Polyphase commutator motors with shunt characteristics Folta, George William 1948

Polyphase commutator motors with shunt characteristics Folta, George William 1948
Calhoun: The NPS Institutional Archive
Theses and Dissertations
Thesis Collection
1948
Polyphase commutator motors with shunt characteristics
Folta, George William
Monterey, California. U.S. Naval Postgraduate School
http://hdl.handle.net/10945/31609
POLYPHASE COMk'UTATOR MOTORS WITH SHUNT
CHARACTERISTICS
G. W. Folta
POLYPHASE COlJl:LUTATOR MOTORS WITH SHUNT
CHARACTERISTICS
by
George William Folta
Lieutenant Commander, United States Navy
Submitted in partia.l fulfillment
of the requirements
for the degree of
MASTER OF SCIENCE
United States Naval Postgraduate School
Annapolis, Maryland
1948
This work is accepted as fulfilling
the thesis requirements tor the degree of
Master of Science in Electrical Engineering
trom the
United States Naval Postgraduate School
Cha.irman
Department of Electrical Engineering
Approved:
Aca.demic Dean
1
PREFACE
This paper deals with polyphase commutator motors having shunt characteristics.
These are adjustable speed (according to NEMA) machines and
consist of two types; the stator fed motor with speed control by an
induction regulator, and the rotor fed motor with speed control by brush
shifting, (Schrage).
The majority of the space will be allotted to the Schrage motor;
nearly every listed reference about this machine was checked and condensed
into this compilation.
The reader must continually keep in mind the weight and space factor
which are so vital in naval ships; for, although one method may give
better speed regulation then another, the equipment necessary may make
the better method useless for marine installation.
I had hoped to get more information on the Schrage motor as used on
hoists and cranes so as to analyze its possibility for use as a cargo
winch.
Dr. FriaUf at the Bureau of Ships told me that Schrage motors were
used for elevators in England and for cranes at the Singapore Naval Base.
r wrote the British Thomson - Houston Company in Rugby, England, tor such
information, but never received an answer.
When I asked one of the engineers at the Bureau of Ships why the
Schrage motor was not considered tor winches, he answered, "commutation
troublest".
Actually, commutation trouble in this motor should be nil,
as will be explained.
I had another reason for choosing this SUbject.
Although the
Electrical Engineering course has always been clearly presented, there
were times that my comprehension of the subject matter was not complete;
such was the Schrage motor.
11
TABIE OF COmENTS
Page
.
I
MErHODS OF SPEED CONrROL OF INDUCTION IDTORS
1. Speed control by the introduction of an emf in the rotor
circuit.
2
2. Speed variation by inserting resistance in the rotor
circuit.
3
3. Speed control by motor clutch.
II
THE: STATOR FED POLYPHASE IDTOR
8
1 • Basic theory.
In
5
2. . Induction regulator.
10
3. Speed range.
11
4.
12
Efficiency.
5. Regenerative braking.
12
6. Reversal.
12
7. Advant.ages.
13
8. Disadvantages.
13
THE HIGGS IDrOR
1. Supposed disadvantages of the Schrage and stator fed
18
machine.
IV
2. General description.
18
3. The rotor.
19
4.
20
The regulator.
SCHRAGE IDTOR
25
1. General.
2. The correlation between an induction motor and the
27
Schrage motor.
iii
TABLE OF CONrENrS
(continued)
Page
V
3. Reversal.
33
4. starting torque and current.
33
5. Speed range.
34
6. Starting and control gear.
35
THEORY OF SCHRAGE MarOO BY cmCLE DIAGRA1f)
1. Operation of machine either as a motor or generator is
explained on the basis of superposition of currents.
41
Operation below synchronous speed.
42
3. Operation above synchronous speed.
45
4. Experimental check on theory.
46
5. Effects of primary' leakage reactance.
47
6. Determination of characteristics from circle diagram.
49
7. Experimental check on theories.
50
B. Brush settings for power factor correction.
59
9. Primary' currents with power factor correction.
60
2.
10. Determination of characteristics when the motor is used
to correct pOvrer factor.
61
li.
64
Results of test with leading cOIDI!DJ.tator voltage.
12. Conclusions.
64
13. Determinations of the primary' currents.
66
14. Obtaining the circle diagram.
67
15. Characteristics from the circle diagram.
68
16. Experimental check on circle diagram theory'.
71
17.
71
Conclusions.
iv
TABlE OF CONTENl'S
(continued)
Pa.ge
VI
COMMUl'ATION
1. General.
SO
2.
81
The emf induced by the rotating field.
3. The emf of self induction.
82
4. Reasons
why commutation is better on Schrage motors than
on stator fed motors.
5. Auxiliary windings on ac commutator machines.
84
85
VII APPLICATIONS OF SCHRAGE AND STATOR FED Ma1'ORS
1. Stokers.
92
2. Feed and separator drives.
92
3. Frequency changing.
92
4.
93
Fans.
5. Pumps.
93
6. Printing and paper making.
94
7.
96
M:isce1J.aneous.
8. Cranes, hoists, lifts.
96
v
LIST OF ILLUSTRATIONS
.
Page
Figure 1.
Vector diagrams for explanations of speed control of
a polyphase motor.
Figure 2.
Connection diagram for a 220 volt and a 440 volt
polyspeed motor.
15
Figure 3.
Schematic diagram of a stator fed motor made by B'm
Co. of England.
16
Figure 4(a)
Speed curves of a 3/1 hp, 1900/630 rpm machine made
by the BTU Co.
16
Figure 4(b)
Torque versus efficiency and power factor curves of
a Brown, Boveri 26 KW, 1410/470 rpm stator ted motor.
17
Typical power factor and efficiency versus speed
curve of a Riggs motor.
22
Figure 6.
Schematic diagram of a Higgs motor.
22
Figure 7.
Torque versus rpm curve, (3.5-1) speed ra.l'lge for a
Riggs motor.
23
Figure 8.
Cross section of rotor winding in a Higgs motor.
23
Figure 9.
Diagram of rotor windings, Higgs motor.
23
Figure 10.
Schematic diagram showing cross oonnection of regulator
for Higgs motor.
24
Figure 11.
Rotation of phase vectors, Higgs motor.
24
Figure 12.
Typical hook-up for a Schrage motor
37
Figure 13.
Speed and torque set up in a Schrage motor.
37
Figure 14.
Internal connections of a Schrage motor, and brush
positions with respect to poles.
38
Difference in magnitUde of
brush position.
38
Figure 5.
Figure 15.
Figure 16.
.
emf
7
due to changing of
Vector diagrams shoWing voltage relationships at sub
synchronous speed and above synchronous speed.
39
Figure 17.
Torque versus power factor curve for a Schrage motor.
39
Figure 18.
Torque versus efficiency curve for a BTH Schrage motor,
and curves showing economy gained by using Schrage
motors as compared with ordinary induction motor.
40
vi
LIST OF ILLUSTRATIONS
(continued)
Page
Figure 19.
Currents and voltages in the secondaries.
52
Figure 20.
Loci 01' secondary currents with brushes set for
approximately 50 per cent 01' synchronous speed.
52
Relations of secondary voltages and currents when
brushes are set to make E2 opposite to Ell and
equal in magnitude at halt speed.
53
Figure 21.
Figure 22(a) Secondary currents retlected to the primary.
53
,
Figure 22(b) Currents taken by primary at no load.
Figure 23.
Figure 24.
53
Circle diagram of primary current tor brush setting
corresponding to 50 par cent synchronous speed.
54
Currant loci of secondary currents - brushes set for
approximately 150 per cent synchronous speed.
54
Figure 25(a) Currents tor approximately 150 per cent synchronous
speed; secondary currents reflected to primary.
54
Figure 25(b) Primary currents at no load.
55
Figure 26.
Circle diagram 01' primary current tor approximately
150 per cent synchronous speed.
55
Theoretical primary current locus compared with test
data - 50 per cent synchronous speed.
56
Theoretical primary current locus compared with test
data - 150 per cent synchronous speed.
56
Comparison 01' prime.ry current locus obtained from noload tests with the true locus obtained by loading.
57
Figure 30.
The circle diagram for the low-speed adjustment.
57
Figure 31.
The circle diagram for high speed adjustment.
58
Figure 32.
Comparison of the theoretical characteristics taken
from the circle diagram with the actual characteristics
obtained by tests for no load speeds. approximately
50 per cent of synchronous speed, and 150 per cent
synchronous speed.
58
Figure 33.
(a) Brushes set to make Ell 180 degrees mIt of phase
with E2 •
73
(b) Brushes shifted to make Ell lead E2 by 90 degrees. 73
Figure 27.
Figure 28.
Figure 29.
vii.
LIST OF ILLUSTRAT IONS
(continued)
Page
Figure 34.
Vector diagrams of secondary voltages and currents when
brushes are set to make (a) Ell 180 degrees out of
phase with E2. (b) Ell lagging E2 by 90 degrees.
(c) Ell leading E2 'by 90 degrees.
73
Figure 35.
Vector diagrams of motor when used to correct power
factor.
74
Characteristic vector diagra~ of motor (primary), Ell
90 degrees ahead of E2.
74
Figure 36.
Figure 3'7.
(a,'b,c)
(a) Circle diagram obtained from no load tests.
Encircled points indicate primary currents taken by
motor under load conditions (determined by loading).
(b) and (c) characteristics obtained by loading and
as predicted from circle diagram.
75
Figure 38.
Efficiency as affected by power factor correction.
76
Figure 39(a) Secondary currents.
76
Figure 39(b) Components of primary current which cancel the mmf of
the secondary currents in the stator.
76
Figure 39(c) Components of primary current which cancel the romf of
the secondary currents in the adjusting Winding.
77
Figure 39(d) The magnetizing current.
77
Figure 39(e) The sum of the component currents in the primary.
77
Figure 40(a) Schematic diagram of motor, showing the brushes set
to retard Ell by ~ degrees. The mm1' of the adjusting winding is ~ degrees behind that of the stator.
78
Figure 40(b) Effective currents, representing the mmf's of the
stator, adjusting windins, and primary windings.
78
Figure 41.
Locus of the primary current, and curves dividing the
power component into the parts allocated to the output and the various losses.
78
Figure 42.(a},(b),(c),(d) Comparison of observed end predicted
characteristics.
79
Figure 43.
91
Armature commutatOr with two brushes.
Figure 44(a} ,(b)
(a) Current in armature coil, de machine.
(b) Current in armature COil, ac machine
viii
91
LIST OF ILLUSTRATIONS
(oontinued)
Page
Figure 45(a), (b)
(a) Armature oommutator for a three phase winding;
(b) Current in armature coil, thr.ee phase.
91
Figure 46.
Simplex armature winding.
91
Figure 47.
Cross section of winding of figure 46.
91
Figure 48.
Duplex armature winding.
91
Figure 49.
Embedded armature winding.
91
ix
TABLE OF SYMBOLS AND ABBREVIATIONS
brush width
induced voltage in rotor
brush emf
primary voltage
stator voltage where specified
voltage induced in one phase of stator
voltage generated in adjusting winding
supplying one phase of stator
bet,~en
brushes
stator voltage at standstill (induced)
emf of self-induction
supply frequency
frequency of emfs induced in rotor, slip frequency
slip frequency
frequency of brush emf
hp
horsepower
K
constant
K
torque constant of the motor
winding factor for the secondary
1
armature length
nwnber of turns in short circuited winding element
speed of rotating field
x.
TABLE OF SYMBOIS AND ABBREVIATIONS
(continued)
n
speed of rotor
the effective primary turns
the effective stator turns
Naw
the effective adjusting winding turns
effective number of secondary turns per phase' included in
stator and adjusting winding circuit. This quantity is
a function of speed adjustment as well as power factor
adjustment
current in winding element
current flowing in stator and adjusting winding resulting
from the voltage E2
current flowing in stator and adjusting winding resulting
from the voltage Ell
standstill primary current
Il
. primary current per phase
exciting current in primary circuit per phase
component of primary current flowing as a result of the
currents in stator and adjusting winding
load component of primary current
maximum value of III and thus diameter of III circle
maximum value of 12 and thus
dia~eter
of 1 2 circle
the resultant secondary current 12 • III
!mag
the flux producing component of the primary current
the component of the primary current which cancels the
mmt of the stator windrrng
the component of the primary current adjustment which
cancels the mmf of the adjusting Winding
magneto - motive force
the no load speed at which the motor would operate if the
brushes were shifted to eliminate Ellsinp- t leRvlng Ellcos ~
unChanged
xi..
TABLE OF SYMBOISAlln ABBPJNIATIONS
(continued)
the developed power
p
poles
R
the resistance of the primary, secondary and adjusting
winding, reflected to the secondary
resistance of stator per phase
resistance of adjusting winding per phase
lr 2a "
ratio of number of turns on the primary to the effective
number of turns on the secondary and adjusting winding
s
%sl1p
s
slip
time of commutation
torque developed (synchronous watts)
the developed torque
surface velocity of al'!llature
impressed voltage
voltage impressed on one phase of primary
stator reactance at standstill
adjusting winding reactance at standstill
x
the standstill reactance of the primary, seCOndary, and
adjusting Winding reflected to the secondary
reactance of stator per phase
reactance of adjusting winding per phase
z
the secondary and adjusting winding impedance
•
xii
-
.
TABLE OF SYMBOIS Arm ABBREVIATIONS
( continued)
phase angle between III and Ell' or 1 2 and E2
angle of lag between Ell and III electrical degrees
angle of lag between E2 and 1 2 electrical degrees
angle between VI and IlL
angle tan-l Naw!N2; the angle in electrical degrees,
(a), between lIs end IlL' (b), between the mmt of the
stator and the resultant mmt of the stator and adjusting
winding, (c), between ssE2 and <ssE2 • Ell)
phase angle between total secondary phase current and
voltage induced in stator
the angle in electrical degrees; (a). between Ell and
ssE2 , (b), between the mmf's of the stator and adjusting
winding. (c), that the brushes must be rota.ted around
the commutator to reta.rd Ell by ~ degrees
magnetic permeanCe of the leakage fluxes per unit length
of armature
flux (total) produced by primary winding
(IZ)s
the component ot the total secondary and adjusting winding
IZ voltage which is in phase with E2
(IZ)t
the component of the total secondary and adjusting winding
IZ· v~ltage which is in quadrature with E2 ~. (IZ) t :
-Ell sin~
the component ot the secondary Il?'R loss which is given by
I (Izl s
the component of the secondary
I
(IZ)t
12a 10s8 which
is given by
INTRODUCTION
Since the motors discussed in this paper are special types of the
induction motor, several methods of speed control of this type motor
are outlined in Chapter I.
The stator fed motor is taken up in Chapter II.
This motor is
better than the induction motor for speed control, but it is not as
good as the Schrage motor.
Chapter III describes the Higgs motor which is a specialized
stator fed motor.
In Chapter IV the Schrage motor is simply explained, whereas
Chapter V explains the motor by the use of circle diagrams.
Commutation and why there should be nocomfuutation troubles is
explained in Chapter VI.
Finally, Chapter VII tells of the many applications of the Schrage
motor.
1
CHAPl'ER I
METHODS OF SPEED CONTROL OF INDUCTION MarORS
1.
Speed control by the introduction of an emf in the rotor circuit.
The following is taken from Liwschitz-Garik and Whipple, (2).
The speed of the induction motor can be made to vary by
impressing across the external terminals of the rotor slip rings
a voltage which is in phase with or direct~ opposite in-phase
to the emf induced in the rotor. If the impressed voltage is
opposite in-phase to the rotor emf, it decreases the rotor current. The rotor, in order to overcome the opposing torque, will
increase its slip, thus causing the rotor current to increase to
an amount sufficient to overcome the opposing torque. The new
slip assume s a value which is sufficient to increase the emf induced in the rotor so that it not only overcomes the impressed
voltage, but also causes the proper value of rotor current to flow.
The greater the impressed counter voltage is, the larger is the emf
to be induced, and therefore the greater the slip is.
If the impressed voltage is in-phase with rotor emf, Le.,
supports the rotor emf, a smaller induced emf is necessary in the
rotor and the slip 'will decrease to such an extent as to cause the
proper value of current to flow in the rotor. In this case, the
greater the impressed voltage, the smaller will be the slip. If
the impressed voltage is made exact~ equal to the rotor emf which
is necessary to produce the proper current, the motor vdJl run at
its synchronous speed and still be able to overcome the opposing
torque. Moreover, if the impressed emf is greater than the
necessary rotor emf, the induction motor operates at a speed higher than the synchronous speed.
Consider Fig. 1. It is assumed for the sake of clarity that
stator and rotor leakage reactances as well as stator resistance
are negligible. To any opposing torque there corresponds a certain
rotor current and therefore a certain emf in the rotor, name~ the
voltage drop I2r2. Also, the rotor speed is fixed by the opposing
torque. If no voltage is impressed on the rotor it runs at a speed
corresponding to this torque, and the emf induced in the rotor is
equal to E2s Q sE2 ti I2r2(fig. la). If a voltage is impressed on
the rotor and the opposing torque does not change, the resultant of
the impressed voltage and the induced emf also must remain unchanged,
namely equal to I2r2. In Fig. lb the impressed voltage V2 is
opposite in-phase to I2r2, Le., to the emf necessary to overcome
the opposing torque. In order that I2r2 (and also the current 12)
have the salm magnitude as in Fig. la, E2s must increase by the sanE
amount V2, i.e., the rotor must increase its slip (reduce its speed).
In Fig. lc the impressed voltage V2 is in-phase ~dth and equal to
I2 r 2. In order that I2r2 and 12 rerrain unchanged, the induced emf
of the rotor E2 s must be zero, i.e., the rotor must run at synchronous speed n s • In Fig • ld V2 is again in-phase with I2r2 but is
larger than I2r2. In this case the rotor emf E2s =: qE2 must be
2
negative, i.e., the slip becomes negative and the motor runs
above 5Y"nchrOnouB speed ..... -. ................•......•......•
The impressed voltage must have the same frequency as the
emf induced in the rotor, i.e., the slip frequency. The polyphase machine which delivers the regulating voltage is connected
with the induction motor either electrically gnd mechanically or
only electrically.
The current and emf of the armature of the regulating machine
are opposite in phase (Fig. lb, V2 opposite to I2) for induction
motor speeds beloW' SYnchronous speed. Therefore, for these speeds
the regulating rnachine acts as a motor. If it is mechanically
coupled to the induction motor it will deliver its mechanical pOYler
to the shaft of the induction motor. If it is only electrically
connected to the induction motor it will deliver its mechanical
power to a third machine which operated as a generator.
'
For speeds above synchronous speed the current and emf of the
regulating machine are in phase with one another (Figs. lc and ld).
Thus for these speeds the regulating machine operates as a generator. If it is mechanically coupled to the induction motor, it
receives mechanical power from the induction motor. If both
machines are only electrically coupled, then a third machine supplies mechanical power to the regulating machine. This power,
transformed into electrical power, is delivered by the regulating
machine to the rotor of the induction motor.
2.
Speed variation by inserting resistance in the rotor circuit.
Rotor resistance may be used to obtain any desired speed (below'
synchronism) for a given torque.
Extra resistance for starting wound
induction motor increases starting torque, reduce.s starting current.
If
resistance is left in during running conditions the speed will be reduced, the slip at a given torque increasing directly with rotor resistance.
This is explained by Liwschitz-Garik and ~'lhipple, (2), in the
follovdng way:
The rotating field exert's a force on the current-carrying
conductors of the rotor and it therefore requires' a certain restraining torque in order to block the rotor. If the rotor is released,
the rotating field then drags it along; the speed increases and
untimely reaches a speed which is almost the same as that of the
rotating field, provided the only torques to be overcome are those
required by the small no-load losses. The rotor cannot travel at
exactly the same speed as the rotating field, for under this condition the rotor conductors would be sta.tionary relative to the field
and no voltage could be induced in them; consequently the rotor then
would carry no current and no force would be exerted upon it. Thus'
the sneed n of the rotor must be less than that of the rotating
field- (ns). The slip s of ·the rotor with respect to the rotating
field is defined as s
ng-n • For low values of slip, i.e., high
ns
rotor speed, the relative velocity of the rotor with respect to the
rotating field is low and the voltage induced in the rotor is small;
conversely, large values of slip produce higher voltage in the rotor.
Any given torque requires a definite rotor current which is proportional to the voltage induced in the rotor; consequently, for a
given torque the slip s must increase liLth the rotor resistance, for
the greater the rotor resistance the greater nmst be the emf required to produce the necessary current.
=
Connection from rotor coils are brought out to a set of three
collector rings mounted on the shaft thru which, by the introduction of
i
i
i
brushes, connection may be made to an outside controller and resistance.
This motor is used for both single and adjustable speed application, the
I
only difference being in that, when used as a single-speed machine the
resistance is introduced into the rotor windings for but short intervals,
I
j
in gradually decreasing steps, until all resistance is cut out of the
rotor circuit windings, and these are short circuited.
j
operates as a squirrel cage at a single speed.
The motor then
For adjustable speed, a
control is furnished with resistance of capacity to carry the load
Il
continuously on any point of the controller at which the handle may be
allowed to remain.
As the amount of introduced resistance is increased,
speed is reduced, and the regulation becomes less stable.
The rotor efficiency, very closely, in
%=1
- s, thus when slip is
increased 25%, the rotor efficiency will be 100 - 25
behaves like a slipping friction clutch.
= 75%.
So the rotor
Speed reduction by additional
resistance in the rotor circuit results in, (a), reduced rotor efficiency
and so reduced motor efficiency; (b), drooping speed characteristic, poor
regulation, and; (c), variation in slip at which maximum torque occurs
without change in value of torque.
This method will give suitable control
4
of the speed where a reduction of not more than 50% is required against
constant torque, but it is inefficient if the motor operates at the lower
speeds for long periods.
The speed can, of course, be reduced still
further, as may be required by fans, when the torque required at the lower
speeds is considerable less than full load value.
Nevertheless, the resist-
ance necessary to obtain these low speeds represent a high proportion of
the total cost of control gear, apart from the energy loss.
This decrease
in efficiency may be described by again referring to Liwschitz-Garik and
Whipple, (2).
The stator power input depends solely on the torque and varies
very little with speed for constant torque, since as the speed
decreases, the increase in rotor iron losses due to the main flux
is compensated by a decrease in windage, friction, and iron losses
due to the rotation. The difference between the power input of the
stator and the losses in the stator vdnding and iron represents the
power of the rotating field. This power does not vary with speed,
at constant torque. Hmvever, the mechanical power of the rotor is
directly proportional to the speed at constant torque. The difference between the power of the rotating field and the mechanical
power of the rotor is the electrical power of the rotor. This
power,which is equal to the slip times the power of the rotating
field, is dissipated in the resistance of the rotor circuit. The
efficiency of the motor therefore decreases as the speed decreases,
and the percent decrease in efficiency is almost equal to the percent decrease in speed.
3.
Speed control by motor clutch.
I included a description of this principle since it is used in the
Magie Winch Assembly made by the Lake Shore Engineering Co.
This unit by the employment of an eddy current or magnetic clutch, provides a range of speeds which are attained by the use of an internal
arrangement of a nagnetic circuit so that the ad.justed output speed of
the motor shaft roy be obtained vath the rotor at all times operating at
full normal speed-- thus the ventilation is constant and the output speed
nay be reduced to a very low value without causing any tendency to over-
5
heat.
In the construction of this motor there is no contact between
the driving and the driven members since the rotor is carried on a
sleeve which rotates on the motor shaft.
On the end of this sleeve
is a drum which runs at rotor speed and carries on its inner periphery
a magnet coil that is excited by dc from a step down transformer and a
rectifier unit.
On the output shaft, a.nd rotating within the magnetic
drum, is an assembly which may be considered as the driven element.
When the magnet coil is fully excited the assembly is rotated at the
same speed as the driving drum and this speed is irn.parted to the output shaft of the motor.
As the excitation of the magnet is reduced"
slip between the two elements occurs and this increases in proportion
to the reduction of excitation.
If no excita.tion were imparted to the
coil, the assembly would remain still even when the driving element
was revolving at full speed.
be obtained.
Thus by varying excitation any speed can
The practical speed range is 10% normal to normal.
disadvantage vdth this system is the complicated construction.
6
The
CHAPTER II
THE STATOR FED POLYPHASE COHMUTATOR lIOTOR
This is a discussion of the 3-pr.ase polyphase commutator motor,
stator fed.
This motor was in competition with the Schrage and was
formerly manufactured by the Crocker-Wheeler Company in this country,
but they have stopped production of these in favor of an electronic
control for speed adjustment.
Many have been built by the British
Thomson-Houston Co. in England.
1.
Basic theory.
The stator has a normal three phase \v.inding and the rotor has a
dc winding with a commutator.
The brushes on the commutator normally
are displaced from one another by 120 electrical degrees, so that a
2-pole motor has 3 sets of brushes.
If ·the stator is supplied with a
3-phase current, a rotating field is set up which rotates at a speed
n s ::. l20fJ!poles, relative to the stator.
Obviously, the frequency of
the emf induced in the stator winding by this rotating field is the
same as that of the line (fl).
On the other hand, the frequency of
the emfs induced in the rotor coils is the slip frequency, f2
/320
= sfl,
where n is the actual rotor speed.
= p(ns-n)
The magnitude of these
emfs is determined by the relative velocity between the rotor winding
and the rotating field.
It is different, however, with the voltages at the commutator
brushes; the frequency of these voltages is independent of the speed
of the rotor and is always the same as that of the stator Winding,
namely, line frequency.
This may be seen as follows; i f the field
remains stationary in space, then at all speeds only a dc voltage will
8
appear at the brushes, just as in the case of a de IJ1..aehine.
The
magnitude of the dc voltage would depend upon the rpm of the armature
and the position of the brushes on the commutator.
But now if the
brushes remain stationary in their original positions on the conunutator and the poles are set in rotation, an ac voltage appears between
the brushes; the frequency of this voltage is independent of the rpn
of the armature and is proportional to the velocity of rotation of the
poles.
Assume this velocity is n rpm, then for a machine having p poles
the frequency of the ac voltages at the brushes is pn/120.
In the ma-
chine being discussed the speed of the rotating field (of the poles)
is n s
=120fl!p rpm;
consequently, the frequency of the voltages at the
brushes is alvvays line frequency, (fl), regardless of armature speed.
Hence, the brushes of this motor may be connected to the same line as
the stator winding without imposing any limitation whatever on the
speed of the armature; therefore, the armature can be supplied ivith
energy directly from the line.
At synchronous speed the emf induced
in the rotor is zero; above synchronous speed the slip is negative and
the emf induced in the rotor ,i.Lnding reverses its direction in relation
to the conditions for sub synchronous speeds.
This applies not only to
the emf produced by the min flux, but also to the emf produced by the
leakage flux.
This leakage emf becomes negative at speeds above syn-
chronous speeds and this improves the power factor.
By means of the
commutator, the slip-frequency, (S£l), emf's induced in the rotor coils
are comnutated to the stator frequency, f1' and their frequency appears
at the brushes.
If N2 is the munber of turns between two brushes the
the magnitude of the voltage betvreen these brushes is E
kdp2
10
a
4.44sflN2
-8 volts, where k-9.p2 is the winding factor for the secondary.
9
The magnitude of this
volt~ge
depends upon the rpm of the armature,
but it s frequency is constant and equa.l to fl.
The follovdng is part of the description by the Crocker-'iVheeler
Co., (1), as to how the motor operates.
Yfuen the motor stator winding is connected to the line a
revolving magnetic field of constant strength is set up. At standstill the revolving magnetic field generates a rra:x:i.mum voltage in
the rotor winding. If the brushes are short circuited, a heavy
current flows in the rotor and the rotor quickly comes up to a
speed slightly below synchronous speed. If instead of short
circuiting the motor brushes, a voltage (exactly equal and opposite to the voltage generated in the rotor by the revolving
magnetic field) is applied to the brushes, no current will flow
in the rotor circuit and the rotor will remain stationary. Now,
if this bucking voltage (which is applied to the motor brushes)
is gradually reduced, the difference between the bucking voltage
and the voltage generated in the rotor winding will cause current
to flow in the rotor. This current develops a motor torque and the
rotor revolves in the same direction as the rmgnetic field. As the
difference in speed between the rotor and the revolving magnetic
field is reduced, and the rotor comes up to speed, the voltage
generated in the rotor by the revolving magnetic field is reduced.
The rotor comes up to such a speed that the voltage generated in it
is just slightly higher than the bucking voltage applied to the
motor brushes. As long as the bucking voltage refl'ains constant,
the motor continues to run at this speed. :8'J adjusting the bucking
voltage the motor can be Jllll.de to run at any speed from standstill
up to a speed slightly below synchronous speed (at which speed the
motor runs when the bucking voltage is reduced to zero and the
brushes are short-circuited).
2.
Induction regulator.
As explained above, the speed of the motor can be regulated above
and below synchronous speed by applying a variable voltage of supply
frequency across its armature.
This variable voltage is obtained from
the regulator which act s as a variable-ratio transformer.
This, reg-
ulator, see Fig. 2, consists of two, single-phase, indudtion type, voltage regulators, placed in one frame, with the two rotors mounted on a
common shaft.
The primary windings are located on the rotors and
connected through flexible leads to the same three phase source of povrer
10
as the stator winding of the motor.
The two secondary windings are
placed on the stationary elements and cormected to form a source of
three-phase voltage vThich is applied to the motor brushes to provide the adjustable voltage for the regulation of the speed.
The sec-
ondary voltage of the regulator depends upon the position of the regulator primary coils with respect to the secondary coils.
When the axis
of a primary coil coincides vrl.th the axis of a secondary coil, the voltage induced in the secondary coil is a maximum.
When the rotor is turn-
ed so that the axis are at right angles, no voltage is induced in the
secondary coil.
If the rotor is turned still further, so that the axis
. of the primary coil coincides with the axis of the secondary coil but in
the opposite direction, a maxinnlm voltage will again be induced in the
secondary coil but it will have a reversed polarity relati.ve to the
primary voltage.
In other set ups, like those made in England, the
stator windings are connected in
windings are connected in series.
p~rallel
to the supply and the rotor
See Fig. 3.
The resultant regulating
voltage from the secondary is of constant phase, but of variable magnitude.
There is no torque on the regulator handwheel because the torques
of the tv'TO halves neutralize each other.
As the handwheel is moved away
from its low speed position and the regulator secondary voltage is
gradually reduced the speed of the motor rises.
As the voltage falls to
zero (by further hand wheel movement) and then increases in the opposite
direction; the motor speed rises above synchronism.
3.
Speed range.
The full-load speed range of the
Crocker~fueeler Polyspeed
for instance, for continuous operation, is from 1720 to 580 rpm.
motor,
By
prOViding a separate, constant-speed, motor-driven blower, the motor can
11
be operated continuously at speeds below 580 rpm.
The percentage drop
in speed from no load to full load, is similar to that of a directcurrent adjustable speed, shunt
motor~
See Fig. 4a.
With a constant-
torque load the drop in speed from no load to full load, in rpm increases
somewhat as the motor speed is reduced.
4.
Efficiency.
The efficiency is relatively high at all speeds; at speeds below
synchronous speed, the slip energy, which in the slip-ring motor is
dissipated in the secondary resistance, is returned to.the line thru
the regulator.
At speeds above synchronous speed, a part of the energy
for driving the motor is fed into the stator and part fed directly into
the rotor.
This nakes particularly effective use of the motor windings.
See Fig. 4b.
5.
Regenerative braking.
This is an inherent characteristic of the root or • When the induction
regulator is moved from a high speed to a low speed position, the motor
is brought dovin to the lower speed with a strong regenerative braking
effect.
This is because the main motor acts as a generator, feeding a
heavy current back into the line.
6.
Reversal.
The direction of rotation can be reversed by interchanging any tv-TO
of the line leads; in changing the direction of rotation of the. motor no
change should be made in the interconnections between the motor and the
induction regulator.
The brush position which gives the best motor
perforwnnce for one direction of rotation is not the best position for
the other direction.
Motors which are to be frequently reversed in
service should have their brushes set in a compromise position vfiich
12
worse.
2)
The commutator potentially requires more maintenance than
in the Schrage because it takes the full current.
3)
The induction regulator takes up as much space as the motor
proper; hence, as far as weight and space is concerned the
13
SJr
Schrage is far superior.
-
CHAPTE.i1. III
HIGGS MOTOR
Supposed disadvantages of the Schrage and stator fed nachines.
1.
Th~
Higgs motor is a stator fed machine.
The speed variation is
against constant torque and gives a horsepower which varies in
proportion to speed.
Messrs. Higgs IvIctors claims that the disadvantage
of the Schrage is the additional brush gear necessary and the increased
commutator wear due to sparking, as it is essential to vary the position
of the brushes continuously with changes of speed, (actually, this is an
exaggeration as will be shovm; there is no sparking and the brush gear
is no more complicated than the induction regulator).
Schrage motors
are also unsuitable for direct connection to high tension mains, O\1.ing
to the presence of slip rings and brush gear in the circuit.
The draw
back of the stator fed rrachine previously described, according to Higgs
is that the rotor energy is "at
101'1
pressure lt
•
Consequently the current
value is high, necessitating a large number of brushes and heavy
COn1'llUtators.
2.
General description.
.
Higgs Motors claims to eliminate these disadvantages, for, though
the fixed brush position and variable ratio transformer are retained,
sparking is eliminated by the use of additional rotor 'winding; such
windings can also be used on the Schrage and the conventional stator fed
motor.
Also the power factor, see Fig. 5, is improved by an auxiliary
winding on the stator, see Fig. 6.
The induction regulator has a movable
rotor, the position of which determines the voltage applied to the
COIl"..ffiutator of the motor.
For anyone position of the regulator this'
18
voltage is, however, constant at all loads, except for a small drop
due to the resistance and reactance of the vdndings.
motor is
practical~
regulator setting.
The speed of the
constant betvreen full load and no-load for a given
See Fig. 7.
As the rotor winding is connected to a
commutator instead of to slip rings, the frequency of the current
collected from the brushes is the same as that of the line, whatever
the speed of the machine.
Excess energy can, therefore, be returned
to the line through the regulator with a corresponding increase in
efficiency.
At speeds above synchronism., on the other hand', energy flows
from the line through the regulator to the l.'1Otor, thus enabling the latter
to develop more power.
The output is, in fact, in proportion to speed.
The normal speed range of 3 to 1 can be increased to 10 to 1 by using a
larger regulator, while by employing a series resistance it can be brought
to a crawl.
Inching can
easi~
be obtained by bringing the rotor of the
regulator to the lowest speed and then operating the stator switch.
As
no main line current is supplied to the rotor, slip rings and their brush
gear are unnecessary.
The number of fL"'C6d brushes for a given size of .
motor is small and both the brushes and commutator require little attention owing to the absence of sparking.
To maintain the temperature with-
in reasonable 'limits at the lower speeds, vdthout excessive vdndage losses
at higher speeds, as occurs when the fan is driven from the motor shaft,
all motors ivith a speed variation of 3 to 1 are fitted ivith a separate
fan.
3.
The rotor.
This is provided with both main and compensating windings, see Fig.
8.
Each coil of the min winding is connected in parallel 'with a coil
of the commutating winding and the pair are connected to the same
19
commutator segments.
They are not, however, placed in the same slot,
and therefore do not undergo comnutation at the same time.
Any. com-
mutating emf in a main coil will thus be discharged thru its associated
commutatingcoil.
Referring to Fig. 9.
same commutator segments as a and b.
A and B are connected to the
The coil a of the aux. winding is
in parallel vdth coils A of the main winding, but as the two coils are
not in the same slot, they do not undergo commutation at the same t:i..m3.
Any cOIIlIlutating emf in the main winding coil A can therefore discharge
round the commutator coil a which is linked by transformer action with
the other coils of the main winding in the same slot, and when the whole
armature is considered together it will be seen that all the coils are
connected in parallel as regards the discharge of the commutating winding emf in the main winding coil undergoing commutation. As, moreover,
this coil is linked by transformer action with the main coil in the same
slot and through it with the other main coils, a path of low impedance
is provided which is sufficient to prevent sparking.
The commutating
winding is placed at the bottom and is separated from the main winding
by a number of insulated steel strips, so as to reduce the nain slot
leakage.
4.
Regulator •
. Two single units are employed on the regulator, a.nd each of these
has a primary and secondary winding, this arrangement being adapted in
order to avoid phase shift between the two.
Primary windings are in
parallel and connected to the mains, see Fig. 10. The secondary windings are connected in series to the motor commutator and comprise the
stator of the regulator.
The cross connection, Fig. 10, causes the
phase vectors of the two secondary windings to rotate in opposite direc20
tion .men the rotors are turned, thus producing a resultant emf, variable,
but always in the same direction.
This resultant emf is the vector sum
of the two individual emfs.
21
CHAPTER IV
SCHRAGE MOTOR
1.
General.
The possibilities of this motor are very ereat.
used
~lone,
It can either be
or in connection vIith induction motors, or as a variable
frequency speed regulating device, especially where a large number of
small motors are regulated simultaneously, as in the spinning industry.
Generally speaking, the commutator motor can be used alone for small
and medium powers, say, up to 300 hp., although machines of this type
have been supplied up to 1000 hp.
For higher powers induction motors
are more suitable, but here again speed regulation above and below
synchronous can be conveniently and very economically obtained by
cascading the induction motor with acomrrnltator motor.
Of course stator
fed com'llututor motors could be used for higher powers.
The supply volt-
age of a Schrage motor may not normally exceed 600 volts.
The commutator
motor both be itself and when cascaded, gives complete speed regulation
over a wide range lvith practically no loss.
The fundamental difference between the ac and the dc motors is that
in the latter, voltage is absorbed by a counter emf and by resistance,
representing output and pO'wer loss; in the former we must take into
account, a new factor--inductance, so that the voltage absorbed, being
wattless, will cause a lowering of the power factor" In the dc motor
designers aim at a strong field, combined with a relatively weak armature,
so as to reduce armature reaction as far as possible.
good power factor is essential.
In the ac motor
Good designing will entail low se1£-
inductance and the combination of a strong armature and a
25
l~ak
field;
consequently some method must be devised to.eliminate the effects of
high armature reaction.
It becomes necessary, then, to reduce the
magnetic flux of armature reaction, or to increase the effective
magnetic reluctance, and this is accomplished by various forms of compensation •. Every commutator motor thus consists of a field winding,
an armature winding, and a compensating winding.
In addition to its adaptability to speed reulation the cOIIJITnltator
motor has a second extremely important advantage; by its very nature
it is capable of providing its own excitation current.
Excitation for
the ordinary induction motor is provided by the mains, and consequently
wattless current is dravm from the alternators, but the commutator
motor, whether employed alone or in cascade with an induction motor, is
self exciting.
It my even be run so as to feed back wattless energy
to the .rrains while absorbing true watts, and thus can act as a po\'w-er
factor compensator.
As a rule the three phase commutator motor is
oP~y
employed for
drives requiring much speed regulation or very frequent starting, as
only in such cases are their advantages fully utilised.
These two points
therefore characterise the type of drive for which they are most suitable,
which include; printing presses (rotary and flat), pulverised-fuel plant,
pumps (centrifugal and ram), ring-spinning frames, rolling mills, mechanical stokers, sugar refining machinery, traveling baking ovens, traction motors, large machine tools, calenders, calico-printing machines,
cement kilns, compressors, blowers and fans, cranes, frequency changers,
high speed lifts, colliery winders and hoists, knitting machines, and
paper making na.chinery.
More examples of how they are used will be
given later.
26
2.
The correlation between an induction motor and the Schrage motor.
In the three phase, slipring, induction motor, the rotor revolves
in the same direction as the rotating field, the latter being set up by
the stator current.
The difference in the speeds of the rotor and the
magnetic field, termed the slip speed, is a small fraction of the
synchronous speed.
(Think for convenience of a motor in which the stand-
still rotor slip-ring voltage is the same as the supply voltage).
If the
supply be connected to the slip rings, and the stator vrl..ndings be closed
upon themselves, the motor will give a somewhat similar performance as
when running connected nornally, and its slip will be the same order as
formerly.
In this case, hovrever, the winding which is producing the
magnetic revolving field, namely the rotor vrl..nding, is itself revolving.
In order then, that the nngnetic field should cut the stator winding at
slip speed, the rotor must turn in the opposite direction to that of the
magnetic field.
If the rotor revolves at the same speed relative to the
frame as the nngnetic field revolves relative to the rotor winding, that
is, at synchronous speed, the magnetic field would then be stationary
relative to the fra.rre, and there Vlould be no slip.
As there must be
some cutting of the stator coils by the field in order that the motor
can do work as such, the rotor must travel at a slightly slower speed.
than the magnetic field but in the opposite direction.
The result is
then that in such a connected machine the magnetic field is revolving
relative to the frame at slip speed, in the opposite direction to which
the rotor is revolving.
The strength of the field is constant, and so
we can look upon it as a revolving field similar to that present in a
synchronous motor, but revolving at only a snnll fraction of the speed
of such· a field system as a synchronous motor working on the same supply
27
•
frequency and having the same number of poles.
Summarizing, the con-
ditions in such a motor are that the magnetic field is cutting the rotor
conductors at synchronous speed and the stator vdndings at slip speed,
while the rotor current is alternating at supply frequency and the stator
current at slip frequency.
Now suppose an ordinary dc vdnding with commutator is fitted on the
rotor also.
The conductors in this winding are also cut at synchronous
speed by the magnetic field.
The voltage at the brushes of a dc gener-
at or depends upon the relative position of the brushes to the axis of
the field.
In an ordinary dc winding, the nnximum voltage (neglecting
the distorting effect of armature reaction) is obtained when the brush
axis is parallel to the axis of the field and the generated field is
zero when the brush axis is at right angles to the axis of the field.
The generated voltage ,iLth the brushes in any intermediate position is
proportional to the angle between the axis of the brushes and the
magnetic field.
If the brushes on a dc generator were caused to revolve
slowly the generated voltage would vary accordingly, the voltage at one
brush varying from positive maximum through zero to negative max:imum
and back again to positive maximum while the brushes revolved through
a distance equal to two pole pitches.
The same effect would be obtained
if, instead of the brushes being moved, the magnetic field turned around
the frame, the brushes reLklining fixed.
Consider now the voltage gener-
ated in the dc winding placed in the rotor slots of the ac motor described
above.
The voltage will vary 'with the position of the revolving field,
and as the latter is revolving at slip speed relative to the fixed brushes,
the generated voltage will also vary at slip frequency.
The commutator
and its winding is acting as a frequency changer; the frequency of the
voltage at the comnutator brushes being at slip frequency, and the
voltage at the slip rings being at the supply frequency.
To obtain
the maximum voltage from a dc generator the brush arms must be an
exact pole pitch apart.
If it "vere possible to decrease the distance
between the brush sets of opposite polarity the generated voltage 'would
be decreased also, and if the minus and plus brush coincided on the
commutator, the voltage iVould be zero.
This is a Schrage motor, see
Fig. 12, a polyphase, rotor fed, induction motor with an additional
commutator vanding which is placed in the same rotor slots and on ,top
of the prinary winding in order to reduce the commutation reactance voltage.
The air gap flux is set up by the primary winding and is practically
constant over the rated load range due to the constantcy of the applied
voltage and frequency.
Now let us examine what happens even more closely; assume the motor
is wound for 3-phase, 2-poles.
Vclhen the motor supply switch is closed,
the current passing thru the rotor primary vdndings creates a magnetic
field which rotates at s,ynchronous speed vdth respect to the rotor, let
this be n s rpm.
If the rotor is locked this field vdll cut across the
stator winding and induce in it emf's and currents.
The interaction of
the stator currents and the rotor revolving field will produce a torque
tending to rotate the rotor in the opposite direction at n rprr, i.e., in
opposite direction to that of the rotating field.
speeds and torques set up.
Fig. 13, shoVls the
The rragnitude of the stator induced emf is
proportional to the rate at which ~ r (field) cuts across the stator
vdnding and the field now moves relative to the stator at (ns-n) rpm.
So stator emf is El = K(ns-n), where K is a constant.
The emf in the
stator of a 2-pole machine completes one cycle per revolution of the
29
rotating field. ,As the field makes ns-n revolution in space per minute,
the frequency, fl, of the stator emf is fl
= (n s -n)/60
cycles per second.
The commutator vanding is cut by the field at synchronous speed. It
follows that the nmdmum emf in any conductor or group of conductors
forming part of the commutator winding is constant and independent of
the motor speed.
The only vray the brush eInf, Eb' can be varied is by
altering the number of segIrents contained between any brush pair.
In
order to get the l'lBximum emf at the terminals of a dc generator the
brushes must be arranged to make contact vr.i.th conductors in the neutral
zone as shovro (bb), Fig. 14.
Let the poles be rotated relative to the
brush center line to positions 2, 3, and h in Fig. 14.
In position 2
the brushes make contact viLth conductors at right angles to the neutral
In position 3 the ma.xi.nn.un emf is again
zone and the brush emf is zero.
generated, but the emf is reversed.
In position h, the emf is again
zero; a further movement through 90 degrees brings the poles back to the
first position.
Thus the brush emf goes thru an ac cycle during the
rotation of the field system.
This is shovm in Fig. 15.
If the pair
of brushes were placed at b'b', a similar emf would be generated, but
its maximum value would be reduced as shown.
generated at each pair of brushes in Fig. 14.
Similarily, an ac emf is
The frequency of the
brush emf vr.i.1l be equal to the number of revolutions in space of the
field per second, fb
= (n s -n)/60
as in the stator winding.
cycles per second, the same frequency
Referring to Fig. 14, the brush center lines
are 120 degrees apart, so 1/3 of a period elapses while the crest of the
rotating field passes from the center line of one brush pair to that of
the next pair.
This gives a 3-phase supply at the three pairs of brushe.s.
It is clear then that the brushes can be connected to the three phases of
30
the stator winding as shown. With the brushes in the position shown,
Eb and El reach their maximum values at the :mme instant, i.e., the
two emf's are in phase.
They oppose one another, hovfever, in
circulating current through the stator winding.
Since the third or regulating winding is carried on the rotor, the
field always rotates at synchronous speed 'with respect to this vfinding,
and the emf induced in this winding is always at supply frequency. When
the motor is at rest with the rotor winding energized from the supply, the
voltage between the two sections of each set of regulating brushes will,
of course, depend on the circumferential distance apart of the brushes on
the commutator.
If the brushes were arranged so that the voltage across
each set 'was equal and opposite to that induced in the secondary windings,
no current Vlould flow through the secondary and regulating windings, and
no torque would be exerted.
Magnetizing current only would .flaw through
the rotor primary windings.
Speed variation is achieved by simultaneously
opening or closing the brush pairs on their center lines.
If the brushes
are on the same segment, E1, .. 0, and the machine rlUlS as an induction motor.
In the lowest speed brush position the regulating voltage at the brushe s is
actually less than that induced in the secondary vfinding so that a secondary current will flow and the motor will start, if free to move.
Referring
to a 2-pole 50 cycle motor, let us assume that 60 volts will be induced in
the stator lrlnding when the motor runs at 1200 rpm (60% slip).
Arranging
the regulating brushes to inject 60 volts into the secondary 'windings in
opposition to the secondary emf will give the motor a no-load speed of
approximately 1200 rpm since at 1200 rpm no current will flow thru the
secondary and regulating windings.
Increase of the injected voltage by
further separation of the brushes will cause further reduction of speed,
-31
for if the brushes are opened out (
motor speed drops tUltil El
X7"1:) Et, opposes El and the
=k(ns-n)
is large enough to circulate enough
stator current to produce the driving torque.
(~) to the position shown,
ed over
If the brushes are cross-
Eb reverses and helps El and
the motor speeds up to just below synchronous, i.e., until El is reduced to a value where El plus
stator.
If
~is
E1> circulate sufficient current
in the
increased until synchronous speed is reached, the
rotating field becomes stationary in space and El
=0, and fb = f 1 =0,
so the motor is fed with dc from the cOIIlnutator. A further movement of
the brushes,
speed.
(;7;:
~),
causes the motor to accelerate above synchronous
The rotating field nOi' reverses and goes in the same direction
as the rotor.
until El
So El reverses and opposes Eb. The motor will accelerate
= K(n-n s )
obtains a value at which Eb - El is just large enough
to circulate the necessary torque current in the stator.
This motor
operates with a shtUlt characteristic, the speed drop on load being
to 10% of maximum speed.
2i
The hp depends on its speed; the primary
current and hp being reduced with the speed, while the full load secondary current is constant at all speeds.
The best method of protecting
the motor is therefore to connect an overload release in the circuit
with the secondary windings and arranged to trip the main switch as
shown in Fig. 12.
Power factor is corrected by rocking the entire brush system round
the comnutator.
w:i.1l be advanced.
80
At sub-synchronous speed, i f Eb is retarded El - Eb
This will ad.vance the phase of the stator current and
improve the power factor.
In order to retard
Et"
the brush system
must be rocked in the direction of the rotating field, i.e., against the
direction of the motor, see Fig. 16. The rotating field will then cut
32
across the center line of each brush pair a little later than it cuts
across the center line of the corresponding stator phase.
At super
synchronous speed the field reverses and travels round in the same
direction as the motor.
The field now cuts the brush center line be-
fore the stator phase center line and, therefore, Eb is in advance of
El; this causes Eb-E I to lead.
In other words a shift of the brush
system against the direction of the motor gives improved power factor
at all speeds.
placement.
An important development is dissymmetrical brush dis-
This means that the brush rockers are not displaced at the
same speed towards or away from each other, so that the axis of the
regulating winding on the rotor is displaced through a small angle with
regard to the axis of the secondary winding on the stator, the displacement being greater the lower the speed.
An improvement in the effi-
ciency and power factor of the motor at loVl speeds is claimed from this
arrangement.
Moreover, a dissymmetrical displacement decreases the
full load current not only in the primary but also in the secondary
circuit, and this is very advantageous as regards temperature rise,
par.ticularly since at low speeds ventilation is necessarily poor.
Fi-
nally, the starting torque is considerably increased by dissymrnetrical
brush displacement.
Examples of the power factor variation can be
seen on curves, Fig. 17 and 18.
3.
Reversal.
The motor can be reversed by changing over two of the supply leads,
but it may also be necessary to move the brushes slightly around the
commutator to obtain the best starting torque and power factor.
4.
starting torque and current.
The starting torque and
~
33
torque are imporved by moving the
brushes in the opposite direction to the motor rotation.
The starting
torque is from 150 to 250% of full load torque with rated voltage and
the brushes at their low speed position.
A usual starting current is
from 125 to 175% of full load lipe current.
5.
Speed range.
Conmutator motors having infinite speed settings can be obtained
with a naximum speed of as much as 15 times the minimum speed, while
speed ranges of 3 to 1 are in connnon use; if the adjusting or commutator
winding is built with a capacity of 50% of the stator winding capacity,
a speed range of 3 to 1, from 50% to 150% of synchronous speed is possible.
It is generally preferable to choose the speed range so that the
top and bottom speed required are equally remote from the synchronous
speed.
In the General Electric f s ACA type motor any creeping speed dovm
to 50% of minimum rated speed may be obtained at rated torque for one
half hour without injurious heating.
A very low speed range for occa-
sional auxiliary duties can be attained by inserting resistances in the
secondary winding, the rotor being designed to give the range demanded
by continuous service.
The insertion of resistance, however, adversely
affects the shunt characteristic, i.e., the speed drop from no load to
full load will be greater than for speed regulation by brush shifting
alone.
Thus, i f speed stability in the lower range is of great impor-
tance, it may be advantageous, with medium-size IOOtors, to employ wider
brush displacement ranges, such as one to eight.
Generally, motors hav-
ing a speed range of 2 to 1 or over are started by switching direct on
the line, with the brushes in the minimum speed position, an interlocking switch being fitted on the brush gear to energize the motor switch
if an attempt is made to start the machine with the brushes in any other
34
position.
6•
Starting and control gear.'
The starting gear usually consists of a three pole switch, con-
tactor, or oil circuit-breaker with under-voltage and over-current
protection; it is thus simple and cheap compared with induction motors
requiring autotransformer or star-delta starting.
For automatic control
a triple pole contactor (with time lag over-current relays) may be used.
For speed regulation the rack and pinion provided on each brush rocker
may be operated either by a handvmeel (mounted on the motor) or by
power, either mechanical or electrical.
Power operating gear may con-
sist of chains, shafts, flexible wires and pulleys, or a pilot
with high ratio gearing.
w~tor
The latter enables remote electrical and auto-
matic control of the main motor speed; the pilot motor must be reversible and must be fitted with a limit switch at the extremes of the brushgear travel, a slipping coupling being sometimes added.
The brushgear
operating mechanism can be pre-set so that the motor accelerates from
standstill to a prescribed maximum.
A typical automatic control equip-
ment cOIIlprises four push buttons (for starting, stopping, accelerating,
and retarding), a main contactor, overload relay, brush-shifting pilot
motor, and a limit switch.
When the start button is pressed, the nnin
contactor is closed, starting the nntor, the limit switch is interlocked vdth the main contactor, so that the latter can only be closed
i f the brushgear is in the correct position whenever the main contactor
opens.
Compared lvith ordinary induction motors, the cost of the Schrage
is relatively high, and the commutator requires extra maintenance.
The
commutator, however, handles only a fraction of the total output, requiring only small voltages and currents to be dealt with, and in modern
35
machines the old commutation troubles practically disappear, provided
that care is taken to use the correct grade and type of brushes.
36
CHAPTER V
THEORY OF SCHRAGE MOTOR BY CIRCLE DIAGRAMS
This chapter consists of four arlicles by Conrad, Zweig, and
Clarke, (10), on the theory of a Schrage motor.
The first arlicle
explains the s:im.ple theory underlYing the circle diagram.
1.' Operation of machine either as a motor or generator is explained on the basis of superposition of currents.
This explanation employs an extension of the application of
the induction motor circle diagram theory. This new theory is most
helpful in explaining some of the motor characteristics such as
power factor correction, design requisites for certain speed ranges,
generator action, and its use for regenerative breaking.
The speed of an ordinary induction motor can be changed by inserting into the secondary element a voltage of slip frequency,
provided this voltage is in such a phase position that it forces a
po'wer component of current into the secondary (that is, this current
produced in the secondary is not maximum when the flux surrounding
the secondary conductors is zero). In the particular motor described
here, this current is obtained from an adjusting winding on the rotor.
The brushes are mechanically coupled so that they are spaced at the
same distance for each secondary phase winding, thereby insuring
equal voltages conducted to each secondary phase. The speed can be
reduced by separating the brushes in a given direction so the voltage collected from the brushes causes a component of secondary current which produced a negative current. The machine can be operated
above synchronism by interchanging their positions (a) to (b) and
(b) to (a), see fig. 19, so that the voltage collected by these
brushes is in such a direction as to force a current through the
secondary which will produce a positive torque. The motor can be
reversed by reversing two of the leads supplying the primary. Speed
adjustment cannot be obtained merely by inserling a voltage into the
secondary from the brushes unless this voltage is in such a direction
as to cause a current that is torque producing. A voltage collected
by the brushes even though large in magnitude may produce little or
no ,change in speed if the current that it produces in the secondary
conductors is in quadrature with the flux surrounding these secondary
conductors. Such a condition lvill materially change the povrer factor
of a motor without appreciable change of speed. Since it is possible
to change both magnitude and direction of this voltage collected from
an adjusting vrinding, there is an infinite number of different brush
settings that may give the same speed adjustment. However, there is
but one setting of the brushes that will provide a given speed at a
given power factor for a given load. Since all of these variables
are interdependent, it is desirable that some form of explanation
which shovro their relation be presented. To show these relations, a
specific brush setting will be chosen and the operation of the motor
41
explained for this setting.
2.
Operation below synchronous speed.
Let us assume that it is desired to operate this motor at
half synchronous speed. This can be accomplished by spacing the
brushes so that the voltage ElJ- induced in the adjusting winding
between the brushes (a) and (b), fig. 19, is exactly 180 degree's
out of phase with the voltage E2 induced in the secondary due to
slippage of the secondary with respect to the flux. The separa-:
tion of the brushes should be sufficient to make Ell equal to
half the standstill value of E2 in order to obtain a no-load speed
equal to ! synchronous speed. In fig. 19, this induced voltage
due to slippage will be designated as E2, and the current that i t
forces thru the secondary and the adjusting winding between the
brushes will be referred to as 12. The voltage induced in the
adjusting vdnding between the brushes vdll be referred to as Ell'
and the current that it causes to flow is designated as Ill. The
total current flowing in the 'secondary for any condition of operation is the sum of the currents 12 and Ill. The mgnitude of
this total current can be best understood by dealing with each
component separately.
Assuming that the flux is constant for constant impressed
primary voltage, the voltage per turn in the adjusting winding
will be constant regardless of the speed. So for any brush setting
En will be constant(independent of slip). The frequency of Ell
changes with slip and at all times is the slip frequency of the
motor. The voltage E2 induced in the stator by the constant flux
is directly proportional to the slip and is of slip frequency.
Therefore E2 and Ell are always voltages of the same frequency.
The current 12 whicli is caused by the voltage E2 is impeded by the
stator phase resistance R2, stator phase reactance 12, the resistance of the adjusting winding Raw, and the reactance of the
adjusting winding Xaw • The current III is limited by the same.
Expressions for these currents are as follows:
:r~
E".:l"
:-
V(f..?. + (\o.4;)?"
I
I,
=
+ (X2., + Xe::tw) ~
£"
VCR,.+
Ra.\AI) • +
("1<2.+
1:2. d~ k
X a.
a:::t
~....,I.crU
)(Cltl<JJ~
5 =% ~
Xqw) . . .
5S£:J...,.
~ ss x~
"k ss X ItW
('2.)
~
:J:"~
ss
=-
V(~~-t-
If'I(W)"Z...
S
S
(3)
+ Cs., Xl. S -t-$SX.(~S)~
Ot"
Xa.
=
ssFz
Y (R,. + tft£fAI) 2- -r ($3 X
Sa.
7-
I"
-
......
l;
i-
$..5
Xeu.. )
~
(4J
If
It 'Will be noted in the above equations that the seconclary currents
are li.m:i.ted by an impedance made up among other things of the .
reactance sSlavf3. This is the reactance offered to currents of
slip frequency, and the voltages that are produced by this reactance
and the secondary currents (I2.ssXawS and Ill-saXaytS) are independent of the pri.mary' supply frequency. At synchronous speed these
voltages become zero and the secondary current is limited by resistance only. Of course l this is on the basis that all the nux cuts
all :3 windings.
On the basis of equations (4) and (5) I the vector diagram of
the secondary circuit including its portion of the auxiliary winding can be constructed as in fig. 20_ From equation (4) it is
evident that the extremity of the vector 1 2 has a locus· following
the path of asemi-eircle as in an ordinary induction J1X)tor.
Equation (5) shows that the vector III also has a circle locus.
From equations (1) and (2) it is evident that the impedance offered to 12 and III are the same, and therefore, these currents must
lag their respective voltages by the same angle; therefore, the
angle 92 = ell- The maximum value of 12 (diameter of circle)is
equal to (ssl!i2)/(sh + s~w) and the ma.x:i.mum value of III is
obtained at 100% power factor of the secondary or at synchronous
43
speed and is equal to ElJ.!(R2 + Raw). The current 12 produces torque
in the direction of rotation (positive) while III produces a negative
torque. The positive torque!! K~2(COS 92). The negative torque
K~Ill(cOS 9],1). The flUX, 9, is aSSlllmd to be constant in magnitUde,
and~that all of it cuts the primary winding, the adjusting winding
and the stator winding. Since Q.z
9u for this brush setting, the
total torque is T = ~(I2-Ill) cos 9 2• For a condition of no load
(zero developed torque) 12 and III are equal and opposite. Or in
other words, the rotor must slip sufficiently to JIBke 12 increase
to a value equal to and opposite Ill- Since the brushes are so
spaced that Ell is ~ ~ at standstill, EQ. and Ell will be equal and
opposite at 50% slip, am the no load speed will be ~ synchronous
speed_ If slippage is increased by further reduction of speed
(loading) 12 becomes large:, III smaller, and a positive torque will
result. This is motor act1.on am it occurs at a SPeed less than no
load speed. If' now the nachine is driven at some speed slightly
above its no-load speed, 12 will be reduced, and. III increased_
Under these conditions, the torque produced by III becomes higher
than that produced by I2 and the total torque is negative. This
condition results in generator action. Thus with this machine
generator action can be obtained at any SPeed within its speed range.
See fig. 21.
Primary current for differenli conditions of load can be determined from the secondary currents and from the no load exciting
current in a manner similar to that used for the ordinary two
element induction motor. However, special consideration must be
given to the turn ratios for a particular motor. Any turn ratio
involving the adjusting winding is a function of speed adjustmenli
(brush setting).
The effect of the secondary currents on the current taken by
the primary can be explained from the diagram of fig_ 22a. This
diagram shows the locus of the components of I2 and I~l when ~
and Ell are 180 degrees out of phase and the machine l.S adjusted
at a no load speed corresponding to approximately 50% slip or !
aynchronous speed. The current ~ flowing in the stator and through
the adjusting ldnding will cause a current to flow in the primary
of sui'ficienli magnitude and direction so as to neutralize the flux
=
=
produced by 12- This componenli of the primary current is -I2 where
i:i=2a
the denominator is equal to the ratio of the number of turns on the
pr1Jmry to the difference of the number of turns on the stator, and
the number of turns in the adjusting lrl.nding that are placed in the
secondary circuit by this particular brush setting, or lr2a
=!!! •
N2a
The currenti ~ passing thru both the stator and the adjusting winding
will produce fluxes in these two windings that are opposite in
direction with respect to the primary circuit. The component of the
primary current that neutralizes the magnetizing etrect of ~ is
44
therefore, a vector that has itsexliremity defined by the path of
-
. the circle -12 shown in fig. 22a.
l r2a
Likewise, the current III will cause a component in the prlma.ry
current whicl\ by the same reasoning as above must be 180 degrees
out of phase with Ill. The magnitude of this primary component is -Ill'
1F2a
and is defined by a current locus which is the path of another circle,
see fig. 22a.
Adding these two vectori.ally, their sum. equals
-~
lr2a -
~;~.
The locus of the resulting current is a circle, the diameter
4.
':l..
is ab and equals V OlL + 0 .Qr
• When the machine is running
at no load the sum. of these currents equals zero. If operated at
synchronous speed, the sum. of the currents equals 00. Resultant
current below bo indicates generator action. Resultant current above
bo, which has a component in the direction of the pri.mary impressed
voltage Vl , indicates motor action. The locus of motor currents and
generator currents is defined by boa. This circle, however, does
not show the total primary current because of no load losses and the
requirements of an exciting current. Fig. 22b shows the no load
loss and exciting components of pr1Inar7 current with respect to the
impressed voltage Vl. The total primary current,Il' can now be
obtained by adding the current de.fined by the circle locus aob of
fig. 22a, to no load current in fig. 22b. This results in fig. 23,
which shows the relation or the primary current to the primary
impressed voltage for different load conditions when brushes are set
to reduce speed to approximately 50% synchronous (saE2 2Ell).
=
3.
.Operation above synchronous speed.
The speed can be raised above synchronous by reversing potential
to stator winding. This is done by transposing brush
positions. In fig. 19, brush ''bit moved to position of brush "a",
and "a" is moved to "b". By reversing the direction of Ell applied
to the stator, the III will have a positive torque and tenCi to drive
the motor at high speed. For speeds above synchronous, E2 reverses
(since the flux reverses its direction of rotation relative to the
stator), and 12 produced a negative torque tending to bring the motor
back to synchronous speed. See fig. 24. It will be noted that both
voltages are reversed with respect to the primary voltage, Vl , from.
their position of fig. 22&.
The current III flowing in the adjusting winding and. stator vd.1l
En applied
45
cause a pri.mary current component -In. For this speed adjustment,
l r2a
-
the fluxes produced by the stator and adjusting winding are in the
same direction with respect to the primary. Likewise, 12 will cause
a primary current component
-~
-lr2a
•
When these currents of fig. 24 are refiected to the primary
circuit, with due regard to turn ratios, they provide a portion of
the total primary current vector diagram shown in fig. 25a. The
total primary current resulting from the secondary currents in the
adjusting and stator is the vector sum of the currenl:is defined by
the two current. loci. Their vector sum equals 2aIl defined by
circle oac. The complete vector diagram for the primary supply can
now be obtained by superimposing the vectors of fig. 25aon those
of the no load diagram of fig. 25b. This gives figure 26. It ldll
be noted that the resultant locus of the primary current. of fig. 26
is moved toward the vector Vl from its -position shown in fig. 23.
Viith this high speed adjustment and with brushes set to make En
opposite E2 , the power factor can be made high at large loads, and
the IlllXimuDi power that can be developed is higher than that of the
low-speed adjustment described previously. This is indicated by
the maximwn power component of the -primary currenli of fig. 26 as.
compared to that of 23.
4.
EJqleriroontal check on theory.
To verify theory thus far advanced, an experimental check was
made on a motor to see how closely the current under load followed
the current predicted by the current loci circles. The motor, a
G.B. BTA, 600-1800 rpm, 6 pole, 60 cycles, 4.1-12.5 hp, was first
adjusted so that its no load speed was 600 rpm (! synchronous).
The brushes were set by opening the secondary connections at the
brushes and adjusting the brushes so that the voltage across them,
Ell, was 1 ~ across the open circuit stator coll at standstill.
This gives only one of the necessary brush adjustment.s. The other
adjustment for, phase position of Ell is obtained by keeping the
brushes fixed with respect to' each other and moving them together
on the surface of the commntator untU Ell is 180 degrees out 'of
phase with E2. With this setting the stat'or coUs can now be
connected to the brushes for operation at ~ synchronous speed for
no load.
The current. taken with this brush setting for different· values
of motor output is indicated by the 'encircled points in fig. 27.
The circle which should pass through these points can be located
by measuring the no load priJpary current (po) and the primary blocked rotor current (pb).
By erecting a perpendicular to the line ob, one diameter of
the current locus is established. If the line ob is extended to the
46
,
point f so that
.2L
bt
=saE2 ,
the point f 'Will fall on the locus of
Ell
\
From points
0
and f the circle 12
can be constructed and
lr2a
consequently the point e is located. A perPendicular to oe will
be another diameter to the primary current circle locus. Intersection of 2 diameters gives the center. The accuracy of this
circle diagram is revealed by proximity of experimental points with
current circle locus cob.
To check the theory for higher speeds the brush positions were
interchanged so that ~l between them at standstill at open circuit
was ! E2 induced in open circuited secondary and in phase with it.
This provided a no load speed of 1800 rpm (150% synchronous). A
current locus circle was determined in the same manner as for the
low speed. This current locus along with experimental points
obtained by loading are shown in fig. 28. The circle locus predicted on the basis of no load and short circuit current provides
a fair determination of the, current characteristics of the machine.
While there appears to be some divergence between this circle and.
the true current locus, the actual differences as would be indicated
on instruments as to power factor, currents, etc. are small. The
diagram of fig. 28, which displays this difference, also with proper
interpretation, reveals the theory sound. Because the difference
between the theoretical and experimental current locus can be explained on the basis of primary leakage reactance.
5.
Effects of primary leakage reactance.
The rotor has two windings; the stator, one. The necessary
spacing of the primary winding with respect to the stator introduces
primary leakage reactance which produces a voltage 90 electrical
degrees ahead of the current in the primary which subtracts from
the applied voltage in such a way that the voltage induced in the
primary' by the mutual flux (mutual to primary and stator) lags the
applied voltage. Consequently at speeds above synchronism, this
mutual 1'lux must introduce a voltage in the stator winding which is
ahead of the voltage that would be presented if there were 100%
coupling between the two windings. The angle of lead ot this
voltage in the stator is eDctly equal to the angle ot lag between
the total pri.ma.ry back emi' and the ba~k emi' produced by mutual flux.
This lead in stator voltage will cause the circle with diameter oe
01' tig.2S, to swing about the point 0 in the direction of the
rotation of the vectors with increase in load on the motor. Since
the leakage between the two rotor windings is small, there is
little shift of circle 01' due to primary leakage fluxes.
It has been found experimentally that the true primary current
locus represented by the points shown in fig. 28 can be predicted
from no, load data of three sets 01' JD8asurements taken on the motor,
as 1'ollmrs:
47
1)
2)
3)
No load input c\trrent and watts.
The standstill input current and watts at reduced voltage •.
The input current and watts on reduced voltage with machine
running.
The first and second determinations above are made in the customary
manner. The third determination can be made by applying a reduced
voltage to the priinary sufficient to rotate the machine at no load
at a speed somewhere intermediate between its no load speed and
standstill. A speed of approximately 50% of no load speed pX!Ovides
fair accuracy in construction of a circle. .
•
From the data of the three above items, it is possible to
calculate the current, power factor, etc. for normaJ. voltage. From
these three values of primary current and their respective power
factors, it is possible to construct the circle locus of the primary
current previously developed, and also to locate it in accordance
with the shift created by the primary leakage reactance. This more
accurate determination of the primary current locus is shown in fig.
29. The current· PO is the no load current, the current PS is the
standstill current for normal voltage determined from data taken
at reduced voltage and the current PR is a current that the motor
would take when running on normal voltage under some load. This
cUrrent PR is determined for a running condition at reduced voltages.
The center of the circle of fig. 29, can be found by erecting
perpendiculars to the chords RS and OR. Once this center C is
located a circle can be drawn through the points O,R, and S which
very accurately predicts the primary current locus with respect to
the impressed voltage. The accuracy of this method of determining
this locus is evident from the proximity of the locus of points
determined experimentally.
The second article shows how the theory so far advanced can
be used to determine such quantities as efficiency, current, torque,
and speed for different conditions of operation.
The theory out-
lined has been checked experimenta.lly and found correct.
There is an infinite nwnber of possible brush settings that
can be made on a Schrage motor. It is possible to move the brushes
so as to change the magnitude or the phase position or both
magnitude and phase position of the voltage collected from the
commutator. Such movements can be used to change speed or power
factor. Any change in brush settinga will materially change the
circle diagram proportions. Therefore, any circle diagram for this
is useful only in determining the characteristics of the machine
for one brush setting; however, an understanding of the use of the
circle diagram for a particular setting of the brushes will be most
helpful in obtaining a general understanding of the operation of
the machine for other setting. The diagrams used for the basis of
explanation will be those corresponding to the settings described
previously, i. e., at 50 and 150% synchronous speed.
The circle diagram can be determined from the 3 sets of readings taken at no load previously listed. From the data of these
3 sets of readings, the circle diagram of fig. 30 can be constructed. This diagram is characteristic of the machine for speed
adjustments below synchronous speed. The line po shows the no load
current and its direction with respect to the impressed voltage Vl.
It is obtained from determi.na.tion 1) above. The standstill current
Iss for normal applied voltage is represented in magnitude and
direction by the line pb.· It is obtained from determination 2).
A third point, x, on the circle is located from determination 3) •
This point is obtained from conversions made on data taken with
reduced voltage. Having the points, o,x, and b, on the circle, it
is possible by erecting perpendiculars to two of its chords to
locate the center r. Using or, as a radius the circle locus of a
primary current oxb can be located.
6.
Determination of cha.racteristics from circle diagram.
Having the currents po and pb in magnitude and direction with
respect to the vector Vl, the lines pe, and oc can now be constructed perpendicular to Vl and the lines be perpendicular to pee The
power supplied to the motor at standstill per phase is the current,
be, multiplied by the voltage VI. The current, ce, multiplied by
the VI is the per phase iron loss of the rotor, the friction and
wiridage losses, and a small amount of iron loss in the stator. By
measurements of primary resistance, the primary copper losses per
phase can be calculated,., for the normal short circuit current.
This loss is represented by the current, cm. The loss in the adjusting winding and in the stator per phase for standstill conditions is (bm). Vl.
,
.The copper losses can be estimated for any value of pritIary
current in a manner somewhat similarily to that normally used with
the ordinary induction motor. Thus for an input current of pf, it
is assumed that .no load current is one component of the loss
associated with it, (ce)Vl' is constant regardless of the magnitude
of the load. The other component of the primary current pf is of,
and the losses associated with this component can be determined by
the relations; loss for cUrrent of • loss for ob
2i
2. Another
ob
method of determining the copper loss for the current pf is to
erect a perpendicular bn from the point b to the diameter of the
circle, and another perpendicular fg to the same diameter. The
copper losses associated with the current of are expressed as loss
of cUITent of
cr
(bc)(sg)V)
(bn)
watts per phase.
the total loss for this current of
with the voltage, this loss caused
laid off on a line fh drawn from f
30, it is labeled hj. Sirnilarily,
49
Having determined
in terms of a current· in phase
by the current of can now be
perpendicular to oc. In fig.
the total copper losses can be
determined from other values of input current. ThuB, for an input
current of pfl, the total copper losses is hljl. BY' successive
determinations of losses the curve ojb can be constructed to show
the change in the copper losses with different values of input
ctlrrent. If each of the distances jh, jlhl are divided so that
='
jk :: jlkl
klhl
kii
bm, a new curve okm can be constructed so that the
me
distance HlkJ. shows the variation in primary copper loss, and kljl
shows the variation in the secondary (adjusting winding and stator)
copper losses for different values of primary current.
The variables for the motor can now be determined for any
value of input current 11.
Input. (ft)Vl watts per phase.
Output = (jf)Vl watts per phase.
Iron loss, friction and windage. (ht)Vl watts per phase.
Primary copper loss = (hk)V~ watts per phase.
Copper loss of secondary {jk)Vl watts per phase.
EfficiencY' = .J! 100%.
ft
Slip
.J.!s 100% of no load speed.
ktSpeed = l-J!f times no load speed.
=
=
kf
Torque
= (kf)Vl synchronous watts per phase.
For speeds above synchronous; fig. 31, shows the circle adjustment for 150% synchronous. The standstill current Iss for normal
voltage is much larger, and the maximum pOller output much greater.
The circle is located from the three sets of no load determinations
described previously. The essential difference of this diagram
compared with that of fig. 30 is the shape of the curves showing
copper losses for different primary currents. The systems of
notations on fig. 30 has been maintained in fig. 31; and the same
explanations hold.
7.
Experimental check on theories.
Tests l'iere made on a G.E. BrA motor, 550-1650 rpm, 4.17-12.5
hp, 6 pole, 60 cycle. The brushes were set by .making the brush
voltage Ell at standstill equal approximately ! ssE2 in the stator
coll to which they were connected. Care was taken to make these
voltages opposite in direction. With this setting the no load
tests described above under 1), 2), and 3) were made to determine
the circle locus of the primary current. The machine was then
loaded and readings of speed, torque, current, watts, and volts
were obtained tor different values of output. The primary resistance was determined by measuring the resistance of the primary winding with direct current. From the no load tests the circle diagram.
of the mchine was constructed and the characteristics determined
from ita proportions.
50
,
A second experimental check was made with the brushe s set
to provide a speed of appro:dma.tely 150% of the synchronous speed.
This ca.n"be done by making Ell across the brushes at standstill
equal to ! ssE2. Also, these two voltages should be in time phase
with each other at standstill to provide proper running speed. No
load determinations similar to those described above were made and
the circle locus of the primar,y current established for the speed
adjustment. From. this circle diagram the characteristics of the
motor were calculated. The machine was then loaded and characteristics again determined e:xperimentally. The circle diagram for
t his is shown in fig. 31.
A comparison of the theoretical characteristics and the actual
are shown in fig 32.
.
This is the end of article 2; the diagrams for articles 1 and 2
.:follow.
After the diagrams articles 3 and 4 will be taken up.
51
This is the third article by Conrad, Zweig, and Clarke and deals
with power factor correction.
8.
Brush settings for povrer factor correction.
Under normal operation of the Schrage lOOtor, the two sets of
brushes which carry the secondary current of the stator coils are
coupled mechanically so that one set of brushes cannot be JlDved
without a corresponding motion of the other in an opposite direction. Thus,· the voltage introduced in the secondary circuit from
the cOlIlIIDJ.tator, while it may vary in nagnitude with speed adjustment does not vary in phase . relative to the voltage induced in the
stator winding. Fig. 33a is a schematic diagram of a two pole 3
phase Schrage motor. The horizontal projections of the vectors
represent the instantaneous voltages induced in the adjacent coils.
Th~ flux, ~, rotates at slip speed, say clockvdse, relative to the
stator. Brushes Al, A2, and A3,are mounted rigidly on an adjustable frame so that a motion of the frame will move them all" through
the same angle; the brushes BlJ B2' and B3 are lOOunted similari.ly
and slso move in unison. These two frames are coupled mechanically
so that when making speed adjustments, the brushes are always equal
distances from the center lines drawn between them. The points of
na:xi.rmlm and minimum potential about the commutator rotate with the
field relative to the stator so that the voltages at the brush pairs
are at slip frequency. If the center lines drawn midway between the
brushes Al and Bl, and A2 and B2, and so on, remain fixed relative
to the stator, the voltage collected from the commutator are fixed
in phase relative to the induced voltage in the stator. The
magnitude of this voltage, Ell, will vary with the degree of separation of the brushes but is appronnately independent of speed.
for a given primary voltage. Interchanging the positions of the
brushes from Al to Bl, Bl to Al, and so on, will change the phase
by 180 degrees ,that is reverse the polarity of the conmutator voltage, Ell. If the coupling between the frames supporting each set
of brushes is removed, either set can be moved in either direction
independently of the other. The various possible brush settings
proved the motor with an extremely wide range of characteristics.
If the flUX, ~, is rotating clockwise relative to the stator, and
the two frames are moved opposite to the direction of rotation of
the flux with respect to the stator by 90 degrees from the position
in fig. 33a, see fig. 33b, Ell, will be advanced 90 degrees in
phase 'With respect to E2 induced in the stator. This procedure will
advance the center lines designating the brush position by 90 degrees
relative to the stator. The phase of Ell collected from. the commutator can be varied only by altering the mean position of the'
brushes as indicated by these center lines.
Figure 348. shows E2 induced in one phase of the secondary when
brushes are set as in fig. 33a, that is when E2 is in opposition of
Ell. Fig. 34b shows what occurs when both sets of brushes are
rotated in a direction so as to advance Ell by 90 degrees as illustrated :in fig. 33b. Since both E2 and Ell. are in series, and force
59
currents thru the same impedance, 12 and III of fig. 34b must lag
their voltages by the same angle. 12 and In are the currents
flowing in the secondary resulting in the voltages E2 and Ell respectively. The two currents and their circle loci are shown if
the center lines are allowed to remin in their new positions of
fig. 33b, and the two sets of brushes are moved across the center
lines, that is, brushes A and B interchange positions, Ell from the
adjusting winding will be shifted in phase by 180 degrees from the
direction shown in fig. 34,b and En will lead E2 by 90 degrees as
shown in fig. 34c. The current 12 lags the voltage E2 by the same
angle that III lags Ell.
If the motor is designed 80 that the effective number of series
e turns in the adjusting winding is ~ the effective nwnber of stator
turns, the largest value of the voltage, Ell, is half the stator
voltage, E2, at standstill. If in figs. 34a, 34b, and 34c, the
brushes of each phase are spaced 180 electrical degrees apart on
the comnutator, En vlill have its largest value. The largest value
of In (its value at synchronous speed) and the largest value of 12
are as determined in section 1 of this series. The resultant secondary current is the vector sum of 12 and In, and, as shown in
figures 34b and 34c, this current is never zero whenever En has a
quadrature component relative to E2. The locus of the extremity of
the vector representing the sum of 12 and III can be derived, and
is found to be another circle shown in fig. 34b and 34c.
In 34b it is evident that the total current flowing in the
secondary lags the voltage E2 induced in the secondary by a relativeJs' large angle at all times. In fig. 34c the total secondary
current leads the voltage E2 over a considerable portion of the
circle. These features enable the machine to generate power at a
lagging power factor when used as a generator excited trom a
synchronous source. Furthermore it can supply these lagging loads
over a wide range of speeds. This section is primarily concerned
with power factor .correction and, therefore, w:Ul deal only with
conditions illustrated in fig. 34c, rather than those of fig. 34b.
9.
PriJmry currents with power factor correction.
The phase current in the primary of this motor can be determined from a knowledge of the mmft s in the adjusting and stator
winding for different values of slip. For each current 12 and Ill'
shown in fig. 34c, there is a corresponding mmr produced in each
of these windings. The mm£ produced by the current III of fig. 34c
in the stator is represented by the vector oa of fig •. 35. Likewise
the mmf produced by current 12 of fig. 34c in the stator is
represented by the vector ob of fig. 35. The total mrnf produced
in the stator by currents 12 and In is therefore the vector oc of
fig. 35, which is the sum of os. and ob. The locus of this stator
mmf, oc, is defined by the circle dce. With this particular brush
setting on a 3-phase motor, there is a mmfproduced by these same
currents, 12 and XlV flowing in the adjusting windings. This mmf
is 90 degrees behind the mmf that these currents produce by flowing
in the stator. The vector oc I therefore represents the adjusting
winding mmf when the stator mmf is oc. The locus of the vector oc I
is the circle d' c I e I . The proportions of this circle are such that
60
•
~'
oc
=.2!!' =.2!!.' = Naw.
od
oe
\'lith the brushes set as described here,
N2
motor action results only at speeds below synchronous. For such
speeds, mmf in the stator and adjusting windings are cancelled
by equal and opposite mm£ in the primary. In fig. 35 the stator
mm:r co is counteracted by the component primary mm:r oc", and the
adjusting winding mm:r oc' is counteracted by the component primary
mmf oc"'. The total mm:r of the primary that is necessary to
counteract the mmf's of the adjusting winding and the stator is
the sum of the oc" and oc"', which is oc····. The locus of the
extremity ot this vector oc"" is defined by the circle d""
c'" 'e"". The primary nm:r must be produced by a current which
is proportional to and in phase with oc,···. In fig. 35 this
current is shown as the vector IlL. The locus of IlL is another
circle having a center coincident with the center of a circle
d I , • 'c' • , •e' • , t. The locus of the primary current can be obtained by adding to this current IlL, the current po. The current
po .is the no load current taken by the motor when the brushes are
set to make Ell zero. The total current taken by the primary,
shown on this diagram, is the vector II, and the locus of its
extremity with respect to the point p is defined by the same circle
that defines IlL. This circle has a diameter passing thru the
point 0, at an angle gamma, the tangent of which is equal to the
ratio of the effective number of turns in the adjusting winding
between brushes to the effective number of stator turns, Naw/N2.
A change in the brush setting will change the total resistance
and reactance of the total secondary circuit and, consequent~,
change the diameters of both the circle loci III and 12. With
the brushes set to obtain the character.i.stics illustrated in fig.
35, the motor will take a leading current, Ill' with respect to
the impressed voltage V1 over a considerable range of loads.
For such conditions of load, it can be used to correct the power
factor of the line supplying the motor.
10. Determination of characteristics when the motor is used to
correct power factor.
On the bases of the theory and the assumptions of section one
of this chapter it was sholm that the currents III and 12 could be
expressed thus:
I1- ::.
Ss
[;20 S
Ell
;- S:z. (
61
:6S
X~+ ssX ........)<!-
- ..
II.
=
ss
Ez..S
III
t""
If the brushes supplying each stator phase are set so that Ell =
sJ!.2, as is illustrated in fig. 34b and 340, then
s~i3.
i
2S and
Ell
at standstill S = 1, and 12
ill
=2.
Since the ration of I2max is to
Illmax is, from equations (l) and (2),
s.s !;.
/:;,
lfz. .,.
(
:>SX2
1(4.~
)
+ "X A ....
a knowledge of the ratio (R2 plus Raw) to (ssX2 plus ssXaw) is all
that is required to determine the diameters of the two component
circles, 12 and Ill, and from them the resultant circle defining
the extremity of the primary current vector as described above.
At synchronous speed 12 is zero and III is a maximum. If the
primary currents for standstill and for some other speed are known,
it is possible to determine the characteristic copper loss curves
for the circle diagrams as is done for the two element induction
motor. At standstill all the energy in excess of the no load
losses is lost in the primary and secondary windings, while for
other speeds these losses vary as the square of the component of
the primary current which is labeled IlL in fig. 35. This involves the same approximation with regard to losses that is made
in plain induction motor theory; namely, that the primary copper
loss varies as the square of the load component of primary current
rather than as the total primary current. The error introduced by
this approximation in the two element induction circle is negligible. The error due to this approximation is somewhat larger in
this motor than in the 2-element motor, but not so large as to
invalidate the method.
The characteristic copper loss curve, bjn is shown in fig. 36.
The point, m, determined by measurement of primary resistance
divides rb into components proportional to the primary and secondary losses. The current ph produces a primary loss which is
represented by mr.· The loss for any primary current, pf, can
be represented by kh which in turn can be determined by the
62
relationship kh
= mr
J2!
2
• The secondary copper loss represented
pb
by jk can be fotmd in a similar fashion. The slip is a ftmction of
the angle (alpha minus gamna.) by which 12 plus III lags E2. This
is also the angle between the vector oc in fig. 35, representing
the IIIIl:f due to 12 plus III in the stator winding, and the vector
labelled E2, the voltage induced in the stator by the air gap flux.
The angle alpha is the phase angle between the voltage VI and the
pr:i.mary current component IlL, or the mm:f oc"". This angle
varies with slip. The angle gamma is determined by its tangent
which equals Naw/N2; thus, gamma is independent of slip. The relation between slip and (alpha minus ga.mma.) is given by
This equation was developed in the original article, but I shall
not include the development here.
Equation (7) may be used to determine the slip at any point
f, see fig. 36, on the circle defining the extremity of II' the
speed may be obtained, since speed equals synchronous speed times
the quantity, (1-5). If the motor is loaded so that the primary
draws a phase current (PF) in fig. 36, the slip will have some
value, S, corresponding to the point, f, and can be determined·
from measurements of alpha and equation (7). If the impressed
voltage per phase is represented by VI' then for the input current,
pf:
=
1)
2)
3)
4)
5)
Input
(tf)Vl watts per phase.
Output '= (jf)Vl watts per phase.
Iron loss, friction, and windage = (th)Vl watts per phase.
Primary copper loss = (hk)Vl watts per phase.
Copper loss of stator and adjusting winding = (kj)Vl watts
per phase.
6)
Efficiency
7)
8)
=J.L times 100%.
tf
The slip is determined by the angle between (of) and V1 (or
between (of) and (oy» using equation (7).
Speed
(1-5) in rpm.
=
The output torque is zero at the point n in fig. 36. For
slips less than that at n, pOlver must be supplied to the machine
through the shaft. At the point v, slightly above synchronous
speed, all the power into the machine comes by way of the shaft.
11. Results of test with leading commutator voltage.
In order to check the validity of the foregoing theory, a
test was made on a G.E. BTA, 550-1650 rpm, 4.17-12.5 hp, 6-pole,
60 cycle motor. With the brushes set so that the commutator voltage was ahead of the induced voltage of the stator by 90 degrees,
the gap between brushes was fixed so that the commutator voltage
was 7.8% of the stator voltage at standstill. The results of the
load test and the blocked rotor test are indicated by the
encircled points on the circle diagram of fig. 37a. The theoretical primary-current circle locus was determined from three noload readings as described in the first section of this chapter.
It is observed that the diameter of the circle lies beneath the
horizontal chord by an angle of approximately ten degrees. Only
about five degrees of this shift is accoWlted for by gamna (the
shift caused by the .rrm.r of the adjusting winding). The remainder
can be attributed directly to the leakage reactance of the primary
circuit which, with high current values, causes the induced voltage, E2, in the secondary to lag the primary voltage. Fig. 37
shows the results predicted theoretica.lly in comparison with the
experimentally determined results.
12.
Conclusions.
The advantages of power factor correction on a line supplying
a motor are well-known. Advantages of power factor correction to
the motor itself result only when these corrections increase the
efficiency of the motor or its horsepower capacity. The effect of
power factor correction on the efficiency of this motor is
illustrated in fig. 3S. Each of the efficiency curve s shown here
was obtained by a load test with brush settings that provided synchronous speed at no load. It is evident that the addition of a
small value of Ell (two per cent of its ma.xiJnum value, that is,
a brush separation of· approximately five electrical degrees) to
improve power factor will increase the efficiency only slightly.
Further increases in Ell will cause excessive secondary currents
and reduce the efficiency. It is evident from the theory developed here that if the primary exciting current is to be reduced
the secondary must carry an additional exciting current to compensate for the reduction in the primary. Maximum efficiency for
a given load will occur when the voltage Ell is adjusted so as to
make the total copper losses in the motor a.m:i.ni.Imlm. Any exciting current flowing in the secondary must be supplied through the
conmutator. For some conditions of ~oa.d this additional current
will cause poor commutation. Thus there is little to be gained
from standpoint of the motor by setting its brushes to improve its
power .factor beyond the va.1ues obtained when Ell is made opposite
to E:l.
.
From intormation reveUed in fig. 37 it is quite evident that
the motor can be adjusted to draw various amounts of leading current.
This feature provides the possibility of improving the regulation
and efficiency of the ~e supplying the motor. These curves of
fig. 37 also show the rehtive high power factor obtainab~e on the
brush-shifting motor as compared with the ordinary well known two
e~ement induction motor.
This is the fourth article by A. G. Conrad, F. Zweig, and J. G.
Clarke and it deals with speed
contro~_with
power-factor
correction.~
It has been demonstrated that the ~cus of the extremity of
the current vector of the Schrage is a circle when the brushes are
set for speed adjustImnt, that is, when the voltage, Ell' collected
from the conmutator is collinear with the induced vo~tage, E:z, in
the stator. Further ~sis has shown that when the brushes are
set to make Ell perpendicular to E2 for the purpose of power factor
correction, that the locus of the extremity of the current vector
is also a circl.e. An anaJ.ysis of these circles has provided a means
of explaining the operation of the motor for these special brush
settings and of predicting all of its characteristics from no-~oad
measurenents.
The developments described above have been made from an analysis
of:
(a). The secondary currents III and 12 produced by the voltages Ell
and F.2 reapective4r.
(b). The mmt1s produced by these currents fiowing in the stator am
the adjusting winding.
(c). The resultant mmt1s produced in the primary ldnding as a result
of the secondary mmf1s.
(d). The primary currents associated with the required primary mm:fl s.
This part;, employing the same lmthods of ~is, extends the
theory of the Schrage and explains it s operation when the brushes
are shif'ted to control speed at the same tin:8 that the JOOtor is used
to correct the power factor of its supply. SpecificaJ.ly, it deals
with the operation of the motor when the brushes are set to mke Ell
out of phase 'With the standstill value of ~ by any angle beta. An
understanding of the developments presented here presupposes a know~edge of the preceding material.
Fig. 39a shows a representative vector diagram of the secondary
currents In and 12' and their ~oci when they are produced by the
voltages En and E2 which are no longer collinear. It can be proved
that the sum of III and 12 for this condition is a vector the locus
of which is defined by another circle. This circle, representing
the locus of the sum. of 12 and I~ is shown in fig. 39a. For the
65.
condition shOYm" the machine will run above synchronous speed at
no load" and the p~rfactor vdJ.l be l~ading. The current vectors
will follow the circle loci shown when the ~ed is varied from
synchronous speed through higher speeds to infinite speed- - in£inite
negative slip. Since the di.a.meter of the III circle coincides
with E"" swingi..ng Ell to various phase posJ.tlons JOOVes the III
circlealong with it. For values of beta ranging from 0 to l.80
degrees" a current is reflected into the pri.mary which has a leading component over the operating range of the motor. The no-load
speed of the DBchine is above synchronous speed i f beta is between
o and 90 degrees" as in fig. 39" and below synchronous speed if beta
is between 90 and 180 degrees.
When the brushes are set to make Ell in the same direction as
ssE2" (beta equals 0)" the- center lines of corresponding stator and
adjusting winding coUs are collinear, and their nmf I said. Ylith
this reference position of Ell' beta is also the angle by' which the
mm:r of the adjusting winding lags the nmf of the stator coils.
This is the angle in electrical degrees through which it is necessary
to rotate the brushes about the conmutator in order to shift Ell bY'
beta degrees (see fig. 40). Only the phase position of En is
changed if· tlie brushes are all rotated about the comnmtator bY' the
same angle, keeping their relative spacing unchanged. ·Positive
values of beta are obtained by moving the brushes in the direction
of rotation from the position where beta O. Values of nmf and of
Ell identical. to those at·aI\V given setting of the brushes can be
obtained by rotating all brushes 360 electrical degrees around the
cormnutator" or by' interchanging the positions of all brush pairs,
and moving them all 1.80 electrical degrees around the commutator.
=
13. Determination of the primary currents.
The primary current resulting from the secondary currents
shown in fig. 39a can be obtained from a consideration of the various
nunfl s imrolved. It the resultant flux crossing the air gap is to
remain unchanged (so that the generated voltage vdll remain approximately equal and opposite to the applied voltage) I the nmfl s
produced by the secondarY' currents flowing in the stator and the
adjusting winding coUs must be cancelled by' component magnetomotive forces produced by component currents flowing in the primary
coUs.
.
While the polyphase current 12 plus 1111 which flows in the
stator coils also flows in the adjusting winding coUs, the nmf t S
produced by'these two sets of coUs in series are not, in general,
in the same time phase with respect to the primary. This is because
the adjusting winding coUs are mechanically displaced from the
stator coUs by beta electrical degrees, as shown in fig. 40. If
the brushes have been shifted to retard Ell by beta degrees (which
advances the prinBry current), the nmr of the adjusting winding lags
the mmfof the stator bY' beta degrees , so that the primary current
cancelling the mrnf of the adjusting winding lags the primary current
which cancels the mrnf of the stator by beta degrees.
Fig. 39b shows a vector diagram of the components of the primary
current necessary to cancel the mn:f' produced by the secondary currents
66
of fig. 39a flowing in the stator. Fig. 40 shows that this component of pr:Lnary current is obtained by a mirror reflection of the
secondary current while the motor is running above synchronous
speed, and by a 180 degree reflection of the secondary current
while the motor is running above synchronous speed, and. by a 180
degree reflection of the secondary current when the machine is
running below synchronous speed. Fig.,39c shows a vector diagram
of the components of primary currents which cancel the mnfls of
the secondary currents flowing in the adjusting wilxling. Fig.
39<1 shovlS the magnetizing component of the primary current. Fig.
3ge shows the SUID. of these component currents, or Il, the total
primary current. At a given load, the currents refiected from
the adjusting winding and the stator winding are separated by the
angle beta. Their sum is separated from the component refiected
from the stator by the angle
Y'=-L-
1
New ~~
N"2.. -t N ltw
<...-
~.
The currents reflected into the prinBry from the stator and the
adjusting winding bear the ratio N2 to Naw •
By adding the reflected current s vectorially, the load component of the primary current is. obtained. The ratio of transformation between the secondary and the primary can be obtained from
the equation
•
14. Obtaining the circle diagram•
. For specific cases, two general methods have been considered
for obtaining the circle locus of the primary current without
loading. the machine. One method makes use of the fact that a
circle is un.i.que~ determined it two points and the slope of the
d.i..aJneter through one of the points is known. This method is
conmo~ used to obtain the circle diagram for the two-element
induction motor. ::It is also applicable for the Schrage JOOtor for
~ value of En ~ The two points conmonly used are the extremities
olthe no load and blocked rotor current vectors. In the twoelement indtlction motor, the ~ter through the no-load point
is 90 degrees behind the impressed voltage, so the circle can be
constructed from these two currents. However, the presence of Ell
in the Schrage causes a shift of the diameter of the secondary
current locus through the no-load point so that it is no longer
perpendicular to E2 (or the applied voltage). As is seen in fig.
39a, the tangent of the angle of slope of this diameter. is
E t..-(.?J
EII~(3
"
$$ E"2"
67
where R and X are the total resistance and reactance at standstill
referred to the secondary. The voltages and beta in this equation
may be determined by opening the brush leads and measuring Ell'
ssE2, and Ell plus saE2. Beta is determined by forming a triangle
of these three voltages. The quantities can also be determined
directlJr if the brush positions and turns ratio are known. The
remaining term, R/X, can be evaluated from the blocked rotor
current. At standstill, the resultant secondary current, III plus
12, lags the resultant secondary voltage, Ell plus ~, by an angle
the tangent of which is x/R. This is the same that ill lags Vl
at standstill. Hence, X/R is the tangent of the angle between
IlL and Vl at blocked rotor. The corresponding di.ameter of the,
primary current locus lags this diameter of the secondary' current
locus by the angle gamma, which was defined earlier.
Erecting a perpendicular bisector to the chord joining the noload and the blocked rotor current extremities gives another diameter, and the center of the prima.ry-eurrent circle locus lies at
the intersection of these two diameters.
This method for determining the locus of the primary current,
from the two points and the direction of the d.ia.m3ter through the
no load current extremity, involves the same approximation that is
made in ordinary induction motor theory- - that this diameter of
the circle passes through the no load point. Actually, limen the
machine is running with no output torque, it is loaded with rotational losses. The circle should be constructed with the diameter
drawn through the t~e no load point, which can be found by supplying these losses mechanically. This refinement produces almost no
change in the circle diagram.
.
The circle can also be located by obtaining three points,
rather than by two points and a diameter. This nethod, which was
discussed' for specific cases earlier, is valid for all brush
positions, since the current locus is always a circle, and three
points deterndne a circle. The necessary data are:
(a) The no'load input current and watts at normal voltage.
(b) The standstill input current and watts at reduced voltage.
(c) The no load input current and watts at reduced voltage, with
the machine running at some speed between the no load speed and
standstill.
This method eliminates the inaccuracies introduced by pri.JIe.ry
leakage reactance and rotational losses.
15. Characteristics from the circle diagram.
When the locus of the extremity of the primary current vector
has been established by the methods discussed above, the characteristics of the motor can be predicted, using nethods similar to
those used in the previous parts of this chapter.
In the ordinary induction motor theory, it is asswned that the
total copper loss of the motor is the loss produced by the no-load
current plus the loss produced by the load component of current
that is reflected from the secondary. The portion of the primary
68
copper loss produced by- the no load current is grouped with the
other no load losses, and the swn of these is assumed to be constant. These assumptions involve two approximations, neither of
which seriously affects the accuracy- of the method for the twoelement induction motor. First, it assumes that the current flowing in the prima.ry' is directly proportional to the current in the
secondary', so that the division of copper loss betlleen the pri.mary'
and secondary' is the same for all loads. Second, it as~s that
the loss for two component currents flowing in a conductor simultaneously is equal to the sum of the losses produced when each
component flows separately-.
CI~~ -t-l,~") R,
-=
(x~
-t-
T'I-)"I.. R,
This is true onJ..y- when the component current vectors are in quadrature. Over the operating range of the ordi..nary' induction motor,
the magnetizing current is nearly- at right angles to the reflected
component, IlL' so that the error due to this approximation is
small. However, in the Schrage motor, the voltage lhl may- be
introduced into the secondary' in such a phase position that the
phase angle of the reflected current may vary- widely with respect
to the nagnetizing current, introducing quite appreciable errors.
The loss curves for the machine can be located without these
appro:xinB.tions. Referring to fig. 41., for some general running
condition when the input current is pf, the current nowing in the
secondar;y is proportional to (of), where po is the magnetizing
current. po can be determined by running the machine at no load
with the brushes set to make En zero. The pri.m.ary copper loss is
proportio~ to (pf) 2, and the secondary' copper loss is proportional to (of) • When the rotor is blocked, ·the total input to the
motor is used in supplying losses. Assuming that the sum of the
iron, friction, and windage losses remains constant from no load to
blocked rotor, the total copper loss can be determined for blocked
rotor. From the resistances of the two windings, the division of
this loss between the pri.mary- and the secondary' can be determined.
Thus, i f the point m is located so that rm ti.mes Vl is the primarycopper loss per phase for a primary- current pb, and mb times Vl is
the secondary- copper loss per phase for a reflected current ob,
then for a.primary- current pf, the primary copper loss per phase is
11k tilms V1 where
~k ~ r~
(fi) ~
and the corresponding secondary' loss is kj times Vl where
This nethod can be used to determine the copper losses at no load.
By- subtracting these copper losses from the no load input, the
friction, windage, and iron loss is determined.
Using these relations, curves can be constructed that divide
69
the power component of the input current into the portions that are
allocated to the output and the various losses. Thus in fig. 41,
for an input current pf:
Input:! (tf)Vl watts per phase.
Output ::: (jf)Vl watts per phase.
Friction, windage and. iron loss
(th)Vl watts per phase.
Pri.ma.ry' copper loss
(hk)VL_watts per phase.
Secondary copper loss = (kj)Vl watts per phase.
=
=
Efficiency
= if
tr
Power factor
= tf
Pi
When Ell has no quadrature component with respect to ~ ,
~ = 0 ), the slip is equal to the secondary loss ~vlded
by the total power across the gap, or
(E~,
4M,.
5 ::::
~
(Z'-R)
1-
100
ot ~ k--I... ~
~o
~ ~ .. ,.,.. + ~ (r /'{)
where I is the total secondary current (~ plus Ill). Under this
condition, a:u of the I2R loss in the secondary is supplied by the
speed voltage, IZ, which varies directly with slip. It can be
demonstrA.ted. that when E" has ~ phase position beta, the voltage
supplying the secondary !2R loss is a combination of a speed voltage
(Xl) s, which varies directly with slip, and a transformer voltage
(IZ)t = Ell ~ ~ , which is independent of slip. Under this
condition, the slip can be evaluated in te;:ms of the output and.
the component of the secondary loss,
(I~), which is assoc:1..ated
with the speed voltage. The expressi8~cfor tile slip is
.
L
s=
(x. ... k) s
4.a..c.
~ ~•. ,... +
/U..c.o
(I"~) s
This slip is expressed in per cent of No, the n07'-load speed at
which the motor would operate i f the brushes were shifted to elim:inate
the quadrature component ( E" II ~ f? ) without altering the inphase
component (til ' - (3 ). Similarly, the torque can be shown to be
proportional to the component of the total secondary power which is
independent of the transfor.Dl'r voltage.
J.lA ...~, • J- ,.." ... Q. __
-
-t-
IQ.A...<)
(::t:. '"- p..") s =-
'2. '1t'
Nd
T tk.v
,.. '74 to
3~..... (/oO
' clw '::. "33. 60
,c
2."R- No "~t.
Q
(~ ~LL.-
..... ALf...<.,
(r 2. R) 5)
To get the quantities e (ra)s and (developed power plus
and s~c(I~)t, i€ Is convenient to draw the dotted line
m k', and. so fOrt.h, on the circle diagram of fig. 41, so that
s,c(I2a)~)
70
sec(~)t
=
=
(k'k)~V watts per phase
e (I""lf.)
(k1j) 1... watts per phase
83.€put pfus sec(I R)s = (k1f)Vl watts per phase
From this, the slip and torque can be evaluated in a manner which
is almost' identical with that used for the ordinary induction motor
circle diagram.
Slip
.
=.!s!.aL times 100%
of No.
k'f
....1t' '" 0 T ("14')
"3.3... 000
= kl f
times watts per phase
0
This involves the sa.ne approximation that is made with the ordinary
induction motor- - that the effect of rotational losses on the slip
is negligible.
16. Experimental check on the circle-diagram. theory.
To check the theory that has been presented here, tests were
made on the same motor described earlier.
The brushes were set for two tests so that beta was approxiJrately 45 degrees and 135 degrees. The po'Wer factor was improved
with each of these settings, am the nachine ran above synchronous
speed when beta was 45 degrees, and below synchronous speed when
beta was 135 degrees.
The magnitude and phase position of Ell were determined
accurately" by the voltage neasurements described earlier, and from
this" from the no-load measurements and from the blocked-rotor
measurements, the circles shown in fig. 42a were constructed. It
is seen that these predicted circles check the observed points.
quite closely. Curves of speed and current against torque, and
power factor and efficiency against output were constructed from
the predicted circle. Fig. 42b,c,d, and e show these predicted
characteristics compared with those actua~ observed.
17.
Conclusions.
1.
Theory and experiments described here have shown that it is
possible to adjust the brushes of the Schrage motor to correct the
power factor of the current supplying it, regardless of the speed
to which it is adjusted.
2.
The locus of the extremity of the priniary current with such
adjustments is a circle. The magnitude and location of this circle
with respect to the prim:\ry voltage vector can be determined from
no load neasurements taken on the motor.
3. The power factor correction is accomplished by causing exciting
current of the motor to flow in the secondary element s instead of
the primary. This causes extra heating in the secondary and reduces
the permissible load current that the secondary can carry. In the
particular machine used in this iIIV'estigation,· values of the quadrature component of Ell' (E II ,....,;... ~ ) in excess of 10 per cent of
71
the standstill value of the induced secondary voltage E2 cause
excessive secondary currents.
4. While the range of Ell sin @ (power factor adjustment) is
limited, Ellcos €I (speed adjustment) is not limited except by
the design of the motor. The voltage Ellcos ~ is opposed in
normal operation by a speed voltage, E2, which limits the flow
of current produced by it. The quadrature component, Ell sin {J ,
causes a secondary current which is opposed only by the motor
impedance and not by the speed voltage. Consequently, a small
angle beta can cause considerable change in power factor in a low
impedance motor.
5. A method of determining the characteristics of the motor
when used to perform the double function of speed adjustment and
power factor correction has been presented. This method employs
the theory and use of the circle diagram. The accuracy of the
predicted characteristics indicates that the theory and description of the operation of the motor as presented here are
essentially correct.
72
CHAPTER VI
COMMUTATION
1.
General.
Since the najor objection brought forth against ac commutator
motors is commutation troubles, it would be wise to analyze commutation
in a Schrage machine.
In ac conmutator motor it might be expected that the c01Ill1Ultation
would be more difficult than in dc machines on account of the fact that
the current in the brushes is alternating instead of direct.
Consider-
ation of the time available for cOIIllm1tation, i.e., the time during
which the current in a coil has to be reversed while the coil is short
circuited by a brush indicates, however, that this does not have any
appreciable effect.
Consider a typical nachine having a comnutator
periphereal speed of 6000 feet per minute (1200 inches per second), a
brush width of 0.375 inches, and a mica thickness of 0.03 inches.
The
time of commutation is,
brush width - mica width
conmutator speed
=0.375 -
0.03
1200
=0.000288 seconds.
At a
frequency of 50 cycles, the duration of. one cycle is 0.02 seconds, so
that the variation of the main current during the commutation period is
very small.
The diagram of Fig. 44 assists in visualising the conditions;
vlith an ordinary dc armature as shown in Fig. 43.
The current in an
armature coil will be as shown in Fig. 44a, the current changing its
direction during the time of conmutation when the segments to which it
is connected are passing the brush.
If the machine of Fig. 43 re-
presents an ac machine such as an ac series motor, the current flowing
at the brushes and in the conductors vr.i.ll be varying sinusoidally.
80
As the current passes the brush, however, reversal takes place as in
the dc machine so that conditions are as shown in Fig. 44b.
It can
be seen that the sinusoidal variation of the current is very slow compared to the variation which has to take place during the commutation
period so that the former can be neglected.
In the three phase
machine the conditions are very similar; the diagram of Fig 45a represents a two pole armature having three brushes which carry three
phase conmutation. As an armature conductor passes from the space
between the brushes a and b to the space between b and c the current
must change from a value shown on curve ab in Fig. 45b to the value on
curve bc at the same instant.
The current in the armature conductor
will thus be as shown by the heavy line in Fig. 45b.
It is thus seen
that, so far as the current reversal is concerned, the conditions in
an ac conmutator machine are very similar to those in a de machine.
There are, however, other factors, certain emfs, which arise to complicate matters.
One of these emfs is produced by the reversal of the
current in the coil which causes a rapid change of the leakage flux
linked lfith the coil; the other is an emf due to the rotating field.
Both emrs are such as to oppose the reversal of the current and therefore tend to hinder the commutation process.
2.
The emf induced by the rotating field.
The emf induced in the winding element by the rotating field
depends upon the magnitude of the rotating flux and the relative velocity of the armature and the rotating field, but since the rotating
field of the shunt motor with current supply thru slip rings has a
constant speed with respect to the armature, the emf induced by it in
the short-circuited winding element is constant and independent of the
81
speed.
Take, for instance, a two pole, 50 cycle motor.
1200 rpm the motor in question will
rwl
At a speed of
at 20 rps counter clockwise;
the nagnetic field will then be rotating 30 rps clockwise with respect
to the stator.
The frequency of the current in the regulating winding
is always at 50 cycles per second with respect to the rotor; and at 20
rps the regulating current is at 30 cycles per second with respect to
the fixed brushes, that is the same frequency as the current in the
stator winding.
Also, the regulating vlindings and secondary windings
are usually designed to work at low voltage so that there is little
risk of flash over at the commutator.
Since the commutator only han-
dles a small proportion of the total power of the motor, the commutation is usually good.
3.
The emf of self induction.
The emf of self induction in the short-circuited winding element
is
proport~onal
to the ampere-eonductors per unit armature circum-
ference and therefore depends upon the range of speed regulation.
is low for a small range of speed variation.
It
At synchronous speed this
emf is zero, for at this speed the brushes of each brush pair are placed
on the same commutator bar, and commutation of current does not take
place.
The following explanation of this is from Liwschitz-Garik and
Whipple (2).
The emf of self induction in the short circuited winding
element is due to the change in the current from plus i a to minus
i a • Usually several winding elements commutate at the ,same time
and therefore the mutual inductance between the short-circuited
winding elements also has to be taken into account. The sides of
the winding elements which are short-circuited by the positive and
negative brushes lie near each other and consequently induce mutual
emfs in each other. The total emf induced in the short-circuited
winding element is thus:
e = _ (LeA..:
~
+
~ Mx J.t·x )
Dlt
82
The magnitude of L is determined by the leakage fluxes of the
armature, Le., by the slot leakage, tooth-top leakage, and end
winding leakage. The same leakage fluxes also determine the
.
magnitude of M•••••••••1f the commutation curve is a straight line,
di/dt is constant and equal to 2ialrc. For any other curve of
commutation, di/dt is a time function but its average value over
the total period of commutation also is equal to 2i~Tc' for the
coil current mst change from plus i a to minus i a in this period.
Calculation of the exact variation of di/dt during the commutation
period is possible only under certain simplifying assumptions which
seldom appear in practice. Therefore it is usual to calculate with
the average value of di/dt, i.e., with 2ia/T c • The absolute value
of the average emf of self induction is then,
If Ne is the nwnber of turns in a short-circuited winding element,
the coefficient of self-inductance may be written as,
where zeta is analogous to the magnetic permeance of the leakage
fluxes per unit length of the armature. If bb is the brush width
referred to the circumference of the armature, and va is the surface
velocity of the armature, the short-circuited period Tc is
Assuming the brush width to be equal to 1 commutator bar, 2Ne i a
Abb, since 2Ne1a ampere conductors fall on 1 commutator bar;
83
=
=
substituting Tc byva , the average value of the emf of se1£induction becomes
where A is measured in ampere-conductors per inch circumference,
va in feet per minute, and 1, the core length (without vent ducts),
in inches. If the brush covers several sommutator bars as is
usually the case, the short-circuited period becomes larger, and
the first equation must be used. The last equation can be used
also for the case when mutual inductance is present, i.e., for the
total emf induced in the short-circuited winding element through
self-induction and mutual induction. The mutual inductance is then
taken into account in the factor zeta whose value usually lies
between 4 and 7.
4.
Reasons
lVhy
conmutation is better on Schrage motors than on stator
fed motors.
In the older types of series or stator-fed motors, generally speaking, commutation is perfect at synchronous speeds, but its quality falls
off rapidly as the speed varies from synchronous.
This is due mainly to
a voltage induced by the rotating field across the commutator segments.
This voltage in the stator fed motor is proportional to the difference
between the speed of the motor and synchronism.
Since the cOl1UIDltatioh
is \Vorse the higher this voltage, the motor develops poor commutation
at low or high speeds.
In order to obtain satisfactory commutation over
a wide range of speeds this induced voltage must be kept
over, constant.
10Vl
and more-
This is the case with the rotor fed motor, where the
induced voltage is practically constant because the rotating field which
creates it is practically independent of the speed of the rotor and has
not only a constant strength but even a constant relative speed of
rotation as regards the commutator winding.
Thus the commutation is as
good at starting as at ordinary working speed.
Consequently, the modern
rotor fed shunt commutator has practically solved the problem of ac
commutation.
The recent and important development of dissymmetrical
brush displacement, already mentioned decreases the full load current
not only in the primary but also in the secondary circuit, and this
is very advantageous as regards temperature rise, particularly since
at low speeds ventilation is poor.·
5.
Auxiliary windings on ac commutator nachines.
The following is taken from the Electrical Times, July 17, 1941,
(14) •
In the Schrage motor the commutator only handles a fraction
of the winding, 'Where as the commutator of the stator fed machine
carries the vmole output and so commutation is more difficult in
this tyPe. Experiments were carried out to obtain increased out
put. High resistance connectors were used between the winding and
the commutator segments. The benefit 'Was only 20 to 30%.
Damping windings are used to provide control of commutation
conditions. Briefly, the interpoles of a dc nachine provide
complete neutralization of the voltage due to the change in the
nain current (reactance voltage), but do not compensate the high
frequency pulsations, while an effective damping winding, although
it only parti.a.1ly damps out the reactance voltage, has also a damping action on high frequency pulsations. Hence, an efficient damping winding can give improved results of the same order as are
obtained by the use of interpoles. The action of a damping winding can best be explained by comparing it with the discharge resistance used to open an inductive circuit such as a generator field
winding. When the current in the coil is reduced from its initial
value to zero by opening the swit ch, the nux linking the coils must
also be reduced to zero i f no discharge circuit is provided. This
rapid change of flux induces a high voltage which causes the switch
to are, thus tending to prolong the period during which current
flows. If there be a discharge circuit, current can continue to
flow in the min coil even after the switch is opened, because the
induced voltage passes a current through the resistance. Hence the
rate of change of flux is considerably less, and the danger of
sparking is reduced. Energy 'Which would otherwise cause an arc is
dissipated in the resistance. The same action takes place in a coil
of a commutator winding as the two segments to which it is connected
pass under a brush. If the voltage induced in the coil, due to the
change in nux linking it, is greater than the total brush contact
voltage which nornally absorbs the energy of commutation, sparking
will result when the circuit (which has been closed by the brush)
is broken. By connedting a coil of a damping or discharge winding
85
in parallel with the nain coil, a circuit is provided in which
current can continue to flow after the brush has ceased to shortcircuit the segments, so that the flux linking the coil need not
change as rapidly as it would otherwise do. Sorm of the energy
of commutation is thus transferred to the discharge resistance.
It should be noted that the use of these discharge or damping
commutator windings does not reduce the number of commutator
brushes except to the limited extent to which the current density can be increased. The commutator voltage is limited by the
losses due to circulating currents under the brush, and camet be
increased except at the expense of excessive temperature rise at
low speeds. This applies to both the Schrage JOOtor and the statorfed motor. Both types must necessarily have approximately the same
aJOOunt of brushgear, irrespective of the method of comection or
whether there are two brush rings or only one. On the average,
assuming equ.aJ.ly safe designs, the stator-fed motor carries no fewer brushes than the Schrage for the same hp and speed range. In
order to obtain the best results, a damping coil J1D1st fulfill two
conditions: it nmst not be linked inductively with the same main
coil to which it is connected. Also, there nmst be no, or only a
snail, circulating current due to the main flux of the machine in
the closed path comprising the min coil and the damping coil. In
all, cases the balance of voltages between the nain and discharge
windings, both with regard to magnitude and phase is, maintained
by suitable choice of the pitch of the coils and their location
round the armture. Three types of winding which fulfill these
conditions and which have been used successfully are described
beloil, the first two being patented by the B.T .H. Co.
Robinson Winding.
In this arrangement, shown in Fig. 46 and 47, and due to Yr.
P. W. Robinson of Schenectady, the main coils are short pitched,
while the discharge coils are over pitched by the same amount as
the main coils are under pitched. Thus both the above conditions
are satisfied, since the two coils lie in different slots, and the
total voltage round the combined circuit is zero. The discharge
coil has a snaller section than the main coil and is wound at the
top of the slot, so that its resistance is greater and its inductance less than those of the main coil. The complete winding
thus consists of two ordinary double layer lap windings, one above
the other and connected to the same commutator lugs. As the damping coil is directly connected to the min coil this winding nay be
termed a direct type of damping winding.
Duplex winding.
An extension of this scheme, shown in the Fig. 48, applies the
same principle to a duplex winding. Here the main coil is of full
pitch, and is connected to segments two apart so as to permit a
larger flux per pole to be used in the machine with out eJ¢eeding
the permissible voltage between adjacent segments. A second set of
main coils is connected to the intermediate segments, thus forming
a duplex winding. The discharge coil has a pitch of 33.3% and is
so located so that the voltage induced by the nain flux in two turns
86
is exactly equal to the voltage in one main coil. This winding
is connected to every segment so that, in addition to its action
as a discharge winding, it also serves to equalize the potential
betlveen the two circuits of the main duplex winding. Constructionally the arrangement is similar to that of the Robinson winding and is also of the direct type.
Embedded winding.
A somevmat different arrangement is used in a winding described by Dr. B. Schwarz in Elecktrotechnik u. Yaschinenbau, Feb.
1934. Here, as may be seen from the Fig. 49, the auxiliary winding is placed at the bottom of the slot belol'l the min winding,
and separated from it by steel shims which complete local magnetic
circuits around the conductors of the auxi.liary winding. The main
coils are of full pitch, while the aux:Ui.ary coil, connected in
parallel 'With a main coil, consists of two turns each of 33.3%
pitch, so that the resultant voltage induced by the main flux has
the same magnitude in two auxiliary turns as in one main coil. In
addition, the windings are arranged so that, considering two main
coils lying in adjacent slots, coils X and Y, the two corresponding
auxiliary coils connected in parallel with them lie in the same
slots as one another, coils x and y. By this means, every main
coil is connected through a small transformer consisting of two
auxiliary coils, to another main coil in another slot; that is, the
action is an indirect one. This second main coil constitutes the
discharge circuit for the first main coil acting through the medium of the auxiliary transformer. In addition to the action as a
discharge winding, a furth~r benefit is obtained due to the fact
that the discharge coil, in which a change in current is brought
about during conunutation of the preceding coil, is the next main
coil to be commutated.
In recent years the BTH Co. has done a considerable amount of
practical work on a.c. commutator machines with damping windings,
both of the direct and the indirect or embedded type, and has come
to the conclusion that better results are obtained with the direct
type of winding. It is evident on theoretical grounds that with a
damping winding of the embedded type the discharge action is a'
good deal less effective than in the first two cases, where the
discharge coils are directly connected to the main coils. In the
first place, the effective discharge resistance, instead of being
the resistance of a single coil, consists of the resistance of a
main coil added to the primary and secondary resistances of the
transformer, each of which consists of two turns of the auxiliary
winding. In the second place, some asynmetry is necessari.l.y
introduced by the method of connection, since, while the voltages
in the two main coils X and Yare different in phase, because they
lie in different slots, the voltages in the two auxiliary coils x
and y are the same, and so cannot balance the min coil voltage
in both cases. Thus, a certain amount of circulating current
inevitably occurs with the winding, and, as a result, a relatively
high resistance in the auxiliary winding is necessary in order to
limit the losses. In both of the direct types of discharge winding,
on the other hand, there is an exact balance of voltages so that
the value of resistance can be chosen to obtain the most effective
damping.
Stator fed motors.
In order to prove the relative merits of direct damping windings, and an embedded damping winding identical motors have been
built, the only difference being in the type of discharge or damping winding. Considering the rotor of a stator fed a.c. commutator
motor rated 300/100 hp. 1,120/320 rpm, 400V, which is a high rating
of hp per pole; this rating was above the limit for a Schrage motor.
As there were six identical machines on this order it was decided
to build the first two rotors with different types of damping winding, and according to which type proved the better, to adopt that
for the remaining four. In both cases the main winding VlaS of the
duplex type; but one was provided with an embedded auxiliary winding located at the bottom of the slot and si.mi.lar to that· indicated,
whereas the second rotor had a discharge wi nding at the top of the
slot. Good commutation was obtained with both rotors, but the
second was appreciably better and this arrangement was adopted for
the remaining four machines. With the winding at the bottom of the
slot, there was noticeable sparking at top and bottom speeds. With
the second mchine using the discharge winding at the top of the
slot, commutation was practically black at all speeds. In addition
to its superiority in commutation, the rotor with the discharge
winding on the top is easier to wind, and makes a much better
mechanical job when complete. On a high speed rotor, the steel shims
in the slot, and the snall vIinding undemeath, on to which the min
winding is pressed by the binding bands, introduce difficulties in
the case of the embedded type of winding. A further comparison has
also been made between a stator-fed motor with embedded damping winding and Schrage motors of corresponding rating. Some Schrage motors
have been built rated 125/65 hp, 1450/1300 rpm. and another rated
100/36 hp 1470/1100 rpm. These motors did not require any special
discharge or damping windings to get good commutation, and have been
in service for several years giving satisfactory operation in each
case. An opportunity arose for building a stator fed motor rated
130/40 hp 1600/1000 rpm 3 phase, 3300 V. It was decided to fit this.
with an embedded damping winding of the type in Figs. 8 and 9. The
auxiliary winding was located at the bottom of the rotor slot belo\V
some steel strips, and was d:iJnensioned to limit the circulating loss
to a reasonable value. The commutation was about equal to that on
the Schrage motors with simplex winding without a discharge winding,
but not as good as would be obtained on a Schrage motor with a
discharge ,vinding. It was evident that this output would have been
difficult to obtain from a stator fed motor without the use of a
damping -winding.
Damping winding on a Schrage motor.
During the last three years (1938-4l) over 100 motors of the
Schrage variable speed type have been constructed, by BTH Co., with
discharge windings in the armature'. All the damping windings used
88
on Schrage motors have been of the direct type, either simplex
or duplex. These are in addition to large numbers of smaller
machines with simple lap or wave armature windings. Where the
Robinson simplex winding has been used it has been mainly to give
increased output to existing designs or increased overload capacity
for severe duties. Many of the larger sizes of motor, which used to
be provided with resistance connectors, are now wound with discharge windings, and give better performance in addition to having
a simpler mechanical construction. Although resistance connectors
were of appreciable benefit in these machines, their main function
was to reduce parasitic circulating currents under the brushes
rather than to influence the commutation of the main current. Moreover, additional losses were introduced in the resistance winding
itself. The Robinson winding, it rray be noted, does not introduce
any circulating current losses, but actually assists by carrying
part of therrain current. As an example one motor is rated 20/'lSJ hp,
1200/400 rpn, 44OV, and is one of several supplied for driving large
machine tools. The commutation of these machines was very good up
to 100% overload. As an emmple of a somewhat special rating
obtained by means of the simplex discharge winding, some motors
rated 10/0 hp 3500/0 rpm may be mentioned. These motors also gave
excellent commutation up to 100% overload. It is correct to say
that on those motors built so far with discharge windings, the
commutation at twice full load has been superior to that of similar
motors, without the discharge winding, running at full load or even
less. It must be appreciated, however, that throughout the world
there are in service very many thousands of Schrage .motors of all
sizes, on which the commutation must be regarded as satisfactory,
even in those machines where visible sparking is present. Visible
sparking does not necessarily mean injurious conunutation, especially
in the case of the Schrage motor where the commutator only handles
the slip power, and where the conunutator voltages are very low.
These conventional simplex windings will continue to be used on the
small and medium sized motors in the future, as there is no reason
to depart from them.
Duplex winding.
This winding permits use of increased fluxes and hence increased output per pole. A Schrage motor of 270 hp using this
type of winding was constructed as early as 1925, and many other
machines have been put into service since that time. The construction was, however, somevThat cwnbersome and expensive, due to
the fact that some means had to be provided for equalizing the two
sections of the winding, and it was only used for machines which
could not be made with a simplex winding. The new duplex discharge
winding illustrated in Fig. 7 however provides with a construction
just as simple as that of the simplex winding, in addition to the
discharge action, which gives even better commutation than the
simplex winding. Many machines, l1hich would previously have been
built with simplex windings, now have duplex windings, thus enabling
a better and more efficient design to be used. An outstanding
example of a machine with the BTH duplex discharge winding, of which
89
nine have been built, are rated 50/0 hp, ?/JOO/O rpn. Here, again
no spa,rld.ng was visible up to 100 % overload. Another interesting
order included nine motors, rated 71/4 hp 1700/100 rpm. for crane
drives. For this duty it is important to have as low a moment of
inertia as possible, because a considerable part of the power is
required to accelerate and retard the motor armature itself. By
using the new duplex discharge winding it was possible to reduce
the stored energy of the rotor to about a half of what it would
otherwise have been. An interesting test was carried out .on one
of these motors. A pilot motor was arranged so as to raise and
lower the speed so rapi~ that the motor took a current of three
times the normal value, the test was carried out repeateclly with
only the least trace of visible sparldng. From the foregoing account
of recent developments in connection with discharge or damping windings it is evident that considerable progress has recently been made
in the design of both Schrage types and stator fed types. For motors
of standard industrial ratings good commutation is inherently easier
to obtain on a Schrage motor than on a stator-fed m::>tor. Taking all
factors into consideration the Schrage motor is inherently the best
motor for ordinary industrial service. Where the supply is 3000 V
or higher or where a separate regulator is preferable, or where the
conditions of output and speed make it more favourable, a stator fed
type of motor is recommended. Further, where a discharge or damping
winding is required in the armature winding the simplex or duplex
winding at the top of the slot is superior to the embedded damping
winding at the bottom of the slot.
90
CHAPTER VII
APPLICATIONS OF SCHRAGE AND STATOR FED MC1rORS
1.
Stokers.
For driving mechanical stokers both the Schrage and stator-fed
are suitable. They both can be totally enclosed.
to drive stokers
Engla.nd.
lvaS
The first Schrage
installed in 1923 at the Greenwich Station,
Twenty-three Schrage motors drive grate stokers at the
Stourport Station in England.
These are totally enclosed.
Recently
an addition of sixteen motors rated at 3.75/1.2 hp, 1480/480 rpm, 400
volts were installed which are provided and fully automatic control
from a combustion regulator.
2.
Feed and separator drives.
Schrage motors have been used for the feed and separator drives
in a station burning pulverised fuel.
An example is the Upper Boat
Station, Great Britain, where Schrage motors rated at 1/0.5 hp, 1000/
400 rpm are used for the six feeders and six motors rated 4/1.6 hp,
1450/ 580 rpm. for the separators. These motors are totally enclosed.
3.
Frequency changing•
.The frequency-changer method of speed regulation is employed when
it is required to vary the speed of a large number of motors and to
keep them all at the same speed. Thus, for sectional drives of big
machines or of several combined machines working on a continuous band
of stuff, automatic speed equalization can be easily arranged for by
making use of an alternator driven by a variable-speed commutator motor •.
Such an arrangement is used in an artificial-silk spinning mill to
supply current to a number of small squirrel-cage motors driving'"
•
92
spinning spindles.
4.
Fans.
A useful application is for fans where the motor is always running,
but for certain periods the outPlt, and therefore the speed is considerably reduced.
Since the speed control of the Schrage motor does not
entail any external loss, the cost for power is reduced to a minimum.
The drives required for boiler house fans are normally within the
capacity of a Schrage motor.
In large commutator motors used in boiler
houses, it is desirable to provide mans for removing the brush dust,
and boiler house dUst, which accumulates over a period of years and
introduces a danger of eventual breakdown of insulation.
Ventilating
systems, using filtered external air or a closed air system are equally
applicable to the Schrage and the stator-fed motor.
There are eight
Schrage motors driving induced draught fans in the Tir John Power
Station, Swansea.
There are also eight Schrage motors in the Leicester
Power Station; two motors rated 250/10 hp, 575/ 210 rpm for forced
draught fans; four motors rated 48.5/ 7.75 hp, 725/420 rpm for the induced draft fans; two motors rated 100/ 36 hp, 1470/UOO rpm for
exhauster fans.
5
These motors are supplied at 415 volts.
Pumps.
Commutator motors are very suitable for dealing with certain pump-
ing problems.
For example, in the pumping equipmant supplYing hydraulic
power for operating lifts in conjunction with an accunnl1ator, the rise
and fall of the latter operates the brush gear of the motor through
chains, so that the motor is accelerated when the reserve power is small
and decelerated when the accumulator is charged. The motor thus increases the capacity of the equipment for a given size of an accumulator
93
i
and minimizes the starts and stops that have to be made.
For borehole
pump drives an approximate constant quantity of water usually has to
be pumped against a total head which varies with the water level in
the well.
The pumps must be run at varying speeds, and since the water
levels often vary considerably, they rray have to run at low speeds for
long periods. The high efficiency of these motors at all speeds makes
them the most economical for duty.
A recent installation, includes two
vertical BTH commutator motors each 420/ 250 hp at 810/645 rpm which
have to pump 1000 gallons per minute against a head varying from 570 to
670 feet and mintain a power factor of not less than 90% through out
the working range. A further example is the Brown Boveri pumping motors
installed at the Giessliweg Station in Basle, Switzerland.
This is a
draining sewage pumping station, in which the volume of sewage water
to be passed in the 24 hours varies considerable, and it is desirable
that this volure of sewage shall be passed as quickly as possible in
order to prevent fermentation.
This pwnping plant is completely
automatic in operation, two commutator motor-driven pumps being used
to take care of the variations in output and head.
6.
Printing and paper naking.
The nu.trerous processes in paper manufacture requiring a variable
speed motor include paper-rraking, reeling, cutting, calendering and
drying.
~
the machines involved in these processes can be advan-
tageously driven by commutator motors.
Paper-rraking machines on ac
systems have hitherto required most elaborate driving units.
The need
for absolutely constant speed at any load necessitated the use of ViardLeonard machine's, with their attendant losses, which occupied valuable
space.
The super calender for news-print can be driven at the maxiJn.um
94
speed possible without causing undue paper breakage; this speed depends
on the quality and strength of the paper, hence the necessity for using
variable speed motors •
.The shunt characteristics of commutator motors, the constant and
uniform acceleration under any printing condition without the least
suspicion of snatching, and the fact that the consumption of power is
proportional to the work done render them ideal for driving printing
presses.
The
~quipment
out resistances.
is arranged for simple push button control with-
Tests made on two similar presses in a printing office,
one driven by an ac commutator motor and the other by a resistance
controlled induction motor, proved that a saving of 25% in time and 50%
in running costs were gained with the former equipment.
The following is taken from the Electrical World, (ll).
Comparison of the BTA type ac adjustable speed brush shifting
motor with a dc motor and motor-generator set combination for
driving two new two-unit gravure presses at the Chicago Rotoprint
Company found the advantage with the ac motor largely on the grounds
of efficiency. Costing roughly the same as a dc motor and motorgenerator set combination, the BTA double-motor drive for the
presses had an over-all efficiency of B2% over a large portion of
its working range. Contrasted with this the motor-generator set
efficiencY alone was B2-B5%. This, when considered with a dc motor
efficiency of B0-B2% gave the entire system an over-all efficiency
of 65-70%. The new presses to be used for printing catalogs and
advertising folders were designed for constant paper (web) speed
of BOO feet per minute, using several roll sizes varying from 33
to 55 inches in circumference, depending on the size of the form
being printed. Neglecting friction and windage, the hp required
to drive such a press operating at constant web speed with different
size of cylinders is constant. However, in a press operating over
a range of web speeds with but one cylinder size, torque required
is constant and hp increases directly with the speed. Therefore,
in selecting the proper size motor for this or any similar
installation, it is only necessary to determine the hp required to
drive the largest cYlinder to be used (55 inches in circumference)
at the speed of 10500 rph. to give the BOO feet per minute web
speed. The rootor selected will then have ample hp to drive the
s.rnaller cylinders at the higher speeds. In the case of the Chicago
Rotoprint Company's presses main drive a BTA motor with a 3:1 speed
range and capable of developing 40 hp over the range from 1000-630
95
.
rpm and 21.2 hp at its minimum speed of 333 rpm is used.
7.
Miscellaneous
In view of the large and steady demand for Schrage type commutator
motors of sizes within the region of 5 hp, the Bl'H Co. Ltd. have developed a 4-pole machine.
It has a maximum rating of 4 hp with a speed
range not exceeding 2050/ 683 rpm; when rated at 3 hp, any speed between
2500/ 0 can be obtained.
The small size of the motor should render it
very easy to accomodate where space is severely restricted, and should
prove very valuable for snail power drives, such as usually required in
connection vdth conveyers, pump driving requirements, textile nachinery,
etc.
The weight of the rrotor is 265 lbs.; length is 29-5/8 inches,
the width is 13 inches, and the height is 15-1/8 inches.
8.
Cranes, hoists, lifts.
High speed lifts have come into common use with the recent devel-
opment of large buildings, and this is one of the most arduous duties
which a motor is called upon to perform.
The motor is required to give
a high starting torque, and maintain a high rate of acceleration and
retardation, the latter by regenerative braking.
It is for lift speeds
of over 200 feet per minute that this tyPe of drive is particularly
applicable and at these speeds the time allowed for acceleration is
very short ,amounting to a few seconds only.
During frequent operation
of the lift the almost continuous speed variation makes a heavy demand
on the commutator and brushgear and this demand is met without undue
sparking or trouble of any kind.
A commutator drive is justified where specially fine control is
required, as in the case of a crane in a poller station engine room, where
accurate control is necessary for maintenance work on steam turbines and
96
generators.
An emmp1e of this type is the crane in the power station
of the St. Anne r s Boardmil1, Bristol, on which the hoist motion is driven
by a Schrage rated 10/ 1 hp, 1750/175 rpm.
On unloading cranes, where
rapid control over a wide range of speeds is the most important
requirement, the Schrage motor has been used with good results.
The
following is from the Power and Works Engineer, (25).
The Vaughn Crane Co. Ltd. of Manchester, England, makes a
special feature of electric overhead and goliath cranes for industrial
purposes and of Variospeed control by means of ac commutator motors.
The Pilot motor is a fractional hp squirrel cage unit. Its drive is
transmitted to the brushgear through spur and bevel gearing, and
there is a drum type limit switch which cuts off current to the pilot
motor at each end of the travel of the brush gear. The brush gear
carries only secondary current at low voltage and the power dealt
with by the commutator is only a fraction of the total output.
Control is by means of a standard tramway type reversing drum
controller, which has three notches in both directions. The first
notch gives creeping speed and the third full speed. Interzoodiate
speeds are obtained by bringing the controller back from the third
to the second notch when the motor has attained the requisite speed,
so giving a close control with any desired incremental adjustments.
Acceleration is 'otherwise automatic. No starting or controlling
resistance are empolyed and the losses associated with their use
are eliminated. Dynamic braking is inherent, and the usual solenoid
is only required to hold a load stationary. Motors with speed
ranges up to 15:1 can be utilized. Motors have shunt characteristics
so that speed barely varies whether the lift is light or heavy. A
starting torque least twice full load torque is available at any
speed within a motor's range. Owing to the saving of resistance
losses, the energy requirements where low speeds are habitually required are strikingly reduced.
The following is from the Engineer, (18), and is a good description
of an ac crane motor, and to me the question arises, "why can't the
Schrage motor be used for an electric cargo winch for marine uses?"
It
can be totally enclosed to protect it from the weather and it does
not require a lot of excess equipment such as an electronic controlled
drive, or a motor-generator set, or hydraulic equipnent, or a magnetic
clutch.
At the same time, as previously explained, it has an infinite
speed adjustment vdthout an induction motor's losses.
97
The equipment required is less than that necessary for the
slip-ring induction motor, since the ac commu.tator motor is arranged
for direct-on-line starting for a~ hp within the requirements of
a crane, provided the brushes are set in the low speed position.
Speed variation is obtained by altering the brush position, giving
an infinite nwnber of graduations from I1creep speed" to "top speed".
I f desired a speed range of 20:1 or more is possible. Top speed is
usually 1000 rpm, and. perfectly controlled working down to 50 rpm
may represent a hoisting or lowering speed of 1 ft. per minute.
This machine has a shunt characteristic with definite speed for each
brush position. By this featUre the crane operator is greatly
assisted when using the "preset" control, since he is able to
estimate the working speed for a~ condition of loading from the
position of the brush gear controller, and provided the crane is
not overloaded, the danger of stalling is eliminated. The commutator motor gives constant torque over the whole speed range, and
so can handle full loads at all speeds. Rate of speed change is
controlled by the pilot induction motor, and it is quite independent of the rate at 'Which the controller handle is operated by the
operator. This condition enables stresses in the crane to be
accurately predetermined, thereby elimina.ting over stressing
attributable to indifferent operation. If, as normally occurs,
when slowing down or lowering a load, the load drives the motor,
owing to the regeneration of the machine, a braking torque is
produced, provided of course, that it is connected to the supply.
As this torque becomes greater as the negative slip increases, the
load is prevented from running away. '¥'lith suitable brush gear
mechanism" the braking effect can be made at least equal to that
of a mechanical brake, and this is advantageous for a hoist motion.
In all equipments so far supplied this feature has been incorporated
and has resulted in considerable saving in brake wear. The motor
reduces the speed of the load to a II creep" when the brake is applied
to bring it to rest. The operation of the flpre-set" follower
control is identical for flforward fl and IIreversefl movement.
Immediately that the controller handle is IOOved from the "off"
position, the main contactor closes for the direction concerned.
Following smoothly on from this operation the controller handle is
turned round to the position which the crane operator knows will
give the desired speed. The contact arm coupled to the controller
handle will have been moved round the drum driven from the pilot
motor" completed the circuit to the pilot motor, and so caused the
main motor brushes to be shifted. When the pilot motor has r0tated the controller drum thru the same angle as the contact arm,
its circuit is broken and the conmutator motor continues to run at
that speed which has been fixed by pre-setting. Reduction in speed
is similarly obtained by bringing the controller handle back towards
the f1offt' position. Provided the motor is connected to the supply
when the load drives the motor, no mechanical braking torque is
necessary e~ept to bring the load to rest.
With the slip ring motor the smoothness of control is limited
by the rotor's resistance graduations, usuall:y limited from 6 up
to 9. The regulation is indifferent, particularly on light loads,
when artificial loading (in the form of a brake) is essential for
98
obtaining a steady creep speed. If the motor is not appreciab~
loaded it will tend to run at full speed. When it is desirable
to slow down and stop an;r motion on the crane, the mechanical
drive is called upon to produce the necessary retarding torque
without an;r assistance from the motor, resulting in heavy wear
of the brake shoes. A number of variables are involved such as
coefficient of friction and pressure exerted by the brake blocks,
producing different stopping distances for the same load and speed,
and considerable shock is transmitted through the crane structure.
The Ward-Leonard scheme, involving variation of the generator
field, gives good stability and smoothness over the speed range,
but it is not recommended from an economical standpoint as a
separate generator is necessary for each crane motion involving
creep control. Another drawback is that the motor-generator set
must be kept running during the time the crane is liable to be in
use.
99
BIBLIOGRAPHY
1. Crocker-Wheeler Co IS. pamphlet on polyspeed motors.
2. Electric JBchinery, Volumes I and II, Liwshitz-Garik and Whipple.
3. Electric Manufacturing, Adjustable speed motors, May 1940.
4. Electric Manufacturing, Adjustable speed motors enter new era,
May
1940.
5. Electrical Review, Ac commutator motors, Sept. 30, 1932.
6. Electrical Review, Control of ac motor speeds, Oct. 12, 1934.
~
7. Electrical Review, Variable speed drives, C. W. Olliver, November
23, 1934.
s.
Electrical Review, Ac commutator motor, C. W. Olliver, November
23, 1934.
9. Electrical Review, Schrage motor, O. E. Mainer, July 16, 1943.
10. Electrical Engineering, Vol. 60, August 1941; Volume 61, July
1942, Theory of brush shifting ac motor, A. G. Conrad, F. Zweig,
and J. G. Clarke.·
11. Electrical World, BTA motor best for gravure press, February 10,
1940.
12.
Electrical Tilms, Polyphase ac commutator motors.
13.
Electrical Times, AC shunt conunutator motor, July 3, 1941•
. 14. Electrical Times, Auxi.li.ary windings on ac commutator machines,
July 17, 1941.
15. Electrician, Vol. 114; page 382, March 22, 1935.
16. Electrician, Vol. 121, page 156, August 5, 1938.
17. Electrician, Industrial Efficiency, October 6, 1939.
18. Engineer, Vo~. 167, page 225-6, AC motor for driving cranes,
February 17, 1939.
19. Engineer, Vol. 167, page 310-11, Three-phase ac commutator motor,
!arch 10, 1939.
20. Engineer, Vol. 168, pages 323-25, Variable speed ac motors for
power stations, September 29, 1939.
100
BIBLlOORAPHY
(continued)
21. Engineering, Volume 140, page 652, December 1.3, 19.35.
22.
Engineering, Volume 147, page 286-7, March 10, 19.39.
2.3. Engineering and Boiler House Review, Motors for power stations,
November 19.39.
24. General Electric COIS. pamphlet on type ACA motors.
25. Power and Works Engineer, page .374, October 19.36.
26.
Power and Works Engineer, Schrage motor versus induction motor,
December 19.36.
27.
Power and Works Engineer, AC commutator motors, October 1945.
101
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