistor-transistor-logic (TTL) gates are listed
in Table 15.2.10. Gates can be combined to form logic devices of two
fundamental kinds: combinational and sequential. In combinational logic,
the output of a device changes whenever its input conditions change.
The basic gate exemplifies this behavior.
A number of gates can be interconnected to form a flip-flop circuit.
This is a bistable circuit that stays in a particular state, a 0 or a 1 state,
D2
R5
V⫺
(b)
Input
R3
⫺
⫹
R1
Table 15.2.9
15-79
Output
R2
Hybrid Circuits
Superdiode
Schmitt trigger circuits
Switched-capacitor filters
Limiters
Dead-band circuits
Logarithmic amplifiers
(c)
Fig. 15.2.31 Hybrid circuits. (a) Superdiode circuit; (b) limiter circuit;
(c) Schmitt trigger circuit.
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15-80
ELECTRONICS
Table 15.2.10
The gates shown in Table 15.2.12 have only two inputs. As was seen
in Table 15.2.10, gates may have as many as eight inputs. In the case of
an AND gate, all its inputs must be 1 in order for its output to be a 1. For
an OR gate, if any of its inputs become a 1, then its output will become a
1. The NAND and NOR gates function in a similar way.
Boolean algebra is the branch of mathematics used to analyze logic
circuits. Boolean algebra has two operators: ⭈ , which indicates an AND
operation, and ⫹ , which indicates an OR operation: The ⫽ has the same
meaning in Boolean algebra as in ordinary algebra. The symbol for ‘‘X
not’’ is X⬘ (or sometimes X ). The identity element for the AND operation is 0; the identity element for the OR operation is 1. The rules for
Boolean algebra can be derived from set theory applied to a system in
which only two numbers exist, i.e., zero and one. These rules are summarized in Huntington’s postulates and DeMorgan’s theorem and are
listed in Table 15.2.13.
To facilitate the analysis of digital circuits and to aid in the application of the rules given in Table 15.2.13, Karnaugh maps are used. Typical two-variable and four-variable Karnaugh maps are shown in Fig.
15.2.32 along with the algebraic expressions represented by each map.
Looking at Fig. 15.2.32b and realizing each of the ones individually
would lead to the Boolean expression:
Digital Integrated-Circuit Devices
Type
54 /74*
No. circuits
per device
No. inputs
per device
00
02
04
06
08
10
11
13
14
20
21
30
74
76
77
86
174
373
374
4
4
6
6
4
3
3
2
6
2
2
1
2
2
4
4
6
8
8
2
2
1
1
2
3
3
4
1
4
4
8
Function
NAND gate
NOR gate
Inverter
Buffer
AND gate
NAND gate
AND gate
Schmitt trigger
Schmitt trigger
NAND gate
AND gate
NAND gate
D flip-flop
JK flip-flop
Latch
2
EXCLUSIVE OR
gate
D flip-flop
Latch
D flip-flop
OUT ⫽ W⬘X⬘YZ ⫹ WXYZ ⫹ WX⬘YZ ⫹ WXYZ⬘ (15.2.11)
NOTE: Example of device numbers are 74LS04, 54L04, 5477, and 74H10. The
letters after the series number denote the speed and loading of the device.
* 54 series devices are rated for temperatures from ⫺ 55 to 125°C. 74 series
devices are rated for temperatures from 0 to 70°C.
Realizing this expression directly would require four four-input AND
gates, and one four-input OR gates. The reduced expression
table, the Boolean algebra equations in Table 15.2.11 are another way to
describe the behavior of the circuits. The JK flip-flop is the most versatile of these three flip-flops because of its separate J and K inputs. The T
flip-flop is called a toggle. When its T input is a 1, its output toggles,
from 0 to 1 or from 1 to 0, at each clock pulse. The D flip-flop is called a
data cell. The output of the D flip-flop assumes the state of its input at
each clock pulse and holds this data until the next clock pulse. The JK
flip-flop can be made to function as a T flip-flop by applying the T input
to both the J and K input terminals. The JK flip-flop can be made to
function as a D flip-flop by applying the data signal to the J input and
applying the inverted data signal to the K input. Some common IC
flip-flops are listed in Table 15.2.10.
Various types of gates are shown in Table 15.2.12. Combinational
logic defined by means of these various gates is used to define the input
to flip-flops, which serve as memory devices. At each clock pulse, these
flip-flops change state in accordance with their respective inputs. These
new states are retained in the flip-flop and also applied to the gates. The
output of the gates change (with only a small delay due propagation
time), and at the next clock pulse the flip-flops will change to the next
state as directed by the gates.
Table 15.2.11
OUT ⫽ WXY ⫹ X⬘YZ
SUM ⫽ AB⬘C⬘ ⫹ A⬘BC⬘ ⫹ A⬘B⬘C ⫹ ABC
CO ⫽ AB ⫹ BC ⫹ AC
Graphic
symbol
Algebraic function
Truth table
Q(t ⫹ 1) ⫽ JQ⬘(t) ⫹ K⬘Q(t)
J K Q(t) Q(t ⫹ 1)
0 X
0
0
1 X
0
1
X 0
1
1
X 1
1
0
Q(t ⫹ 1) ⫽ TQ⬘(t) ⫹ T⬘Q(t)
T Q(t) Q(t ⫹ 1)
0
0
0
0
1
1
1
0
1
1
1
0
Q(t ⫹ 1) ⫽ D
D Q(t) Q(t ⫹ 1)
0
0
0
0
1
0
1
0
1
1
1
1
S
JK flip-flop
Clock
J
⬍
K
Q
Q
R
T flip-flop
Clock
T
⬍
Q
Q
D flip-flop
Clock
D
⬍
Q
Q
(15.2.13)
(15.2.14)
Adders are available, not for a single bit, but rather for 4 bits, 8 bits, or
16 bits. There is only one carry bit for the whole device. The devices in
Table 15.2.14 will be described in words, but the specifications for each
of these devices has a truth table as a part of the product description. An
arithmetic logic unit (ALU) or accumulator can perform several simple
logical or arithmetic operations, such as add, complement, shift left, and
shift right. In addition, the set of instructions which command these
operations will also be decoded by the ALU. As one might expect, the
Boolean expression for a 16-bit or 32-bit ALU can be formidable. A
Flip-Flop Sequential Devices
Name
(15.2.12)
requires only two three-input AND gates and one two-input OR gate, a
simpler and less expensive logic circuit.
Some of the more common large-scale integrated circuits are listed in
Table 15.2.14. The adder can add two binary numbers together and
output the result. For a single bit adder which adds a bit from word A, a
bit from word B, and a carry bit, C, the Boolean expressions for the two
outputs of the adder are
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DIGITAL INTEGRATED CIRCUITS
Table 15.2.12
Name
Combinational Gate Logic
Algebraic
function
Graphic symbol
Truth
table
Q ⫽ xy
x y Q
0 0 0
0 1 0
1 0 0
1 1 1
OR
Q⫽x⫹y
x y Q
0 0 0
0 1 1
1 0 1
1 1 1
Inverter
Q ⫽ x⬘
x Q
0 1
1 0
Buffer
Q⫽x
x Q
0 0
1 1
Q ⫽ (xy)⬘
x y Q
0 0 1
0 1 1
1 0 1
1 1 0
AND
NAND
Q ⫽ (x ⫹ y)⬘
NOR
Q ⫽ xy⬘ ⫹ x⬘y
⫽x⫹y
EXCLUSIVE-OR
x y Q
0 0 1
0 1 0
1 0 0
1 1 0
x
0
0
1
1
y Q
0 0
1 1
0 1
1 0
parity generator/checker is a circuit that can make a cyclic redundancy
code which can be used to check the accurate transmission of data. Shift
registers perform only the shifting functions of ALUs. Counters are
made in many configurations — up-down counters, binary-coded-decimal counters, Johnson counters, etc. A decoder can best be explained by
a simple example. A three- to eight-line decoder will be described. The
three input lines can have eight binary variations, 000, 001, 010, 011,
100, 101, 110, and 111. For each of these input conditions one, and only
one, of the output lines will be set. The input signal is said to have been
decoded. The encoder performs the related operation of encoding from
Table 15.2.13
Algebra
Rules for Boolean
X⫹0⫽X
X⫹1⫽1
X ⫹ X⬘ ⫽ 1
(X ⬘ )⬘ ⫽ X
X⫹Y⫽Y⫹X
X ⫹ (Y ⫹ Z) ⫽ (X ⫹ Y) ⫹ Z
X ⫹ X⭈Y ⫽ X
X⭈1 ⫽ X
X ⭈ X⬘ ⫽ 0
X⭈X ⫽ X
X⭈0 ⫽ 0
X⭈Y ⫽ Y⭈X
X ⭈ (Y Z) ⫽ (X ⭈ Y)Z
X ⭈ (X ⫹ Y) ⫽ X
DeMorgan’s theorem:
(X ⫹ Y)⬘ ⫽ X⬘ ⭈ Y⬘
(X ⭈ Y)⬘ ⫽ X⬘ ⫹ Y⬘
15-81
eight to three lines. Depending on which one input line is set, an appropriate output bit pattern is set. Encoders and decoders are available in 2
to 4, 3 to 8, and 4 to 16 line configurations. The multiplexer/demultiplexer performs a similar function. The multiplexer has a data input
terminal and control input terminal and several data output terminals. It
switches a single train of information, ones and zeros, on an input line,
and routes this information to one of many output lines depending on
the bit pattern placed on a control input. The demultiplexer performs the
reverse operation. Multiplexers/demultiplexers are also configured in 2
to 4, 3 to 8, and 4 to 16 line devices.
Fig. 15.2.32
Karnaugh maps of typical logic functions.
There are a wide variety of display controllers and drivers. These are
devices which interface between digital circuits and displays which
humans can use. Light-emitting diodes, liquid crystals, and fluorescent
displays have different drive requirements. The digital device may perform in binary, but the display requires decimal. In many cases, the
display driver is separate from the display itself, but more recently the
display driver has been incorporated in the display.
The complexity of integrated circuits is growing continually. The
simple gates and flip-flops discussed so far are still used to some extent.
Logically, these gates and flip-flops can be integrated into larger singlechip circuits called large-scale integrated circuits (LSI). Combinations
of these circuits are grouped together on single chips called very large
scale integrated circuits (VLSI). VLSI chips contain 500,000 to more
than 2,000,000 gates. VLSI chips have enabled the explosive growth of
personal computers, by allowing ever-increasing functionality at lower
cost. These circuits are custom-designed VLSI circuits which are developed for large-volume applications. Bridging the gap between LSI and
custom VLSI circuits are a series of linear and digital user-designed
circuits.
The first of these user-designed circuits is called programmable logic
arrays (PLAs) or programmable array logic (PAL). The PLA grew out of
the programmable read-only memory (PROM). A programmable readonly memory is a computer memory which can be programmed one
time, and then can be read many times. The PROM consists of a series
of gates, initially all ones, each of which can be electrically altered to be
a zero. A PROM might be logically organized to be 1 bit wide by 64K
bits long, or 4 bits by 16K bits, 8 bits by 8K bits. The example just given
is a series of 64K-bit PROMs. PROMs are available up to 4M bits on a
single chip. The PLA is also field programmable, but has a more limited
programming capability than the PROM. The limited capability makes
Table 15.2.14 Large-Scale Digital
Integrated Circuits
Adder
Arithmetic logic unit
Shift register
Multiplexer
Demultiplexer
Display controller-driver
Accumulator
Parity generator-checker
Encoder
Decoder
Counters
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15-82
ELECTRONICS
it less costly to program. The PLA is adaptable to replacing Boolean
functions in both combinational logic and sequential logic circuits. The
PLA can replace 10 to 40 gate and flip-flop chips, resulting in a simpler,
more reliable, less costly design.
There are two other approaches to application-specific integrated circuits, (ASICs). These are field-programmable gate arrays (FPGAs) and
standard cells. The FPGA is sometimes referred to as a ‘‘sea of gates.’’
The chip is designed with a series of identical gates, but without any
interconnections between them. The user specifies the interconnections,
and thus makes the chip specific to the application. The manufacture of
the chip cannot be completed until the interconnections have been specified, however. There was initially some concern that the circuit designer would be losing design control, but this has largely disappeared
now that alternative sources of supply are available. The customizing
production drawings and specifications can be pulled from one manufacturer, and sent to another. The standard cell device is similar to an
FPGA, except that some interconnections are specified to connect gates
to form subcircuits. The design by the user consists of selecting the
individual cells, AND gates, OR gates, flip-flops, etc. and specifying the
interconnections of these standard cells. The standard cells may be as
simple as an AND gate or as complex as a 16K-bit memory or a microprocessor. The standard cell also allows the user to specify light-current
cells or heavy-current cells. The standard cell gives the user much more
flexibility in the design. Standard cells have also been extended to include linear circuits, op amps, difference amplifiers, limiters, etc. Some
standard cell chips are suitable for hybrid digital and linear circuits on a
single chip.
Two problems arise as more complex circuitry is incorporated in
ASICs, and also in microprocessors (which will be discussed in the next
section). These devices require more input and output data pins than are
available in dual in-line packages (see Fig. 15.2.24), and they generate
more heat on the chip. One solution to the pin problem is the pin array
package, shown in Fig. 15.2.33. These packages still maintain 0.1-in
spacing between pins, but the pin count has been increased to 132 and
208 in the examples shown. The package size has increased from dual
in-line packages, and the force necessary to install and remove the
Pin array
with 132 pins
Bottom view
Pin array
with 208 pins
Bottom view
Fig. 15.2.33 Typical pin grid array packages. (All pins are on a 0.1 in grid.)
package is not appreciably greater than for dual in-line packages. There
are additional microcircuit packages at various stages of development
and standardization that have up to 500 pins and 0.05-in pin spacing.
Higher-speed and higher-density digital electronics lead to higher
heat dissipation in devices. One solution is to provide heat sinks and
fans to remove heat. A more recent development is the heat pipe. The
heat pipe consists of boiling fluid/vapor refrigerant in a closed chamber.
The semiconductor boils the fluid to change it to the vapor state. A pipe
conducts the vapor to a vapor-to-air heat exchanger, where the vapor is
condensed. The heat pipe also provides a return path for the fluid to
return to the semiconductor. An attractive feature of the heat pipe is that
there is no requirement for a mechanical compressor.
A further problem for complex electronic circuits is proof of design
and proof of manufacture testing. To adequately test a device, it is
necessary to have access to additional test points in the circuit, beyond
those needed to interface in actual operation. With a limited number of
output pins and highly complex internal logic, the design of the circuit
must include a fairly long test vector that will excite the various circuit
functions. In addition, a fairly long results vector will be generated by
the circuit. One approach to test design is called boundary scan design,
described in specification IEEE 1149.1 of the Institute of Electrical and
Electronics Engineers. This specification outlines a four- or five-wire
test circuit, and the internal circuitry, which allows a test vector to be
entered serially onto the chip, and allows a results vector to be serially
unloaded from the chip. Internal circuitry is required to accept each bit
of the input test vector and to generate each bit of the results vector. The
serial transfer is a tradeoff between the time required to enter and retrieve the test vectors and the cost of additional input/output pins for the
device. Serial transfer would be unacceptable for normal operations, but
is all right for test purposes.
Computer Integrated Circuits
One of the devices which has become feasible as a result of VLSI is the
microprocessor. A complete computer can be built in a single IC device.
In most cases, however, several devices are employed to build a complete computer system. In most computers, the cost of the microprocessor is negligible compared with the total system cost. Not long ago, the
central processing unit (CPU) was the most expensive part of a computer.
The microprocessor provides the total CPU function at a fraction of the
earlier cost.
The power of a microprocessor is a function of its clock speed and the
size and number of its registers. Clock rates vary from 1 to 200 MHz.
Common register sizes are 8, 16, 24, 32, and 64 bits. Most personal
computers currently use 32-bit registers. Commonly microprocessors
have 12 to 16 registers. Other factors affect processor power and speed.
Most microprocessors have a data bus, an address bus, and a set of
control and power supply terminals. The number of each of these buses
may vary. A small inexpensive microprocessor might have 8 data lines,
16 address lines, and 8 control/power supply lines. A large sophisticated microprocessor might have 64 data lines, 24 address lines, and 20
control/power supply lines. Some microprocessors have 32-bit internal
registers and only 16 data bit terminals. The 16 data terminals are timemultiplexed to allow the complete 32-bit word to be entered onto the
microprocessor chip in two read cycles. Once in the microprocessor, the
32-bit registers communicate with each other over a data bus which is a
full 32 bits wide. This scheme reduces the number of terminals on the
microprocessor, and reduces its cost, with a slight timing penalty.
Clock speed is limited by clock skew and parasitic capacitance. There
is a finite time required for electric pulses to move along wires and for
gates to change state. Typically electric pulses move along wires at
about half the speed of light. Gates change state in a few nanoseconds.
Parasitic capacitance is the stray capacitance that exists between any
circuit and ground. When the gate changes state, this capacitance must
be charged. The charging current increases linearly with switching rate
and voltage. Because of the small circuit components and short wire
lengths, the parasitic capacitance is much smaller for circuits on a chip
compared with circuits on printed boards. Modern microprocessors use
an on-chip clock rate that is 3 times as fast as the off-chip clock.
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COMPUTER APPLICATIONS
The microcontroller is a device similar to the microprocessor, in that it
is also a CPU. Microcontrollers are configured for controlling applications. In general, they have greater input and output capability. It is very
easy to control a single output bit for turning on an electric relay. With a
microprocessor, 8 bits are switched together, but with a microcontroller
individual bits can be switched. Some microcontrollers have 256 internal registers, rather than the 15 to 20 of a microprocessor. Microcontrollers are often stand-alone single-chip devices, and are a less
expensive system than a microcomputer. Typical applications for microcontrollers are microwave oven controllers, washer and dryer controls, and automobile engine controls.
Memory is an important part of a computer system. Memory can be
divided into two broad classes: volatile and nonvolatile. Volatile memory forgets what it contains when the power is removed from it. Nonvolatile memory retains its contents when power is removed. Randomaccess memory (RAM) is volatile memory that is sometimes called main
memory. This is the memory that the CPU uses for most of its operations. RAM is a semiconductor memory. RAM may be dynamic memory or static. In a static RAM the memory elements are flip-flops, and
retain their memory for as long as power is applied. Dynamic RAMs are
also semiconductor memories, but hold the information as charges on
capacitors. This charge will leak off even though the power is still
applied to the memory. To retain the memory in a dynamic RAM, it is
necessary to continually refresh the memory. This is typically done
every 10 ms. Dynamic RAMs are much cheaper and have less loss than
static RAMs. Large banks of memory are made with dynamic RAMs.
Small memories, or memories with special requirements, are made with
static RAMs. Typical dynamic RAMs are 256K, 1M, and 4M bytes.
Typical static RAM may be as small as 1K bytes and can be as large as
1M bytes.
Nonvolatile memory can take several forms: ROMs, disk storage,
compact disks, and tapes. A read-only memory (ROM) is a set of flipflops which are programmed to take a particular state. This programming may be installed at the time of manufacture. Some devices are
programmable by the user (PROMs). A PROM is typically all ones as
shipped. The individual memory cells may be programmed to a zero or
left a one. Programming memory cell is done by applying a high voltage
to a programming pin when that cell’s address is set. Normally the
ROM or PROM is written once and read many times. An EPROM is an
erasable, programmable read-only memory. The EPROM is programmed in the same way as a PROM. To erase the memory, the
EPROM is placed under a high-intensity ultraviolet light. When the
EPROM is erased, all of the memory is lost, so the entire program must
be reentered.
Disks and tapes are magnetic storage devices. When power is removed from these devices, the memory is retained due magnetic retentivity. Magnetic memory can only be read (or written) by moving the
magnetic medium under a read/write head. Compact disks are a form of
read-only memory. In the compact disk, the information is stored optically rather than magnetically.
The magnetic tape is a removable nonvolatile memory device. Hard
disks are usually not removable. Floppy disks are removable, but for
large quantities of data, the tape medium is cheaper and more convenient. Tape is either in a cartridge or on a reel. Newer tape cartridges
have large volumes, 50M bytes to 1000M bytes. The trend is that cartridge tape is replacing reel tape as a storage means. Magnetic tape can
store computer data for over a year, and with a periodic refreshing can
store data indefinitely.
In addition to the CPU and storage devices, other integrated circuits
have been designed for computer systems. These are listed in Table
15.2.15. The tristate buffer is a switching amplifier which can be a zero,
a one, or high impedance. When a computer bus line can be driven by
several devices, all of the devices are high impedance except the one
which is controlling the bus line at that time. Typically eight tristate
buffers are contained in one IC package. The tristate transceiver is similar to the tristate buffer except that it allows the bus signal to be received
as well as written. The parallel interface adapter is a device that permits a
parallel output device, such as a printer, to be connected to a computer
Table 15.2.15
15-83
Digital Computer Integrated-Circuit Devices
Tristate buffer
Parallel interface adapter
Universal asynchronous transmitterreceiver
Analog-to-digital converter
Programmable timer
Programmable interrupt controller
Tristate transceiver
Programmable peripheral interface
Floppy /hard disk controller
Digital-to-analog converter
Direct memory access controller
bus. There are several variations of the parallel interface adapter. Some
control two, three, four, or more parallel devices. Some have been designed to work with Intel microprocessors, others to work with Motorola microprocessors. The programmable peripheral interface is a parallel
interface adapter designed for use with Intel computers. The universal
asynchronous receiver-transmitter (UART) is a device which will allow a
serial output device to be connected to a computer bus. The UART has a
parallel connection to the computer bus, but provides a serial output to
the peripheral device. The UART is programmable, so that it can be set
for 7-bit or 8-bit transmission. It can be set for even or odd parity
checking, and the transmitting/receiving speed can be set. The floppy/
hard disk controller is a programmable device which can handle the
interfacing requirements for floppy and/or hard disks. Analog-to-digital
and digital-to-analog converters allow the digital computer to interface
with analog signals. The programmable timer is a timing circuit which
can be programmed by the computer either to generate an interrupt
periodically, or to generate an interrupt after a programmed time. An
interrupt is a signal on one of the control lines of the computer bus. The
direct memory access controller (DMA) is a CPU type of device which can
control the computer bus. The direct memory access controller generates a DMA request signal on a computer control line. The main CPU
will complete the portion of the instruction that it is executing, and
generate a DMA grant signal. The DMA controller then transfers data
into or out of main memory or disk memory. Since the CPU is not
involved in the memory transfer, it can proceed with the instruction it is
executing, as long as that instruction does not require the computer bus
that the DMA controller is using. DMA controllers can off-load the
CPU, allowing the overall system to have more power. The programmable interrupt controller is a general-purpose semiconductor which can be
used in various types of computer printed boards to service interrupts.
Computer Applications
The computer is universally used, and a complete discussion of computer usage is beyond the scope of this section. Computer techniques are
used in electronic design so extensively that some description of these
electronic applications of computers are necessary in order to understand modern electronic design.
Computers can be used as control devices. The computer might be a
controlling device to simply turn a pump on and off. On the other hand,
a variable displacement pump might incorporate a microcontroller in its
feedback control loop in place of pilot hydraulic circuits. Entire processes may be controlled by computers. For example, modern paper
machines use computers throughout for control purposes. Computer
terminals at various locations show the operation conditions throughout
the machine. Entire chemical-producing plants are under computer
control.
Some automotive applications include control of air, fuel mixture,
and spark timing in an automobile engine. This is typical of an application in which there are several input variables that must be measured,
and several controlled outputs. The input variables that must be measured are air temperature, manifold temperature, crankcase temperature,
engine speed, fuel flow, etc. The outputs which are controlled by the
computer are ignition timing and fuel/air mixture. Automobile brakes,
transmissions, and suspensions are automotive subsystems that incorporate microcontrollers for improved automobile performance. Microcomputers may also be used to reduce the complexity of automobile
wiring. In such a system, battery power is distributed to all parts of the
car along a single wire. Intelligence for the control of lights and acces-
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15-84
ELECTRONICS
of the filter to a previous part. The IIR filter incorporates feedback from
a later part of the filter to an earlier part. The FIR filter normally requires a greater length to accomplish the filtering objective than does
the IIR filter. Because the IIR filter incorporates feedback, it is possible
for it to be unstable. The FIR filter cannot be unstable. Both FIR and IIR
filters employ a clock to generate a fixed delay from point to point in the
filter. The clock delays are shown as ⌬T in the figure. Each delay is a
clock period. FIR filters are often 45 to 75 delays long. IIR filters are
usually 8 to 10 delays long. The effective delay for an IIR filter is longer
than the actual number of delays, because of the feedback in these
filters. Although the infinite response suggests that the filter never completes its response, in a practical sense the IIR filter response is finite.
The design approach for FIR filters and IIR filters is quite different. For
an FIR filter, the filter coefficients A0 through An are designed to optimally fit a desired filter response curve. The response of this optimal
filter often has excessive sidebands, that is, high output in the nonpass
frequency range. The sidebands can be reduced by applying a window
function, which modifies the filter coefficients. The effect of the window function is to broaden the transition region between the passband
and the stopband, and reduce the magnitude of the sidebands. The response equation, or transfer function, for the FIR filter with even symmetry is
sories flows along a single control wire. Relays are provided at each
lamp, switch, etc. to interface between the control and battery wire.
Messages along the control wire are time-multiplexed so that all devices
share the same control wire.
Testing, data acquisition, and analysis is another use of computers.
Modern testing systems make extensive use of computers. Either digital
or analog signals can be incorporated into a test system. Analog-to-digital converters convert analog signals into digital signals which the computer can use. Within the computer, a model of the process being tested
can be constructed, and monitored by the test data being accumulated.
Within the computer, signals can be generated which are not being
actually measured. For example, the velocity of a mass may be measured, but the acceleration or the position of the mass may be calculated
within the computer. Highly reliable systems can be developed using
electronic control and computers, if redundancy is incorporated into the
measuring and computer systems. In the aerospace industry, these systems are called fly-by-wire control systems. The space shuttle and supersonic fighters are examples of craft using fly-by-wire. In such systems,
failure of a computer, a controller, or a sensor must not cause a failure of
the craft.
Data acquisition systems give the user much greater insight into the
data being taken. Limitations and anomalies of the measured data become much more apparent than with analog data. The need for signal
conditioning becomes imperative in a digital data acquisition system.
Signal processing is an integral part of any digital data system. Analog
signals can be filtered as a part of signal conditioning, but after the
signal has been converted to digital data, digital filtering may also be
used. Digital filters can be broken into two classes: finite impulse response (FIR) and infinite impulse response (IIR) filters. As can be seen in
Fig 15.2.34, the FIR filter does not have any feedback from a later part
H(␻) ⫽
冘 A cos (␻i)
i⫽n
i
(15.2.15)
i⫽0
In this equation, the frequency ␻ is a continuous variable in radians per
second. Although the filter is a discrete device, its frequency response
can be viewed as continuous. The digital signal is discrete, and when a
signal is converted from digital to analog, this conversion adds its own
modification of the signal.
A0
A1
A2
A3
A4
An⫺1
Input
⌬T
⌬T
⌬T
⌬T
⌬T
⌬T
An
⌺
Output
⌺
Output
(a)
A0
A1
A2
A3
Input
⌺
⌬T
⌬T
⌬T
An⫺1
⌬T
⌬T
B1
B2
B3
Bm⫺1
Bm
(b)
Fig. 15.2.34 (a) Finite impulse response (FIR) and (b) infinite impulse response (IIR) digital filters.
An
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POWER ELECTRONICS
The design approach for an IIR filter is quite different. For an IIR
filter, the first step is to design a low-pass analog filter for a particular
filter response, such as a Butterworth, Chebychev, or Cauer response.
The analog filter can then be converted to a low-pass digital filter using
the bilinear transform. Finally, the low-pass digital filter can be converted to a high-pass, band-pass, or band-stop filter. The resulting filter
has a transfer function given by
冘 A cos (␻i)
H(␻) ⫽
1 ⫺ 冘 B cos (␻i)
i⫽n
i
i⫽0
i⫽m
(15.2.16)
i
i⫽1
A more complete discussion of digital filter technology can be found in
the references at the beginning of this section. Further help in digital
filter design can be obtained by using a computer program, such as
MATLAB.
Digital filter calculations consist of a series of mathematical operations which follow a consistent pattern. Each calculation is performed
by retrieving a number, multiplying it by a coefficient, adding it to
another number, and storing the result. This is the operation at each of
the summation points in the FIR and the IIR. A microprocessor called a
digital signal processor (DSP) has been developed for this purpose. DSPs
complete a retrieve, multiply, add, and store in a single clock pulse (30
ns), several times faster than a general-purpose microprocessor. DSP
microprocessors are widely used in test equipment, telephone equipment, television, and military equipment. Because of the wide usage,
DSP microprocessors are quite inexpensive. DSPs were developed in
order to increase the bandwidth of digital signal processing. It should be
further noted that digital filters can employ parallel processing, in which
several DSP chips can operate in parallel to further increase bandwidth.
Digital signal processing is not limited to analog and digital filtering.
Digital data may also be improved by using smoothing algorithms, or by
removing inconsistent data points, such as outliers. Insight into data
content can also be obtained by statistical analysis. Digital signal processing is also used to improve the quality of voice signals, and to
enhance video signals.
Digital Communications
The bandwidth required to transmit information depends on the digital
pulse rate. A fundamental relationship exists for the bandwidth required
for the transmission of digital or analog information. For analog signals,
the bandwidth depends on the highest frequency component in the signal. For digital circuits, the bandwidth depends on the maximum pulse
rate. The relationship between the two is known as the Nyquist criterion. The minimum pulse rate pr necessary to transmit an analog signal
digitally must be twice the highest frequency f:
pr ⫽ 2f
(15.2.17)
This is a bilateral relationship. The frequency bandwidth BW of a transmission system must be equal to at least half the pulse rate:
BW ⫽ pr/2
(15.2.18)
These conditions are minimum requirements. A transmission system
that has greater bandwidth can support slower pulse rates, and similarly,
a high-pulse-rate system can be used to pass a low-frequency signal. A
higher pulse rate than the Nyquist criterion will approximate an analog
signal waveform more accurately, but is not necessary to pass the information. Voice-grade lines have a bandwidth of 4,000 Hz, and will pass
8,000 bits per second (bps). Twisted-pair cable, properly terminated,
can pass about 1 MHz or 2 Mbps. Coaxial cable can pass 100 to 200
MHz. To obtain greater bandwidths, light is being used to transmit
information, rather than electricity. Inexpensive fiber-optic devices can
be used for distances up to several hundred feet. For longer distances,
phase-locked lasers and single-mode fiber-optic cable is being employed. Single-mode fiber is quite small and somewhat hard to terminate. Tooling to facilitate connecting single-mode cable is becoming
more readily available.
15-85
Electric instrumentation has several communication standards which
facilitate the connection of measuring systems. A low-speed instrument
bus, IGIB, has been developed and described in a standard, IEEE 488.
For higher communication rates, a CAMAC crate has been standardized. These standards give physical characteristics, such as connectors
and plugs, mounting dimensions, printed board sizes, etc. These standards also define communications protocol, bus timing, and connector
pin assignment.
In most computers, the interconnection of computer printed boards is
accomplished using a ‘‘mother board.’’ The mother board may contain
the CPU, semiconductor memory, and several input/output devices.
While these devices are somewhat arbitrary, it is essential that the
mother board contain a computer bus structure. The computer bus is
defined by a standard which defines the type connector, the connector
circuit configuration, and the computer communication protocol. There
are several agencies which define computer bus standards in the United
States and around the world. Some of the more common IEEE bus
standards are listed in Table 15.2.16.
Power Electronics
Power electronics can be divided into three major groups: motor drives,
power supplies, and power amplifiers. Motor drives can be divided into
dc motor drives and ac motor drives. Power supplies include power
supplies for electronic circuits, battery back-up systems, electric power
applications, and induction/dielectric heating supplies. Power amplifiers cover other power conditioning topics.
The power for dc motor armatures can be derived from thyristor
circuits like those shown in Fig. 15.2.9. Single-phase bridge circuits are
used for 5-hp drives and smaller. Three-phase bridge circuits are used
for drives larger than 5 hp. A single set of six thyristors can supply
power for about 300 hp. Above 300 hp, multiple sets of thyristors must
be used in parallel. Mill drives have been built with more than
10,000 hp provided by thyristors.
The control of dc motors whether powered by thyristors or by dc
generators is accomplished electronically. Control of individual drives
can be accomplished by tachometer feedback or by armature voltage
feedback. The speed-regulation accuracy for armature feedback is 5
percent; for tachometer feedback speed-regulation accuracy is from 0.1
to 1.0 percent. When two drives must be coordinated with each other, as
in a continuous-web processing machine, they can be regulated to control torque, speed, position, draw, or a combination of these parameters.
Torque controls can be achieved using dc motor armature current for a
feedback signal. Speed-control signals are derived as for single motors.
Position or draw control can be accomplished by using selsyn ties or
dancer rolls. A dancer roll is a weight- or spring-loaded roll that rides on
the web. It is free to move up and down, and as it does, a signal is taken
from its position to serve as a feedback for the drive regulator before the
dancer or after it.
Coordination of the motions of two or more drives requires tracking
of the drives in both steady-state and transient conditions. Linearity of
the control and feedback signals determine steady-state tracking. Provision must be made for both low-speed and high-speed matching signals.
Transient matching requires that signals not only be the right magnitude
but also arrive at the right time. An example will serve to illustrate this
point. Suppose it is desired to have two drives with tachometer feedback
Table 15.2.16
IEEE Computer Bus Standards
Identifier
Common
name
Description
IEEE 488
IEEE 796
IEEE 961
IEEE 1014
IEEE Proposal
IEEE Proposal
GPIB
Multibus
STD bus
VMEBUS
VXIBUS
MXIBUS
General-purpose instrument bus
General-purpose computer bus
Standard computer bus
European computer bus
Instrumentation adaptation of VMEBUS
Extension of VXIBUS
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ELECTRONICS
which have a continuous web between them. One way to accomplish
this would be to designate one drive as a master and the other as a slave.
The tachometer on the master drive would serve as its own feedback
signal and as the reference or command signal for the slave drive. The
slave drive would have its own feedback from its own tachometer and
so its regulator would try to minimize the difference between the two
tachometer signals. On a transient basis the master drive will always
start before the slave. An alternate and more common arrangement is to
provide a common reference for both drives and let each drive receive
its command signals at the same instant.
Digital computers are being used on-line in mills and continuous
processing industries. DC motors can be controlled by either analog or
digital regulators. With the greatly reduced cost of integrated circuits,
digital regulators are being increasingly used.
For digital speed control, feedback is taken from a digital tachometer,
which generates a pulse frequency proportional to motor speed. An
adjustable frequency reference is compared with the feedback, and the
difference is applied to an up/down counter. The output of the counter is
converted to a dc control signal for the motor controller. In this system
the dc drive functions as a form of phase-locked loop, and in fact, a
phase-locked loop can be substituted for the up/down counter in digital
speed control. For digital position control, a shaft encoder is used for
feedback. In a shaft encoder, a single turn of the motor shaft is encoded
in a binary code. For example, an eight-track shaft encoder can define
28, or 256, parts of a revolution. The output of the shaft encoder is
compared with a digitally derived reference code. The difference between the feedback and the reference is converted into a dc signal which
controls the motor, and holds the desired position. Sometimes the position encoder is mounted on the driven machine. The accuracy of sensing
the machine position is improved. Accurate milling machines have
feedback from machine position, rather than from the drive motor.
Computers can be used to derive a speed reference or a position reference for several motors, and thus, an entire process line can be controlled. Modern high-speed paper making machines are one example of
coordinated computer control of several motors. Graphic terminals,
which display the entire paper making machine, are located at various
positions allowing several operators to coordinate the operation of the
machine.
DC motors can also be supplied from a fixed dc voltage such as a
battery or a fixed dc power supply. Inherently, fixed voltage on the field
and on the armature of a dc motor implies constant-speed operation. The
armature voltage can be controlled by a proportional device such as a
transistor. Proportional devices are limited to low-power applications.
At low speeds, the dc motor requires low voltage and full current to
produce full torque. The losses in the control device are proportional to
the product of voltage and current. The full battery voltage is applied to
the control device and the motor in series. Thus, the control device has a
high voltage drop across it at low motor armature voltages and low
motor speeds. To reduce controller losses, a chopper may be used. The
chopper is thyristor, power MOSFET, or IGBT circuit which can rapidly switch on and off. When the chopper is on, the loss in the control
device is low because the voltage drop is small. When the chopper is
off, the loss in the control device is small because the current through
the device is very small. Large amounts of power can be controlled with
low losses by means of a switching controller. For large dc motors, the
switching rate of the chopper is made high enough that the inherent
inductance of the motor armature maintains a nearly constant motor
current.
The speed range of a variable-speed dc motor is limited by its armature resistance, which produces IR drop. For a very small motor, the IR
drop can be as much as 20 percent of rated armature voltage. The
resistance-to-inductance ratio of small motors is much greater than for
large motors. To extend the speed range of small motors, a low-frequency chopper can be used which will cause the armature current to be
discontinuous. Discontinuous conduction reduces the effective IR drop
for the motor allowing its speed range to be extended.
DC motors have been widely used for variable-speed applications
because of their excellent characteristics. AC motors have been used
primarily for constant-speed applications. The control schemes described above are equally applicable to ac motors (except of course for
armature voltage and armature current feedback). If power circuitry is
properly handled, the control of an ac motor is just as flexible and
versatile as that of a dc motor.
AC motors can be supplied either from phase-controlled circuits or
from inverter circuits. Phase control uses a simpler electronic circuit than
the inverter, but it results in high loss in the ac motor. The phase control
switches at line frequency, and so the frequency applied to the motor is
constant. This means that the synchronous speed of the induction motor
is constant. The difference between the operating speed of the motor
and synchronous speed produces losses in the rotor of the motor proportional to the torque demanded of the motor. For this reason, phase-controlled ac motor drives are limited to applications with slight speed
change requirements or to loads that require much lower torque at low
speed, such as pumps or fans. Inverters are used for ac motor drives
which require constant torque operation over a wide speed range. Several different schemes have been used for ac motor inverters. Some of
these are pulse width modulation (PWM), six-step inverters, and current
link inverters. Most modern drives are PWM inverters. In a PWM inverter, the ac line voltage is converted into a fixed dc voltage, and then
this dc voltage is inverted into a variable frequency and a variable
voltage. Figure 15.2.35 shows a block diagram of PWM inverter. Since
the motor is an ac motor, both positive and negative voltages must be
applied to each motor phase. The motor shown in Fig. 15.2.35 has three
phases. Consider the operating conditions at only one of the phases of
the motor. To apply a positive voltage to phase A-B of the motor,
switches SW1 and SW4 are turned on. To apply a negative voltage to
phase A-B, SW2 and SW3 are turned on. To apply zero volts to phase
A-B, either SW2 and SW4 or SW1 and SW3 are turned on. A similar
operation takes place for phase B-C and phase C-A of the motor. The dc
voltage in the PWM inverter is constant. To generate an approximate
sine wave of voltage, the inverter must produce a low voltage during
part of the cycle and high voltage during another part of the cycle, and it
must do this on both the positive half of the sine wave as well as the
negative half. On the positive half-cycle, switching occurs alternately
between switches SW1 and SW4 and switches SW1 and SW3. To control the speed of the ac motor and keep its losses low, one must vary
both the frequency of the motor voltage and its magnitude. The motor
requires an approximately constant volts per hertz rate for full torque
and low losses. Although this control scheme seems very complicated,
PWM inverter drives have been developed which are far more dependable than dc drives. The commutator and brushes required for a dc
motor limit its reliability. The motor frequency for an ac motor inverter
drive varies from nearly zero to a maximum of 120 Hz typically. To
reverse the rotation of the ac motor, the phase sequence applied to the
motor is reversed. The actual switching frequency of the inverter is
approximately 10,000 Hz, allowing the inverter switch on and off several times during a cycle applied to the motor. The pulse width is varied
or modulated, to change the output voltage, hence the name pulsewidth-modulated inverter. The inductance of the motor keeps the
current variations in the motor from being too great.
Switching power supplies use technology similar to ac motor inverters. In Fig. 15.2.35, SW1 and SW2 are called an inverter leg. The
inverter leg is fundamental to switching power supplies. The dc chopper
described earlier could be made with a single inverter leg. Regulated dc
power supplies are also made using an inverter leg. A higher intermediate voltage is produced in an ac-to-dc rectifier. This voltage is then
chopped to make a lower output voltage. Feedback is taken from the
load point of the power supply to control the pulse width of the inverter
leg, or chopper. Precise computer power supplies are often switching
power supplies of this type. They maintain precise dc output voltage
under changing load conditions and changing supply voltage. Because
the power supplies switch, the loss in these power supplies is much
lower than for proportionally controlled power supplies.
The switching behavior of the inverter leg is actually more complex
than simply turning on one switch and turning off another. Each switching action involves a resonant transient. At each switching point an
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15-87
⫹ DC
SW1
SW3
SW5
A
AC
line
B
C
Fixed
rectifier
SW2
SW4
AC
motor
SW6
⫺ DC
Fig. 15.2.35 Pulse width modulation inverter.
R-L-C circuit rings from one voltage to another. At very high switching
rates, a second transient starts before the first transient is over. This
leads to a power supply with a parade of transients. Such power supplies
are called resonant converters. These power supplies may be used for
dc-to-dc, dc-to-ac, ac-to-dc, and ac-to-ac conversions. By using the
ring-up inherent in resonant circuits, voltage levels can be increased as
well as decreased. Induction heating power supplies are typical of these
kinds of circuits. Induction heating is produced by the magnetic losses
generated in nonmagnetic and magnetic metals. If the frequency and
voltage are increased sufficiently, dielectric heating may also be produced in high-frequency power supplies. High-frequency, high-voltage
power supplies are also used to produce a corona discharge to treat the
surface of plastic materials. Plastics which are normally hydrophobic
can be made hydrophilic by corona discharge treatment.
Communications
The Federal Communications Commission (FCC) regulates the use of
radio-frequency transmission in the United States. This regulation is
necessary to prevent interfering transmissions of radio signals. Some of
the frequency allocations are given in Table 15.2.17. The frequency
bands are also classified as shown in Table 15.2.18. Very low frequencies are used for long-distance communications across the surface of the
earth. Higher frequencies are limited to line-of-sight transmission. Because of bandwidth considerations, high frequencies are used for highdensity communication links. Orbiting satellites allow the use of highfrequency transmission for long-distance high-density communications.
A radio transmitter is shown in Fig. 15.2.36. It consists of four basic
parts: an rf oscillator tuned to the carrier frequency, an information-input device (microphone), a modulator to impress the input signal on the
carrier, and an antenna to radiate the modulated carrier wave.
A radio wave can be modulated in several ways. It can be amplitude-,
frequency-, or phase-modulated. These are examples of analog signal
modulation. A radio wave may also be digitally modulated. Examples
Table 15.2.17 Partial Table
of Frequency Allocations
(For a complete listing of frequency allocations, see ‘‘Reference
Data for Radio Engineers,’’ published by Howard Sams & Co.)
Frequency, MHz
Utilization
0.535 – 1.605
27.255
54 – 72
76 – 88
88 – 108
174 – 216
460 – 470
470 – 890
Commercial broadcast band
Citizen’s personal radio
Television channels 2 – 4
Television channels 5 – 6
Frequency-modulation broadcasting
Television channels 7 – 13
Citizens’ personal radio
Television channels 14 – 83
Table 15.2.18
Frequency Bands
Designation
Frequency
Wavelength
VLF, very low frequency
LF, low frequency
MF, medium frequency
HF, high frequency
VHF, very high frequency
UHF, ultra-high frequency
SHF, super-high frequency
EHF, extremely high frequency
3 – 30 kHz
30 – 300 kHz
300 – 3,000 kHz
3 – 30 MHz
30 – 300 MHz
300 – 3,000 MHz
3,000 – 30,000 MHz
30,000 – 300,000 MHz
100 – 10 km
10 – 1 km
1,000 – 100 m
100 – 10 m
10 – 1 m
100 – 10 cm
10 – 1 cm
10 – 1 mm
NOTE: Wavelength in meters ⫽ 300 /f, where f is in megahertz.
of digital modulation include on-off, binary phase-shift keying (BPSK),
frequency-shift keying (FSK), quadrature phase-shift keying (QPSK),
and quadrature amplitude modulation. Regardless of which method of
modulation is used, the frequency content in the signal requires a bandwidth of frequencies to support the transmission. If a 10-kHz signal is to
be broadcast, the transmitted radio wave will consist of a center frequency, called the carrier frequency, and an upper and lower sideband,
Fig. 15.2.36
Radio transmitter.
for a frequency bandwidth of 2 times 10 kHz, or 20 kHz. It is possible to
suppress the upper or lower sideband leading to single-sideband transmission, but this is a special case. Regardless of the modulation scheme,
the bandwidth required for signal transmission is the same for a given
signal bandwidth. Higher frequencies or higher digital pulse rates require greater bandwidth for transmission.
A radio receiver is shown in Fig. 15.2.37. This is called a superhetero-
Antenna
TRF
Local
oscillator
Loudspeaker
Mixer
IF
Demodulator
TRF ⫽ tuned radio-frequency amplifier
IF ⫽ intermediate-frequency amplifier
AF ⫽ audio-frequency amplifier
Fig. 15.2.37
Superheterodyne radio receiver.
AF
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ELECTRONICS
dyne receiver because it utilizes a frequency-mixing scheme. The tuned
radio-frequency amplifier is tuned to receive the desired radio signal.
The local oscillator is adjusted by the same tuning control to a lower
frequency. The mixer produces an output frequency which is the difference between the incoming radio-signal frequency and the local-oscillator frequency. Since this difference frequency is constant for all tuning positions, the intermediate-frequency amplifier always operates
with a constant frequency. This allows optimum design of the intermediate-frequency (IF) amplifiers since they are constant-frequency amplifiers. The IF frequency signal is modulated in just the same way as
the radio signal. The demodulator separates this audio signal, which is
then amplified so that the loudspeaker can be driven.
The term radar is derived from the first letters of the words ‘‘radio
detection and ranging.’’ It is essentially an echo system in which the
location of an object is determined by sending out short pulses of radio
waves and observing and measuring the time required for their reflections or echoes to return to the sending point. The time interval is a
measure of the distance of the object from the transmitter. The velocity
of radio waves is the same as the velocity of light, or 984 ft/␮s, so that
each microsecond interval corresponds to a distance of 492 ft. The direction of an object can be determined by the position of the directional
transmitting and receiving antenna. Radio waves penetrate darkness,
fog, and clouds, and hence are able to detect objects that otherwise
would remain concealed. Radar can be used for the automatic ‘‘tracking’’ of objects such as airplanes.
A block diagram of a radar system is shown in Fig. 15.2.38. The
transmitting system consists of an rf oscillator which is controlled by a
modulator, or pulser, so that it sends to the antenna intermittent trains of
rf waves of relatively high power but of very short duration, corresponding to the pulses received by the modulator. The energy of the
oscillator is transmitted through the duplexer and to the antenna through
sending pulses in order that the distance to the target may be determined. This is accomplished by synchronization of the sweep circuit of
the oscilloscope with the pulses by the master timer.
Displays Conversion of the received radar signals to usable display
is accomplished by a cathode-ray oscilloscope. The simplest type,
called the A presentation, is shown in Fig. 15.2.39a. When the pulser
operates, a sawtoothed wave produces a linear sweep voltage (Fig.
15.2.39b) across the sweep plates of the cathode-ray tube; at the same
time a transmitter pulse is impressed on the deflection plates and the
return echoes appear as AM pulses, or ‘‘pips,’’ on the screen, as shown
in Fig. 15.2.39a. The distance on the screen between the transmitter
pulse and the pip caused by the echo is proportional to the distance to
the target, and the screen can be calibrated in distance such as miles.
Fig. 15.2.39
Type A presentation.
[The return of the spot to its initial starting position, produced by the
sweep interval cd (Fig. 15.2.39b), is so rapid that it is not detectable by
the eye.] The direction of the target may be determined by the angular
position of the antenna, which can be transmitted to the operator by
means of a selsyn. Different objects, such as airplanes, ships, islands,
and land approaches, have characteristic pips, and operators become
skilled in their interpretation. A bird in flight can be recognized on the
screen. Also a portion of the scale such as ab can be segregated and
amplified for close study of the characteristics of the pips.
Plan Position Indicator (PPI) In the PPI (Fig. 15.2.40) the direction
of a radial sweep of the electron beam is synchronized with the azimuth
sweep of the antenna. The sweep of the beam is rotated continuously in
synchronism with the antenna, and the received signals intensity-modulate the electron beam as it sweeps from the center of the oscilloscope
screen radially outward. In this way the direction and range position of
Fig. 15.2.38 Block diagram of radar system.
either coaxial cable or waveguides. The receiver is an ordinary heterodyne-type radio receiver which has high sensitivity in the band width
corresponding to the frequency of the oscillator. For low frequencies the
local oscillator is an ordinary oscillator for frequencies of 2,000 MHz;
and higher a reflex klystron (hf cavity oscillator) is used. A common
intermediate frequency is 30 MHz but 15 and 60 MHz are also frequently used.
In most radar systems the same antenna is used for receiving as for
transmitting. This requires the use of a duplexer which cuts off the
receiver during the intervals when the oscillator is sending out pulses
and disconnects the transmitter during the periods between these pulses
when the echo is being received.
The antenna is highly directional. By noting its angular position, the
direction of the object may be determined. In the PPI (plan position
indicator), the angle of the sweep of the cathode-ray beam on the screen
of the oscilloscope is made to correspond to the azimuth angle of the
antenna.
The receiver output is delivered to the indicator which consists of a
cathode-ray tube or oscilloscope. The pulses which are received, corresponding to echoes from the target, must be synchronized with the
Fig. 15.2.40
Plan position indicator (PPI) of southeastern Massachusetts.
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COMMUNICATIONS
an object can be determined from the pattern on the screen of the oscilloscope, as shown in Fig. 15.2.40.
There are two methods by which the angular direction of the cathode
spot is made to correspond with the angular position of the antenna. In
one method, used on board ship, two magnetic deflecting coils are
rotated around the neck of the tube in synchronism with the antenna, by
means of a selsyn. In the other method, used on aircraft, two fixed
magnetic deflecting coils at right angles to each other and placed at the
neck of the tube are supplied with current from a small two-phase
synchronous generator whose rotor is driven by the antenna. Thus a
rotating field, similar to that produced by the stator of an induction
motor, is produced by the magnetic deflecting coils. These two rotating
fields, although produced by different means, are equivalent and cause
the cathode beam to sweep radially in synchronism with the antenna.
Circular coordinates spaced radially corresponding to distance are obtained by impressing on the control electrode short positive pulses synchronized with the transmitted pulse but delayed by time values corresponding to the desired distances. These coordinates appear as circles
on the screen. Since the time of rotation of the antenna is relatively
slow, it is necessary that a persistent screen be used in order that the
operator may view the entire pattern. In Fig. 15.2.32 is shown a line
drawing of a PPI presentation of Cape Cod, Mass., on a radar screen,
taken from an airplane.
The applications of radar to war purposes are well known, such as
detecting enemy ships and planes, aiming guns at them, and locating
cities, rivers, mountains, and other landmarks in bombing operations. In
peacetime, radar is used to navigate ships in darkness and poor visibility
by locating navigational aids such as buoys and lighthouses, as well as
protruding ledges, islands, and other landmarks. It can be similarly used
in air navigation, as well as to operate altimeters for determining the
height of the plane above ground. It is also used for aerial mapping.
Sonar is closely related to radar. The difference is that sound is used
as the transmitting signal. Since the velocity of sound in water is so
much slower than radio waves in free space, the timing for sonar signals
is much slower than for radar signals. A 1-s delay of a sonar return echo
implies that the target is 2,500 ft away. A 1-s delay of a radar return
echo would imply that the target is 193,000 miles away. Although A
displays and PPI displays are used with sonar signals, much reliance is
placed on hearing. Transmitting a sonar pulse is an excellent way of
disclosing your presence to an enemy vessel. Today more reliance is
placed on passive detection of enemy vessels, by signature analysis.
Signature analysis is signal analysis using fast Fourier transforms and
filters.
Television is accomplished by systematically scanning a scene or the
image of a scene to be reproduced and transmitting at each instant a
current or a voltage which is proportional to the light intensity of the
elementary area of the scene which at the instant is being scanned. The
varying voltage or current is amplified, modulated on a carrier wave,
and then transmitted as a radio wave. At the receiver the radio wave
enters the antenna, is amplified, and demodulated to give a voltage or a
current wave similar to the original wave. This voltage or current wave
is then used to control the intensity of a cathode-ray beam which is
focused on a fluorescent screen in a cathode-ray reproducing tube. The
cathode-ray beam is caused to move over the screen in the same pattern
as the scanning beam at the transmitter and in synchronism with it. Thus
each small area of the receiver screen is illuminated instantaneously
with light intensity corresponding to that of a similarly placed area in
the original scene. This process is conducted so rapidly that owing to
persistence of vision of the eye, the reproduction of each instantaneous
scene appears to be a complete picture and the effect with successive
scenes is similar to that produced by the projection of successive frames
of a motion picture.
Scanning and Blanking In the United States the ratio of width to
height of a standard television picture is 4 : 3, and the picture is composed of 525 lines repeated 30 times a second, this last factor being
one-half 60, the prevalent electric power frequency in the United States.
The scanning sequence along the individual lines is from left to right
and the sequence of the lines is from top to bottom. Also, interlacing is
15-89
employed, the general method of which is shown in Fig. 15.2.41. The
cathode-ray spot starts at 1 in the upper left-hand corner and is swept
rapidly from left to right either by a sawtooth emf wave applied to the
sweep plates or by the sawtooth current wave applied to the sweep coils
of the tube. When the spot arrives at the right-hand side of the picture,
the sawtooth wave of either emf or current in the sweep circuit acts to
return the cathode-ray spot rapidly to point 3 at the left-hand side of the
picture. However, during this period the cathode-ray is blanked, or entirely eliminated, by the application of a negative potential to the control
grid of the tube. At the end of the return period, the blanking effect
ceases and the spot appears at point 3, from which it again is swept
Fig. 15.2.41
Pattern of interlaced scanning.
across the picture and this process is repeated for 262.5 lines until the
spot reaches a midpoint C at the bottom of the picture. It is then carried
vertically and rapidly to B, the midpoint of the top of the picture, the
beam also being blanked during this period. This process of scanning is
then repeated, a second set of lines corresponding to the even numbers
2, 4, 6 being established between the lines designated by the odd numbers. These lines are shown dashed in Fig. 15.2.41. This method or
pattern of scanning is called interlacing. The two sets of lines taken
together produce a frame of 525 lines, which are repeated 30 times each
second. However, owing to interlacing, the flicker frequency is 60 Hz
which is not noticeable, 50 Hz having been determined as the threshold
of flicker noticeable to the average eye. In Fig. 15.2.41, for the sake of
clarity, the distances between horizontal lines are greatly exaggerated
and no attempt is made to maintain proportions.
Frequency Band In order to obtain the necessary resolution of pictures, television frequencies must be high. In the United States, VHF
frequencies from 54 to 88 MHz (omitting 72 to 76 MHz) and 174 to
216 MHz are assigned for television broadcasting. A UHF band of frequencies for commercial television use is also allocated and consists of
the frequencies of from 470 to 890 MHz (see also Tables 15.2.17 and
15.2.18).
In order to obtain the 525 lines repeated 30 times per second, a bandwidth of 6 MHz is necessary. The video, or picture, signal with the
superimposed scanning and blanking pulses is amplitude-modulated,
amplified, and transmitted. The carrier frequency associated with the
sound transmitter is 4.5 MHz higher than the video carrier frequency
and is frequency-modulated with a maximum frequency deviation of
25 kHz.
In scanning motion-picture films a complication arises because standard film rate is 24 frames per second, while the television rate is 30
frames per second. This difficulty is overcome by scanning the first of
two successive film frames twice and the second frame three times at
the 60-Hz rate, making the total time for the two frames 1⁄12 (2⁄60 ⫹ 3⁄60)
or 1⁄24 s average per frame.
Kinescope The kinescope (Fig. 15.2.42) is the terminal tube in
which the televised picture is reproduced. It is relatively simple, being
not unlike the cathode-ray oscilloscope tube. It has an electric gun operating at 8,000 to 20,000 V which produces an electron beam focused on
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15-90
ELECTRONICS
a fluorescent surface within the front wall of the tube. The picture is
viewed at the front wall. The horizontal and vertical deflections of the
beam are normally controlled by deflection coils, as shown in Fig.
15.2.42.
Fig. 15.2.42 Kinescope for television receiver.
Television Receivers A block diagram for a television receiver is
given in Fig. 15.2.43. It is in reality a superheterodyne receiver with
tuned rf amplification, the separating of the sound and video or picture
channels taking place at the intermediate frequency in the mixer. The
sound channel is then conventional, a discriminator being used to demodulate the FM wave (Fig. 15.2.21). The object of the dc restorer is to
make the picture reproduction always positive, and it consists of applying a dc voltage at least equal in magnitude to the maximum values of
the negative loops of the ac waves. The synchronizing pulses for both
the vertical and the horizontal deflections are delivered by the dc restorer to an amplifier and the two pulses are then divided into the V and
H components. The integrating and differentiating circuits are necessary
to separate horizontal and vertical synchronizing signals.
As stated earlier, at any instant the magnitude of the current from the
pickup tube varies in accordance with the light intensity of the part of
the scene being scanned at that instant. This current is amplified and,
together with the sound and synchronizing current, is broadcast and
received by the circuit shown in Fig. 15.2.43. The video current is
detected by rectification, is amplified, and is then made to control the
intensity of the kinescope electron beam. Tubes produce a scanning
pattern, identical with that in the pickup tube, and these tubes are triggered by the synchronizing pulses which are transmitted in the broadcast wave. Hence, the original televised scene is reproduced on the
fluorescent screen of the kinescope.
Color-television transmission is similar to black-and-white television,
and the two signals must be compatible with each other. The kinescope
for color TV has three electron guns, one for each primary color. The
fluorescent screen has a matrix of three different colors of phosphor and
a mask with many small holes in it. The intensity signals for each color
are phase-shifted from each other so that the proper phosphors are excited by each electron stream at each mask point over the entire screen.
A black-and-white signal does not have the same synchronizing signal
as a color signal. The color receiver has circuits which recognize this
state and switch it to black and white reception.
Telephone Communications
Telephone communications is an important aspect of electronics because it provides a source of low-cost electronic devices which can be
adapted for electronic circuit design. One of the big problems in telephone system design is switching. Each long-distance phone call requires that several circuits be joined together, and thousands of phone
calls take place simultaneously. Older telephone exchange buildings
had rack upon rack of telephone relays. These are being replaced by
semiconductor switching. Older trunk lines were coaxial cable or microwave radio links. These are being replaced by fiber-optic cables.
Fiber-optic cables have a tremendous bandwidth. A telephone voice
circuit has a bandwidth of 4 kHz. Thousands of these circuits can be
routed through one fiber-optic cable. Separating these voice circuits into
communication channels by frequency separation and filtering is not
feasible. Communication on fiber-optic cables is done digitally, and in
packets. Each packet is a few hundred bytes long, and includes a header
with an address and other control information, the message, and a trailer
with error-correcting codes. The address is a session address to identify
where the packet is from and where it is going. Users time-share the
fiber-optic cable, and all of this signal processing must be done at a high
enough rate to appear to be instantaneous to the subscriber.
Any form of signal modulation can be considered to be data encryption. The information is coded or encrypted onto the transmitting medium. At the transmitting end, a clear signal with no noise is sent, but at
the receiving end of the medium, the signal contains the signal plus
unwanted noise. Since the coding method is known, and since the noise
is known to be random, separation of signal and noise is possible. This
process of separating signal from noise will be repeated several times
for long-distance communications. Each time, the noise is discarded and
only the signal is sent on. Such a scheme of data encryption, or modulation, is also warranted in data telemetry, in order to allow noise to be
removed from the desired signal.
Long-distance communications takes several different forms. Historically, cables with hundreds of twisted-pair circuits were used for longdistance communication. These are used for local subscriber service
today. The next step was a cable made up of many wideband microwave
coaxial cables. The coaxial cables were replaced by microwave radio
links. Then, fiber-optic cables replaced microwave links. Wireless communication, using laser-generated signals, is the latest form of high
volume data transmission. In a telephone system, all of these systems
must coexist and work together.
There is great interest in cellular communications. This is a microwave broadcast system. The cellular communication system is evolving
today, and promises to allow mobility of telephone communications.
Global Positioning Information
Navigational aids take several different forms. Shoran (short-range navigation) and loran (long-range navigation) are methods by which ships
or planes can locate their position. A loran station consists of a master
Fig. 15.2.43 Block diagram for television receiver. TRF: tuned radio frequency; IF: intermediate frequency.
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GLOBAL POSITIONING INFORMATION
transmitter and slave transmitter located about 200 mi apart. A pulse is
broadcast from the master transmitter and, when received by the slave
transmitter, causes it to also broadcast a pulse. The loran receiver, located on a ship or airplane, receives both the master and the slave
pulses. The receiver allows the user to measure the delay between receiving the master and slave pulses. The delay defines a line across the
surface of the earth. By picking three loran stations, determining the
delay of each of the transmitter pairs, and using loran charts, three lines
can be defined, allowing the user to determine the triangle which defines the receiver’s position. Very high frequency omnirange (VOR) is
used for short-range aircraft navigation, often in conjunction with direction measuring equipment (DME). VOR stations are located approximately 100 mi apart. The receiver in the aircraft receives a signal from
the VOR transmitter with two pieces of information, which allows the
pilot to determine the bearing from the VOR station. The first piece of
information is a 30-Hz amplitude-modulated signal which is broadcast
from a rotating directional antenna. The antenna rotates 30 times per
second, and broadcasts in the 108- to 112-MHz range. The second piece
of information is a frequency-modulated signal which varies between
9,480 and 10,440 Hz. This FM signal changes frequency at 30 Hz, and
forms a reference for the VOR system. A receiver on the aircraft compares the 30-Hz amplitude-modulated signal and the 30-Hz frequencymodulated reference, and translates the difference in phase between the
FM signal and the AM signal to give the airplane its bearing. The VOR
also broadcasts station identification information in the 112- to 118MHz range. Most VOR stations can also broadcast a distance measuring
equipment (DME). The aircraft has a DME transmitter which broadcasts
a pulse which activates a ground mounted transponder. The aircraft
sends an FM signal in the 1,025- to 1,150-MHz range. The DME ground
station responds with a delayed signal in the 962- to 1,024-MHz or
1,151- to 1,213-MHz range. Distance is determined by the time delay
required for the propagation of the signals. The depressed, straight line
distance from the aircraft is measured by the DME equipment. To relieve the pilot of the job of correcting for altitude in the DME signal and
to allow the pilot to fly any heading, a computer system called RNAV
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was introduced. RNAV uses the VOR and DME information, plus wind
velocity and airspeed, to calculate the required aircraft heading and the
estimated time of arrival at the destination.
Most major airports have an instrument low approach system (ILS).
This produces a set of signals in the 108- to 112-MHz region. There a
right-hand antenna lobe, relative to the aircraft, which is modulated at
150 Hz, and a left-hand lobe modulated at 90 Hz. In addition, in the
329- to 335-MHz range there is a vertical set of antenna lobes, with the
upper lobe modulated at 90 Hz and lower lobe at 150 Hz. The display
on the aircraft is an azimuth needle and a glide path needle. The pilot
simply flies the aircraft to center the needles to determine a correct
landing approach. There are also ILS marker beacons to tell the pilot
how close the aircraft is to the airport. Other aircraft navigation systems
are based on ground-operated radar systems. Still others are based on
aircraft-mounted Doppler radar. Doppler radar is a velocity measuring
system.
Satellite positioning systems are the newest development for long-range
and short-range navigation. The global positioning system (GPS) is a system involving 21 to 24 satellites which broadcast accurate time marks.
By knowing which satellite has broadcast the time mark, and measuring
its delay to the aircraft, one can determine a spherical surface in space.
By determining the delay from four satellites, four spherical surfaces in
space can be determined. Worldwide navigation accurate within 50 m
in longitude and latitude and within 100 m vertically is being realized
commercially. The military capabilities are better than the commercial,
in the submeter range. The commercial timing signals are deliberately
disturbed with a random timing variation to degrade them. The capability of the military system was demonstrated by the accuracy of ‘‘smart’’
bombs during the Iraq-Kuwait war.
GPS is being used for commercial truck fleets to provide better fleet
utilization. GPS is also being used by the Tennessee Valley Authority to
monitor power system performance, by determining the phase angle of
the 60-Hz voltage wave throughout its system. GPS is also being used
by search and rescue teams. GPS receivers for search and rescue cost
$300 to $600, and are dropping in price at this time.
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