of electrical machinery
Lecture Note ForJune
Principal of electrical machinery and optimization of electrical power
SuthipongThanasansakorn
Southeast Asia Fishery Development Center, Training Department Thailand June 2010 Part 1
Direct current generator
By
Suthipong Thanasansakorn
1/6/2010
Southeast Asia Fishery development center
Training Department
Thailand
2
DC GENERATOR
The direct current (DC) generator (Dynamo) is only used for special application or local
power generation. The limited of implementation is weaken on commutation required to rectify
the internal generated AC voltage to DC voltage; thereby the operating characteristics of DC
generators are still importance, because most concept can be applied to all others electrical
machine.
We can classify the generator machine into two parts.
1. The stationary part generally called “stator”.
2. The rotating part usually called “rotor” or the armature referred to
a DC machine.
1. Field winding connection
The four brushes ride on commutator. The positive brushes are connecting to terminal
A1 and negative brushes are connecting to A2 of the machine the brush are positioned
approximately midway under poles.
The four-excitation or field poles are usually join to produced a magnetic path in series
and their ends to terminal F1 and F2 They are connected such that they produced north and
south poles
Fig. 1 The general arrangement of brushes and field winding of four-pole DC generator
3
2. Type of DC generator is characterized by the manner in which field excitation is
provided. In general the method employed to connect field and armature winding has classify
into two groups.
2.1 Separately excited generators. These kind of generators has provided field exciter
terminals which are external DC voltage source is supplies to produce separately magnetic field
winding (shunt field) for magnetize of the generator as illustrated in figure 2 as below.
Fig. 2 Separately excited generators.
2.2 Self excited field generators. This type of generator has produced a magnetic field
by itself without DC sources from an external. The electromotive force that produced by
generator at armature winding is supply to a field winding (shunt field) instead of DC source
from outside of the generator. Therefore, field winding is necessary connected to the armature
winding. They may be further classified as
a) Shunt generator.
This generator, shunt field winding and armature winding are connected in
parallel through commutator and carbon brush as illustrated in the figure 3
Fig. 3 Shunt generator
b) Series generator
4
The field winding and armature winding is connected in series. There is
different from shunt motor due to field winding is directly connected to
the electric applications (load). Therefore, field winding conductor must
be sized enough to carry the load current consumption and the basic
circuit as illustrated below.
Fig. 4 Series generator
c) Compound generator
The compound generator has provided with magnetic field in combine with
excitation of shunt and series field winding, the shunt field has
many
turns of fine wire and caries of a small current, while the series field
winding provided with a few turns of heavy wire since it is in series with
an armature winding and caries the load current. There are two kinds of
compound generator as illustrated in figure 5 and 6.
Fig. 5 A short-shunt compound generator
5
Fig. 6 A long-shunt compound generator
3. Characteristic of separately excited generator
The generated electromotive force (EMF) is proportional to both of a magnetic density of
flux per pole and the speed of the armature rotated as expression by the relation as following.
Eg = κ φ n
Where
K
=
Constant for a specific machine
φ
=
The density of flux per pole
n
=
Speed of the armature rotation
Eg
=
Generator voltage
3.1 By holding the armature speed (n) at a constant value it can show that generator
voltage (Eg) is directly proportional to the magnetic flux density. Which, flux density is
proportionately to the amount of field current (If). The relation of field current and generate
voltage as impressed by figure 7.
Fig. 7
6
From the figure 7 when the field current (If) is become zero a small generate voltage is
produce due to a residual magnetism.
As the field current increases cause to increase generated voltage linearly up to the
knee of the magnetization curve. Beyond this point by increasing the field current still further
causes saturation of the magnetic structure.
3.2 Generator voltage (Eg) is also directly to the armature speed. The formula and a
magnetization curve can be both impressed about this relation.
Eg ' = Eg ×
n'
n
Where
Eg
=
Generator voltage or the value of EMF at speed n
Eg '
=
Generator voltage or the value of EMF at speed n’
n
=
Speed of the generator armature ( n’ ≠ n )
Example:
The open circuit terminal voltage versus the field current for a separately excited DC
generator with provided the following test data at revolving speed 1400 rpm as show by the
table1 below.
Voltage (V)
6
30
58
114
153
179
Ampere (A)
0
0.1
0.2
0.4
0.6
0.8
Table 1 No-load characteristic for 1400 rpm and 1000 rpm
Figure 8 Magnetic curve for example 3.1
7
Solution
Curve (a) in figure 8 shows the characteristic at revolving speed 1400 rpm obtained by
the data as show in table 1. To obtain the characteristic at 1000 rpm, is made of the relation as
Eg = Kφn
For instance, at a field current of 0.4 Amp the terminal voltage is 114 volts, when the
speed is reached to 1400 rpm and kept its field current constant at this value, the open circuit
voltage at 1000 rpm becomes.
Eg = 114 ×
1000
= 81.40 Volts
1400
4. Voltage Regulation
When we add load on the generator, the terminal voltage will decrease due to
4.1 The armature winding resistance is mainly of armature resistance. It is cause
directly decrease in terminal voltage as following relation.
Vt = Eg - Ia R a
Where
Vt
=
Terminal or output voltage
Ia
=
Armature current or load current
Ra
=
Armature resistance
Figure 9 (a) Load characteristic of
Figure 9 (b) Circuit diagram
a separately excited DC generator;
8
4.2 The decrease in magnetic flux due to armature reaction. The armature current
establishes a magneto motive force (MMF), which it distorts to main flux, and makes result
in weakened flux. We can put inter-pole between main field poles to reduce the armature
reaction.
To have some measure by how much the terminal voltage change from no-load
condition and on load condition, which is called “voltage regulation”.
Voltage regulation
Vnl − V fl
x 100
Vfl
=
= %
Where
Vnl
=
No-load terminal voltage
Vfl
=
Full-load terminal voltage
Remark:
A separately excited generator has disadvantage of requiring an external DC source. It
is therefore used only where a wide range of terminal voltage required.
Example 2
The separately excited generator of example 1 is driven at revolving speed 1000 rpm
and the field current is adjusted to 0.6 Amp. If the armature circuit resistance is 0.28 ohm, plot
the output voltage as the load current is varied from 0 to 60 Amp. Neglect armature reaction
effects. If the full-load current is 60 Amp, what is the voltage regulation?
Solution
From example 1, Eg = 153 volts when the field current is 0.6 Amp, which is the open
circuit terminal voltage. When the generator is loaded, the terminal voltage is decreased by
internal voltage drop, namely.
Vt =
E g - Ia R a
Vt =
153 - (40 × 0.28)
For a load current of, say 40 Amp.
=
141.80 Volts.
This calculation is for a number of load currents and the external characteristic can be
plotted as show in fig. 10 at full load the terminal voltage.
Vt =
153 - (60 × 0.28)
9
=
136.20 Volts.
Therefore the voltage regulation is
Voltage regulation
=
Vnl − V fl
x 100
Vfl
=
153 - 136.2
× 100 = 12.3 %
136 . 2
= %
Figure 10 Calculated load characteristic of an example 3.2
5. Load characteristic
5.1 Self excited DC shunt generator
A shunt generator has its shunt field winding connected in parallel with the armature
so that the machine provides it own excitation. For voltage to build up, there must be some
residual magnetism in the field poles. There will be a small voltage (Er) generated.
Figure 10a Shunt generator circuit
Figure 10b Shunt generator
load characteristic
If the connection of the field and armature winding are such that the weak main pole
flux aids to the residual flux, the induced voltage will become larger. Thus more voltage applied
to the main field pole and cause to the terminal voltage increase rapidly to a large value.
10
When we add load on the generator, the terminal voltage will decrease due to.
a) The armature winding resistance
b) The armature reaction
c) The weakened flux due to the connection of the generator to aids or
oppose to the residual flux.
Example: A shunt generator has field resistance of 60 ohms. When the generator delivers
60 kw the terminal voltage is 120 volts, while the generated EMF is 135 volts. Determine
a) The armature circuit resistance
b) The generated EMF when the output is 20 kw and the terminal voltage is
135 volts.
Solution
a) The circuit diagram when delivering 60 kw is as show in figure 11 the load
current is
IL =
60,000 watts
120 volts
= 500 Amperes.
The field current supplied by the armature is
If =
120 volts
60 ohms
= 2 Amperes
Therefore,
IA
= If + IL = 500A + 2A
= 502 Amperes.
Since,
Vt = Eg – Ia Ra
Ra =
Eg − V t
Ia
=
135 - 120
= 0.28 Ohms.
52
Figure 11 Circuit diagram for the solution of example 3
b) For a load of 20 kw when the terminal voltage is 135 volts, therefore the
11
load current is
IL =
20,000 watts
= 148.1 Amperes.
135 volts
If =
135 volts
60 ohms
And the field current is
= 2.25 Amperes
Ia =
148 + 2.25 = 150.25 Amperes
Eg =
Vt + Ia Ra
=
135 + (150.25 x 0.28)
=
177.07 volts
5.2 Series generator
The field winding of a series generator is connect in series with the armature winding.
Since it carries the load current, the series field winding consists of only a few turns of thick
wire. At no-load, the generator voltage is small due to residual field flux only. When a load is
added, the flux increase, and so does the generated voltage.
Figure 12a. Circuit diagram of
Figure 12b. Load characteristic of
a series generator
a series generator
Figure 12 shows the load characteristic of a series generator driven at a certain speed.
The dash line indicated the generated EMF of the same machine with the armature opencircuited and the field separated excited. The different between the two curves is simply the
voltage drop (IR) in the series field and armature winding.
Vt = Eg - Ia ( R a + R f )
Where
Rf = The series field winding resistance
Ra = The armature winding resistance
The series generators are obviously not suited for applications requiring good voltage
regulation. Therefore, they have been used very little and only in special applications for
12
example, as voltage booster. The generator is placed in series with a supply line. When the
current consumption is increase, the generated voltage of the series machine goes up because
the magnetic field current is increases.
5.3 Compound generator
The compound generator has both a shunt and a series winding. The series field
winding usually wound on the top of a shunt field. The two winding are usually connected such
that their ampere-turns act in the same direction. As such the generator is said to be
cumulatively compound.
Figure13 A simply circuit of compound generator
Figure 14 Terminal voltage characteristic of compound generator
-
Curve s is represent the terminal voltage characteristic of shunt field winding
alone.
13
-
Under-compound, this condition the addition of series field winding too short it is
cause the terminal voltage no rise to certain value and reduce while increasing in
load current
-
Flat compound by increasing the number of a series field turns. It is cause to rise
up in terminal voltage and when no-load and full load condition a terminal voltage
is made nearly same value or equal.
-
Over-compound, if the number of series field turns is more than necessary to
compensated of the reduce voltage. In this case while a full load condition a
terminal voltage is higher than a no-load voltage. Therefore over-compound
generator may use where load is at some distance from generator. Voltage drop in
the line has compensated by used of an over-compound generator.
-
If a reversing the polarity of the series field occur this cause to the relation
between series field and shunt field, the field will oppose to each other more and
more as the load current increase. Therefore terminal voltage will drop, such
generator is said to be a differentially compound.
The compound generator are used more extensively than the other type of dc
generator because its design to have a wide variety of terminal voltage characteristics.
Example 5 Determine the number of series turn required on each pole of a compound
generator to enable it to maintain the voltage at 240 volts between no load and full load of 20
kw. Without the series winding, it is found that the shunt current has to be 4 amps. On no load
current and 5 amps at full load, to maintain the terminal voltage at 240 volts The number of
turns per pole on shunt field winding equal 600 turns.
Solution.
Figure 15 gives the circuit diagram of the compound generator.
A long shunt
connection is assumed. On no load the field excitation current is supplied by the armature.
The resulting MMF of a shunt field pole is
= n1 = 600 x 4 = 2400 At
The small voltage drop across the series field winding will be assumed negligibly small.
On full load the shunt field current is 5 Ampere used to maintain the terminal voltage
at 240 V.
In other words, the required MMF is taken = n1 = 600 x 5 = 3000 At/pole. It is
the difference in MMF that would have to be supplied by the series field winding to give flat
compounding while keeping the shunt field at he initial value of 4 A. Thus the MMF series field
14
is 3000-2400 = 600 At/pole. The full load-current which also flows through the series field
winding is
IFL =
PL = 2 0,000 = 83.3 A
V1
240
Therefore, the number or turns required on the series field to provide an MMF of 600 At is
Series field turns =
600 At / (83.3 + 4)
=
6.9/pole
Figure 15 Compound generator at full load for Example 5
Or we can solve by
Ns =
Nf Δ If
Is (rated)
Where
Ns
= number of series turns required
Nf
= number of shunt field winding
Δ If
= the increase in shunt field current required to maintain the terminal
voltage from no-load to rated –load without the series field
Is
= rated load or armature current flowing through the series field
winding
15
Machine Efficiency
The efficiency of any machine is the ratio of the ratio of the output power to the input
power. The input power is provided by the prime mover to drive the generator. Because part
of the energy delivered to the generator is converted into heat, it represents wasted energy.
These losses are generally minimized in the design stage; however, some of these losses are
unavoidable.
Efficiency
=
Efficiency
=
Output power
___________________
Input power
x 100%
or
Output power x 100%
____________________________
Input power + losses
The losses of generators may be classified as
1)
Copper losses
The copper losses are present because of the resistance of the windings.
Currents flowing through these windings create ohmic losses.
The windings that may be
present in addition to the (I2 R ) armature winding are the field windings, inter-pole and
compensate windings.
2)
Iron losses
As the armature rotates in the magnetic field, the iron parts of the armature as
well as the conductors cut the magnetic flux. Since iron is a good conductor of electricity, the
EMF s induced in the iron parts courses to flow through these parts. These are the eddy
currents.
Another loss occurring in the iron is due to the Hysteresis loss is present in the
armature core.
3)
Other rotational losses consist of
3.1
bearing friction loss
16
3.2
friction of the rushes riding on the commutator
3.3
windage losses
Windage losses are those associated with overcoming air friction in setting up
circulation currents of air inside the machine for cooling purposes. These losses are
usually very small.
Example 6 A 10 kW 125 V compound generator has rotational losses amounting to 580 W.
The shunt field resistance is 62.5 Ω , the armature resistance 0.12 Ω , and the series field
resistance 0.022 Ω . Calculate the full-load efficiency.
Solution The full-load current is
IL =
10,000 / 125 =
80 A
Assuming a long-shunt connection, the field currents is
IF =
125/62.5
=
2A
Then IA = 80 + 2 = 82 A, which is the current through the series winding. The I2 R losses can
now be determined:
Armature 822 x 0.12
=
Series field 822 x 0.022 =
807 W
148 W
Shunt field : 22 x 62.5
=
250 W
Rotational loss
=
580 W
=
1,785 W
Total loss
Hence
η
=
P
out
/ (Pout + P
loss)
= 10,000 x 100/ 11,785
17
= 84.9%
An alternative way to solve for the efficiency is as follows:
EG =
V1 + IA ( Rs + RA )
= 125 + 82(0.022 + 0.12) = 136 .6 V
the power developed by the armature is
pd =
EgIA
= 136.6 x 82 = 11.205 W
Therefore, the power input at the shaft is pd + rot = 11.785 W and
η
=
(10,000 / 11,785) x 100
as above
18
= 84.9%
Part 2
Direct current motor
By
Suthipong Thanasansakorn
1/6/2002
Southeast Asia Fishery development center
Training Department
Thailand
19
Direct Current Motor (DC motor)
DC motor is similar to dc generator; in fact the same machine can act as motor or
generator. The only difference is that in a generator the EMF is greater than terminal voltage,
whereas in motor the generated voltage EMF is less than terminal voltage. Thus the power
flow is reversed, that is the motor converts electrical energy into mechanical energy. That is
the reverse process of generator.
DC motors are highly versatile machines. For example, dc motors are better suited
fore many processes that demand a high degree of flexibility in the control of speed and
torque. The dc motor can provided high starting torque as well as high decelerating torque
for application requiring quick stop or reversals.
DC motors are suited in speed control with over wide range is easily to achieve
compare with others electromechanical.
Counter EMF in DC motor
When voltage is applied to dc motor, current will flow into the positive brush through
the commutator into the armature winding. The motor armature winding is identical to the
generator armature winding. Thus the conductors on the north field poles are carry current in
one direction, while all conductors on the south field poles carry the current in opposite
direction. When the armature carry current it will produce a magnetic field around the
conductor of it own which interact with the main field. It is cause to the force developed on
all conductors and tending to turn the armature.
The armature conductors continually cut through this resultant field. So that voltages
are generated in the same conductors that experience force action. When operating the
motor is simultaneously acting as generator. Naturally motor action is stronger than
generator action.
Although the counter EMF is opposite with the supplied voltage, but it cannot exceed
to applied voltage. The counter EMF is serves to limit the current in an armature winding.
The armature current will be limited to the value just sufficient to take care of the developed
power needed to drive the load.
In the case of no load is connected to the shaft. The counter EMF will almost equal to
the applied voltage. The power develops by the armature in this case is just the power
needed to overcome the rotational losses. It’s mean that the armature current IA is controlled
and limited by counter EMF therefore.
Ia
VL − Ea
Ra
=
20
Where:
VL
=
Line voltage across the armature winding
Ra
=
Resistance of the armature winding
Ea
=
Induced EMF or generated voltage
Ia
=
Armature current
Since, EA is induced or generated voltage it is depend on the flux per pole and the speed of
the armature rotate (n) in rpm. Therefore
K φn
Ea =
Where:
K
=
the constant value depending on armature winding and
number of pole of machine.
φ
=
Rotation of the armature
And,
Z×P
a
=
K
Where:
Z
=
Total number of conductor in the armature winding
a
=
Number of parallel circuit in the armature winding between
positive and negative brushes.
For wave wound armature “a”
=
2
Lab wound armature “a”
=
P
Example
A dc motor operated at 1500 rpm when drawing 20 amps from 220 volts supply, if the
armature resistance is 0.2 ohms. Calculate the no load speed assumed Ia = 0 amp (This
amount to assuming the brushes and rotation loss are negligible)
Solution
When load condition
Ia
=
Ea
And
Ea
20 amps.
=
VL – Ia Ra
=
220 – 20 (0.2)
=
216
=
kφn
21
Volts.
At no load condition
Hence
216
=
Kφ × 1500
Kφ
=
216
1500
=
0.144
Ia
=
0 Amp.
Ea
=
VL
Ea
=
kφn
n
=
220
kφ
=
220
0 . 144
=
1528 rpm.
=
220 Volts.
Mechanical power develop in dc motor (Pd)
Pd
=
Mechanical power develop
T
=
Torque exerted on the armature
Pd
=
ωT
⎛ 2π n ⎞
=⎜
⎟T
⎝ 60 ⎠
Where: T
=
=
Therefore:
T
=
Pd / ω
Ea × Ia
2π n / 60
=
Kφ n × Ia
(2π n ) / 60
Kφ IA
Example: from the motor that mentions before, determine
Solution:
Pd
T
=
EA IA
=
216 • 20
=
4320 watts.
=
Pd / ω
=
27.51 NM
22
Classification of dc motor
There are generally three type of dc motor namely
1, Series motors
2, Shunt motors
3, Compound motor
The series motor is widely used because its excellent starting torque
characteristics, but each type of dc motor has very definite operating characteristics. It is
essential to know construction and requirement before a proper motor4 selection.
Starting of dc motor
At the instant of start up, the armature is not rotate, therefore the counter EMF EA
is zero because no any flux which induced to the armature winding. If we start the motor as
mention before with direct across the line 220 volts supply. That armature winding tend to
reach search current equivalent to 220/ 0.2 = 1100 A. This current subject to the armature to
produced as mechanical shock and would blow fuse and disconnect itself from supply. It is
therefore necessary to insert some resistance in series with the armature circuit to limit the
current flow through the armature winding.
As The motor come up to speed, this resistance is taken out in steps because EA
rise as the motor come up to full speed. This resistance arrangement is called starter. If the
resistance has induced into the motor as mention above is to be limited at 100 amps. The
total resistance of the starter plus the armature winding resistance of 2 ohms, the starter
circuit as illustrate below.
Figure 1.The starter circuit
23
When the main switch is turn on by starter arms is moved to contact 1 the motor
is starting to rotate. The starter arm has to slowly move to the following contact until the
final position. Motor is full speed condition, this whole process shout a few seconds
depending on the size of motor.
The starter must be rated on horsepower, voltage and current that is used. The
motor less than 1 hp. direct on line full voltage starting is allowed.
In the final position no added the external resistant connected to the armature
circuit, so the current is directly apply to the armature path but the external resistant still
connected to maintain the field circuit. The starter arm is continuously kept closed to the final
contact by the holding coils. (Magnetized force) When there is a failure power or the field
circuit is opened accidentally. The spring will return starter arm out off position, automatically
shut down. The amount of current consume with starter connection becomes.
I
start =
Vsource
R A + Rstarter
In the separately excited machine the field winding is connected to a different
supply. If the field winding is connected in parallel with the armature winding we called a
shunt machine. The series machines has a field winding is caries the load current, It is
necessary to capable the winding to carry the load current without the excessive heat loss in
its.
In case of compound motor it has both of winding conductors. This field winding
may be connected so that the field aid one another, depending on flux oppose. Thus, the
compound motor may have long-shunt or short shunt field winding. It is arrangement,
whether the shunt field is connected before or after the series field winding.
When the motor is loaded the speed tends to slow down. This is known from
practical experience. The amount which speed is slows down at full-load, as compare with
no-load condition will depend on type of connection employed.
24
Figure2. Illustrated the current on different kinds of dc motor
The armature current on the starter position 1 is can be define as following.
I1
V supply
R A + R starter
=
Since torque is directly proportion to the armature current and flux it is cause
accelerated to the start up of the armature. During the acceleration period the counter
electromotive force is increase (Ec) but the armature current tendency to decrease to some
value. When the starter arm is moved to position 2 with sufficient resistance the current that
flow into the armature is rise approximate to I1 again. This operation is continued until the
last contact is reach, assume that the motor is steady-state speed and current.
Example
Calculate the require resistance for a four-step starter to limit the starting current
of a DC shunt motor to 150% 0f rate current. Assume that all four steps have an equal
resistance value. The motor has rating as 25 Hp, 240 Volts, 860 rpm. And armature
resistance is 0.08 Ohms, the overall efficiency 90%. Determine at each speed of the starter
resistance must be take into the starting circuit to maintained the rate value during start up
period. Assume that the field current is negligible compared to rate armature current.
Solution:
Motor starting circuit. At full load the motor input power is as computed by the following.
Pin
=
Hp × 746
η
=
10 × 746
0.9
And the line current is
IL
=
8,288
= 34.53 Amps.
240
25
=
8,288 watts
The designed rate current is limited at 150% of nominal rating, the maximum current will be
Imax
=
1.5 ×
=
34.53 Amps.
51.79 Amps.
The current-limiting resistance at startup is
Imax
=
V supply
R A + 4R
240
=
51.79 × ( 0.1 + 4R)
240
=
240 - 5.17
=
R
=
=
240
0.1 + 4R
= 51.79 A
5.17 + 207.16R
207.16R
234.84
=
207.16
1.13 Ω
Furthermore, at rated speed and current,
=
=
V - IA Ra = kΦn
240 - (51.79 × 0.1)
234.8
=
kΦ × 860
kΦ
=
0.273
Ec
Therefore, k Φ = 0.273 which is constant under stated assumptions. On the startup period,
since n = 0 rpm there are four resistors connected in series with the armature winding then
the total resistance and current will be.
Imax
Then
=
IR
240
=
IA (0.1 + 4R)
The motor will accelerate and the current decay until IA = I
A rated
at this point the
starter is place in the contact 2 there are 3 resistors in series with the armature-winding and
IA = IA max Then
240
=
=
=
=
Ec
Where :
n
=
51.79 (0.1 + (3 × 1.13)) + Ec
(51.79 × 3.49) + Ec
180.6 + Ec
59.4
E c = kΦ n
59.4 = 0.273 n
59.4
≈ 218 rpm.
0.273
At time t1 the motor has produce counter EMF 59.4 volts and accelerate up to 218 rpm.
Similarly on contact 3 with 2 resistors connected in the circuit.
26
240
=
51.79 ( 0.1 + (2 × 1.13 )) + Ec
=
122.22 + Ec
117.78 volts
Ec
=
n
=
117.78
≈ 431.42 rpm.
0.273
At time t2 Ec = 118 volts and n = 431 rpm.
On contact 4, with a single resistor connected in the motor circuit
Ec
=
181 volts and n
=
664 rpm. at time t3.
At contact 5, the final position the starter-arm connected without resistor no additional
resistance in the motor armature circuit. Motor is directly across to the sources.
Ec
=
235 volts and n
=
860 rpm. at time t4
When the motor has reach to a steady condition. The time interval of the revolution reach to
full speed condition is depending on motor plus load inertia the variation of armature current
and time will somewhat similar to the curve as giving.
Automatic starter of DC motor
The apparent disadvantages of manual starter of DC motor are.
-
Bulkiness of the starter.
-
Lack of remote operation.
-
Possibility of improper operation.
-
Non-uniform acceleration.
An automatic starter has covered all these disadvantages in order to providing other
reliable control feature such as over and under voltage protection, over speed protection.
To illustrate these matters let refer to a simple automatic starter connected to a dc
shunt motor as illustrate as following
Figure3. Illustrated an
automatic starter
connected to a dc shunt
motor
27
This starter circuit is called a “counter-EMF starter”. The starting resistance is added
to cut out the current in a single step; the contact M is a magnetic contact. The magnetic is
an electrical switch which has a holding coil placed on an iron armature core. When the
current flows through the holding coils, the coils become magnetize. This attracts to move
iron armature that carries a set of contacts, which has an electrically insulated from each
other. Thereby closing the circuit the holding coil energized, contact M are closes and
complete the circuit by pressing the start button and disconnect the circuit after pressing the
stop button, which together the start and stop button may be remotely located from the
motor.
When the magnet is energized contacts M and M1 close, contact M1 is called “
maintaining or holding contact” it function is keeps magnetic contact energized after the start
button is released the motor continue operates until the stop button is pressing.
It is cause to the motor connected to power supplied with the starter resistor R
connect in series with the armature winding because at the standstill the counter EMF is zero,
the starter resistor is provided for limited the starting load current. The contact T is called
“accelerating contact”
When the motor is speed up the counter EMF is increase as the voltage across the
armature (Ec). Whenever Ec reach about 80% of supply voltage contact A closes and
associated short out the starter resistor R. The contact T is controlled by a definite time
presetting called timer-delay relay, because the counter EMF is relate to the acceleration of
the motor. So now the motor is directly cross to the supply voltage. To obtain smoother
accelerate and high performance.
In the starting circuit overload protection has provided by the thermal overload relay,
there are two types basically both are operate by the heat generate in the heating element
which is connect in series with the motor cause the amount of motor current flow through its.
One type is bimetallic strip and the other is melting a strip of solder. Both types act to open
the motor circuit when the over current is produced.
To stop the motor under normal operation, the stop button is presses thereby open
the control circuit. This is cause by the stop button is disconnect the holding coil, the holding
coil is de-energized cause the contact M are disconnect, thereby motor is separated from the
sources.
Speed characteristic of DC motors.
When the mechanical load is removing from the motor, the motor speed will
increase. The amount by which it increases depends on the type of motor. The speed of
shunt motor increase about 8%, for a compound motor it rise approximately 15 to 20%. And
for the series motor it would rise rapidly and it is for this reason the series motors must
always drive a load. To make clear and understand why this happens let consider the relation
as following.
28
n
=
V - IA R A
kφ
In shun motor the flux is only slight affected by the armature current, while the IA RA drop
rarely exceed 5% of line voltage. Therefore the maximum change in speed must be the same
order as IA for a shunt motor. Shunt motor is fairly constant machine as indicated by a curve
as following.
Figure 4. Illustrated shunt motor current characteristic
For a compound motor, when the load is removed there are two factors affected the
speed and flux. Unlike the shunt motor, the effect of series field is removed under no-load
condition, thereby weakening the overall of field flux. The result is larger increase in speed,
since the speed is inversely proportional to the flux as indicated as the above formula. This is
representing by curve b. We can now see why the series motor will run at danger high speed
when the load is removed. There would not be any flux because the flux is depend upon load
current The curve c illustrated a load current behavior of the series motor. For this reason the
series motor is not used in instance where the load can be disconnected by accidentally such
as belt-coupling drive should never be used with series motor drive.
In similar the torque-load characteristic for the shunt, compound and series motor
can be compare the figure as illustrated below shows for the 3 different type of dc motors. In
series motor, the developed torque depends on the load current and fluxes; because the flux
in turned depend on the current under this condition.
T
=
kφ I A
29
Figure 5. Illustrated DC motor speed characteristic
If magnetic saturation is considered, the graph will become a straight line at heavy
load, since the flux will not change with increase loading. Figure 5 it is evident that for a
given current below full-load value, the shunt motor develops the largess torque. For currents
exceeding rated current, the series motor develops the largess toque. It is for this reason that
in applications requiring large starting current. Such as for hoist, electric trains, and so on,
the series motor is the most suitable machine.
Example
A 240-Volt shunt motor has an armature current of 0.25 ohms. Under load, the
armature current is 20 A. Suppose that the flux is suddenly decrease by 2.5%, what would be
the immediate effect on the develop torque?
Solution. When the current is 20 A
T =
=
kφ I A
Kφ ื
20
and
Ec =
=
=
VL - I A R A
240 - (20 ื
0.25)
235 volts
If φ is suddenly decreased by 2.5%, Ec is also decrease, since Ec = K φ n and the speed
cannot change instantaneously, thus
Ec
=
235 - ( 235 ×
=
235 - 5.87
=
229.13 volts.
=
VL - E c
RA
2.5
) = 235 × 0.975
100
The new armature current is
I'A
240 - 229
0 . 25
=
30
= 44 Amps
The new value of develop torque is
T
=
k ( 1 - 0.025 φ ) I A
=
=
K ( 0.975 φ ) 44
40.7 Kφ
This is corresponding to an increase in torque as
T'
T
40.7 Kφ
20 Kφ
=
=
2.035 times
Thus slight decrease of flux almost doubles the torque. This increased torque causes the
armature to accelerate to higher speed. At that point the counter EMF limits the armature
current just enough to carry the load at this new speed.
Speed regulation.
The motor classification on the basis of speed change with load is particularly
important in the selection of motor. If the speed of motor is relatively constant over it normal
range, the phenomenon of motor as having good speed regulation. Speed regulation is
usually expresses ass a percentage as found as following.
Speed regulation =
=
no - load speed - Full - load speed
× 100
full − load speed
Nnl
-
Nfl
Nfl
×
100
%
%
Where:
nfl
=
Full-load speed
nnl
=
No-load speed (Both express in r/min, rpm.)
Note. Speed changes cause by loading condition, there are no resulted by speed adjustment
made by manually or by personnel.
Speed control of DC motor
In order to change in speed of dc motor there are three quantities that may be
considered as parameter:
-
The armature resistance IA.
-
Flux φ density.
-
And terminal voltage V.
The armature resistance method by adding resistor in series with the armature
effectively increases the armature circuit resistance. Let considered the equation as follow.
31
n
=
V - (I A R A)
kφ
There is indicates the results in reduction of steady-state speed, since it is
proportional to the counter EMF, except for no-load condition. This method is the field current
is kept constant. Figure. Illustrate the characteristic of different value of armature circuit
resistance. As apparent, this method of speed control is relatively simple and inexpensive.
Anyway it has some disadvantage such as.
1. Adding resistance in the armature circuit the speed of the motor, compare to that
without any resistance adding, however it is always lower
2. This method of speed control is ineffective at no load condition due voltage drop
cause by IA and R is less.
3. Adding resistance means increase in I2 R losses, therefore waste power due to
heat generated. As a rule of Thumb, the percentage reduction in speed is equals to the
percentage of input power that is consumed in the added resistor.
4. The constant-speed characteristic of the motor is loss. In generals the speed
control of this method is limited about 50 % of nominal rated speed.
Example. A 200 volts shunt motor run at 1000 rpm when the armature current is 60
A, the armature circuit resistance is 0.2 ohms. Find the required resistance to be added in
series with the armature to reduce the speed to 800 rpm when the armature current 40 A.
Solution
Ec
=
=
=
V -
(IA R A)
200 - ( 60 × 0.2)
188 volts.
Since the field current has remained constant, the counter EMF is proportional to the speed.
Therefore the counter EMF at 800 rpm is.
E'c
=
kφn
=
=
150.4 volts
188 ×
800
1000
Therefore, the voltage drop in the armature circuit is equal to the voltage drop at the resistor
plus voltage drop by the armature resistance; they are computing as follow.
VL
=
VR + V R A - E c
VR + VR A =
200 - 150.4 = 49.6 volts.
The total armature circuit resistance is obtaining by.
R + RA
49.6 volts
40 Amps.
=
32
=
1.24 Ohms.
Therefore, the additional resistance required in the armature circuit for reduced the speed
from 1000-rpm into 800-rpm compute by.
R = 1.24 – 0.2 = 1.04 Ohms.
Figure 6. Illustrated shunt motor armature resistant characteristic
The second method of speed control is by changing the fluxφ. The additional resistor
is employ to connect in series with the field winding. Normally used a variable resistor and it
is generally called “field rheostat” Therefore the field current is directly proportional to the
field rheostat.
The speed of motor is increase by reduction in flux (increase resistance
value). This method has some disadvantage. One is it can only increase speed at the motor
normally run at a light load. Another disadvantage is speed is increase without corresponding
reduction in shaft load. The motor will overload condition. However at a light loads or under
no-load condition the speed can be above nominal rating speed reach about 200%. And the
field current control as shown in figure7 bellow.
Figure 7 Illustrated DC motor field resistant characteristic
33
The third method of speed control is by changing the terminal voltage V of the
motor. This is normally the most frequent application of control, at least for shunt motor,
where the field winding is separately excited as represent by the figure 8 as bellow.
Figure 8 Illustrated DC motor terminal voltages characteristic
The voltage control method lowers the speed in a similar as the armature resistance
control method; but it does not drawback. Due to the no-load and full-load speed can be
reduced all the way down to zero. And there is not much power wasted. There are various
ways to obtain a variable DC voltage. Some modern ways are using solid-state device.
Finally if the speed control above and below nominal speed are desired, a
combination of two methods. For example, field control combined with armature voltage
control would achieve this mode of control.
Solid-state controllers
The Dc speed is function of the armature and field voltages. Control and adjustment of
armature voltage cause results in a constant-torque drive, while constant horsepower is
obtained by field-voltage control. Therefore all DC drives essentially consist of a controlled
voltage supply and a feedback network, which controls the dc voltage as a function of speed.
Recently a solid-state or computer control is used for this purpose. In this section to
introduce some of concepts without going too much detail of electronic controls to electrical
machines requires a sound knowledge of both electronic and machines.
Power switching principles
A wide range of speed control for dc motor can be achieved by controlling the field and
armature current by adding resistance in the corresponding circuits. The easily of control is at
the expense of reduced efficiency. Commercial electronic controller for dc motor is used ac
line alleviating the need for a separately dc source. This controller consists of solid-state
circuit and electronic components such as thyristors (SCR), diode, and choppers are essence.
34
They control the average current supplied to the motor. The combination of control system
and motor is usually referred to as the drive system.
Figure9. Show a simple switching circuit in principle. The switch is opened and closed
at specified intervals; in fact this be done by electronically. If voltmeter were connected
across the load resistor R the average voltage read by a dc voltmeter would be.
VR
=
t1
E
t2
volts.
Generally, the ratio t1 / t2 is called “duty cycle” of the waveform and by controlling it;
the amount of power consumed by R is controlled. For example as below the on time is 2ms,
while off time is 3ms. Therefore the average current in the circuit is.
IR
=
E
t on
×
=
R
t off + t on
2
12
×
2 + 3
2
=
2.4 Amps.
For the specific switching rate of one cycle equal to 5 ms, therefore 200 of on-off operation
of the switch per one second are required. It can be appreciated that a these switch rates or
greater, electronic switching techniques must be resorted to for this either power transistors
or SCRs are usually used to handle the required power. There are numerous varieties of
circuits that accomplish. The method of speed control by controlling the duty cycle will be
illustrated in the following.
Figure 9 Show a simple switching circuit in principle
Example. A Dc motor is supplied from 120 volts dc switch source as showing. The
duty cycle is 50% and the input power is 3 kW at 600 rpm. The armature circuit resistance is
0.05 ohms. Determine
a. The delivered shaft horsepower.
b. The new speed and output horsepower if the duty cycle is increased to 0.6
Solution,
35
a. If the duty cycle is o.5 the average motor voltage is Vm
And the average input current is Idc =
=
3000 / 120 =
0.5 x 120 = 60 volts
25 Amps.
Since the average power is the same on either side of the switch, the average motor current
is
120 x Idc
=
60 x IA
IA
=
2 x Idc
=
50 Amps.
The average counter EMF is
=
60 – (50 x 0.05) volts
=
57.5
Ec
=
Kφn
Kφ
=
57.5 / 600
=
0.0958
Ec
Hence;
=
Kφn
Therefore, the average output power is
W
=
57.5 x 50
=
2,875 watts.
=
3.85 Hp,
b. When the duty cycle is adjusted to 0.6 the average motor voltage becomes 0.6 x 120 =
72 volts. With the load remaining the same, the torque will remain constant, since T = Kφ
IA, The field current being constant. Thus IA remains the same.
Hence;
EC
=
72 – (0.05 x 50)
=
69.5 volts
And the new speed
n'
=
69.5 × 600
57 . 5
=
725 rpm
The output power
Po
=
EC IA
=
=
4.66 Hp.
69.5 x 50
=
3475 watts
As example above the controlling of dc motor speed by controlling of duty cycle is
rather than changing of armature resistance or field resistance. Control by electronic means
results in insignificant added losses their force. High system efficiencies can be maintained at
all speeds. Considering that at a duty cycle of 1.0 that is, when full voltage is applied, the
speed becomes 1200 rpm. The speed of the motor can be controlled over a wide range. Of
course additional control is available by changing the field current as well as.
When a high inductive circuit is interrupted abruptly, the inductance temps to keep
the current flowing. It can do so only when a path provided. Therefore a diode is added in
the circuit as shown, normally referred to as a freewheeling diode.
36
Diodes
A silicon diode is two terminal devices consisted of a thin layer of silicon, doped so as to
provide a P-type layer and N-type layer. A diode allows unidirectional flow of current. That is
conducts well in only one direction. The figure shows the circuit symbol and static
characteristic of diode. Current flows through the diode when the voltage at the anode (Ptype materials) is positive with respect to the cathode (N-type materials). The allow
configuration of the symbol shows the direction of conventional current flow and the diode is
then open circuited.
Thyristors (Silicon- controlled rectifiers)
A silicon-control rectifier (SCR) is a three-terminal device made from four layers of
alternating P-and N-type materials. Normally, the device blocks current flow in both
directions, using one of P-N junction’s forms by these layers. Common N1 layer performs
most of the blocking duty, so that the block capacity is symmetrical. A second layer N2 forms
the cathode and the other P1 layer forms the anode. The gate is connected to the P2 layer. A
positive pulse of current from the gate to cathode floods junction J2 (which is responsible for
forward blocking) with carries so that it looses its blocking capability and start to conduct in
the forward direction. Once start the gate signal can be removed, as this process is selfgenerating and forward blocking cannot be recovered without turning off the current flow.
Normally with gate current applied, the thyristor will perform essential the same as a diode.
Gate pulses are of a few microseconds’ duration and insignificant power compared to
that controlled. Conduction ceases when the positive voltage is removed from the node, after
control by the gate is reestablished.
AC rectification
In the section on power switch principles, it was shown that dc motors could be
energized from dc source using electronic switching methods. As the result current or voltage
waveform indicates, the ripple in the armature current will generally by high. Thus some
additional filtering in the circuit is necessary to keep the motor losses to acceptable levels,
although the armature inductance may provide some of this filtering.
To have some measure of ripple present, the form factor is used. It is measure of
departure from pure dc and is defined as the root-mean-square (rms) value divided by its
average value of the current. For half-wave rectifier current the form factor is 1.57; for fullwave it is 1.11 pure dc has a factor of 1.0 or unity.
The form factor is an important consideration with motors designed to operate form
rectified ac power supplies. The increase in motor heating for a constant out is approximately
proportional to the square of the form factor. For example a motor operating from half-wave
rectified dc will have about 2.5 times the heat rise of the same motor operating on pure dc
voltage source.
37
To accommodation the increase heating effect, a larger motor is generally required
for such applications. However, with motor ratings increase, larger amounts of power are
needed, which are available only from ac sources. Alternating current is then converted into
direct current using rectifiers, thyristors, or a combination of such devices.
Figure 10 Show a simple electronic switching circuit in principle
38
Marine Engineering Section
Part 3
Direct current shunt motor
By
Suthipong Thanasansakorn
22/3/2001
Southeast Asia Fishery development center
Training Department
Thailand
38
Marine Engineering Section
Direct Current Shunt Motor
Introduction: The principle of dc shunt motor is the current carrying conductor in the magnetic
field. The exciter field of the motor forms the magnetic field here by the exciter field of motor; the
armature winding embodies by the conductor coil.
The super positioning of the exciter field and
magnetic fields around the conductor coil produces a force (Torque) on the moving conductor loop or
armature. This torque is proportional to the magnetic field and the armature current.
M
∼
Φ IA
The conductor or conductor rotates in a motor with two poles, the rotating motion would cease at
90°, Since at this point both force directions are indeed parallel different from the other rotation
direction. For the rotation that is to proceed, one of the magnetic fields must be reversed at this
moment. The collector also called commutator or current reverse carries out in this function. It
continuously reverses the current direction in the armature coil so that the current direction remains
constant below the north and south poles of the exciter field. This produces is continues rotating
motion of the armature. The zone, where the current reverse it takes place, is described as the
neutral zone.
Fig.1 The current carrying conductor
Fig. 2 the current carrying conductor
coil in the magnetic field
coil in the magnetic field with commutator
39
Marine Engineering Section
Design
The dc machines consist of the stator and the rotor. The stator is constructed either of solid
steel or in modern motors out of laminated sheet metal. It contains windings which are located on
the pole pieces and which produce the exciter field. In a dc shunt motor, the exciter winding is
parallel to the armature winding, that is, in shunt connection. It consists of many windings of thin
wire. The exciter winding of a dc shunt wound motor can also supplies a separate voltage source. In
such case one speaks of a separately exciter motor.
There are also dc motors with permanent
magnetic excitation up to approx. 30 KW. The armature is constructed out of laminated sheet and
contains the armature winding in grooves located on the armature. The ends of the armature coils
are soldered on to the bars of the collector.
The current flow to the exciter winding and to the armature is provided by making the
corresponding connections on the motor terminal board, whereby the current flow to the armature
during rotation is carried out via the brushes located on the commutator.
Fig. 3 Cross- section of a 2-pole dc motor
Starting and starting current
A back e.m.f. is induced in the coils of the armature winding which are rotating in the exciter
field. This voltage is opposed to the applied voltage. The current in the armature is therefore
determined by the difference between the mains' voltage and the induced back e.m.f. Since the
armature is at a standstill at the moment of switch-on, the back e.m.f. is absent. As a result the
armature current would too high when switched on directly: This could lead to damage the brush
40
Marine Engineering Section
switchgear, and the commutator. Therefore a starter resistor is connected in series to the armature
in order to limit the current.
Back e.m.f. is the induce voltage (VA) is produced according the law of induction. This voltage is
opposed to the applied terminal voltage (VT) in accordance with Lens’s law. The induce voltage is
dependent on the magnetic field and the speed of the armature.
VA
∼
Φn
Commutator’s winding.
The magnetic field formed by the coils of the armature winding is called the armature cross-field
because it is located perpendicular to the exciter field. Both magnetic fields are superposition and
form the resulting total field. This effect of the cross-armature field on the exciter field is also called
the armature reaction. The affect is not constant but it depends on the load. The brushes of the
commutator are arranged in the neutral zones. These neutral zones alter their position with every
load: this causes a shift against the rotation direction in the motor and damage to the commutator,
brushes.
To avoid the commutator faults, machines larger than 1 KW are equipped with
commutating poles. The commutating poles are arranges in the magnet frame located in the neutral
zone.
The commutating pole winding consists of a few turns of thick wire and is connected to the
armature winding in series. The commutating poles are connected so that they are in opposition to
the armature cross field in the neutral zone and thus cancel it out.
Fig. 4 Shift of the magnetic field in loaded dc motor a and b and
and cancellation by means of commutating poles
41
Marine Engineering Section
In dc motor which are subject to strong load fluctuations, and fast running machines, an
additional compensation winding apart from the commutating poles may be required to cancel out
the armature reaction. It is arranged along the grooves in the pole pieces of the exciter field. It is
also in series with the armature winding. See Fig. 5 and Fig. 6
Fig. 5 Cross section of a shunt motor with main
Fig. 6 Main pole with grooves for
and commutating pole and compensate
compensate winding
winding Connection and Rotation direction
The connection designations and the current directions determine the rotation direction of a
machine. The rotation can be determined by looking at the drive side of the motor. There are two
types of rotation in machines and they are referred to as clockwise and counterclockwise.
connection of dc machine has the same designations for motors and generators.
Connection designation:
A1 - A2
=
Armature winding
B1 - B2
=
Commutating winding
E1 - E 2
=
Shunt winding
F1 - F2
=
Separately excited winding
42
The
Marine Engineering Section
a) Clockwise rotation
b) counterclockwise rotation
Fig. 7 Shunt wound motor diagram with commutating poles and starter
In this case the alphabetical numerical system for example A1 , B1 , E1 , F1 stands for the
beginning of the winding, then the symbol with the same letter but the next digit for example A2 , B2 ,
E2, F2 stands for the end of the same winding. It has been determined that for clockwise rotation,
then the current flow every winding from the beginning to the end, that is from the lower digit to the
higher digit. In the shunt wound rotor, a current flows from A1 to A2 and from E1 to E2 for clockwise
rotation
Reversal of the Rotation Direction
As an explanation the rotation direction of the armature is determined by the direction of the
exciter field and the armature cross-field. Therefore, a reversal of the rotation direction is achieved
by changing the direction of one of the two magnetic fields; or rather one of the currents that
generates them. However, should the exciter current and the armature current be simultaneously
reversed, then the previous rotation direction is determined. The exciter winding of dc shunt-wound
43
Marine Engineering Section
motor has a high inductance because it consists of many windings. As a result, high self-inductance
voltages could arise during switchover that could easily lead to damage of winding insulator. Hence,
Reverse of motors rotation on the armature winding.
In this case, the current direction in the
commutating winding and, if necessary, the compensation winding must also be reversed.
Speed and Exciter current characteristic
The speed of dc shunt motor can be decreasing or increasing beyond the normal speed by
the exciter field. In the practice this is carry out by adjusting the exciter current by using a rheostat
in the exciter circuit.
Separately excited dc shunt wound motors; the voltage source for the exciter field itself can
also be change if necessary. Since the exciter fields together with the armature current generate the
torque as mention the torque of dc shunt motor decreases when the magnetic field is weaken.
However a reduction of power is compensated for by the increasing speed of the rotor itself. The
following can be concluded for the speed of the motor:
N
=
VT - IA . RA / (Φ. C) Or
N
=
VA
IA
=
Armature current
VT
=
Terminal voltage at the armature
VA
=
Induce back e.m.f. in the armature
RA
=
Internal resistance of the armature circuit
Φ
=
Magnetic flux density of the exciter field
N
=
Revolution
C
=
Machine constant (conductor length, number of
/ (Φ. C)
Where:
winding)
44
Marine Engineering Section
Fig. 8 The Separately excited circuit with
Fig.9 Speed and exciter current characteristic
Commutating poles, starter and field regulator.
Caution:
of dc shunt wound motor.
Should the manufacturer of the dc shunt motor provide no special instructors
concerning the field regulating range, then the increased of rotor speed caused by the weakened
fields may not exceed the normal speed by more than 10%. In dc shunt wound motor operating at
no-load, a field that has been weakened too severally can lead to an extremely high rotor speed, the
motor “ races “. As a consequence the motor is destroyed by the centrifugal force that is generated.
For special applications, however, the dc shunt wound motor can be completely adapted for
speed setting ranges of 1: 3 and more under load with constant power.
A very weakened field, for
example a break in the armature circuit, also causes extreme damage in dc shunt wound motors
under load.
The load torque does indeed prevent the racing of the motor but the lack of large
portion of t he excitation results in a very small back e.m.f. in the armature. A slight armature
resistance and increases to a very high value only limit the armature current. If the motor is not
immediately switch off, the commutator, the brushes and also the winding are damaged.
45
Marine Engineering Section
Speed and armature voltage characteristic of dc shunt motor
A second possibility to alter the speed of a dc shunt wound motor is offered by the reduction
of the armature voltage VT
, the
change of terminal voltage at the armature circuit can be realized
using a rheostat connected to the armature circuit (starter) as well as using a variable dc voltage.
Fig. 10 Speed changing of a separate excited of dc shunt motor using variable transformer in the
armature circuit to below nominal speed and separate field regulator.
For this the rheostat must be suitable for continuous operation below nominal load.
fashion, the speed can be basically reduced from the nominal speed to zero.
In this
Since the excitation
remains unchanged here, the motor can produce its nominal torque, however at that point the power
reduces because of the reducing speed.
.
Naturally, as the motor speed decrease, the ventilation of the motor decreases in relation to
the square of speed, and the rotation of the armature is dependent on the construction of the motor number of grooves, laminations of the commutator. These aspects could reduce the speed setting
range and motor application possibilities in the range below the normal speed.
found by proving separate ventilation and special armature construction.
46
Assistance is here
Marine Engineering Section
Difference between the methods
Speed controlled with variable voltage source, an armature voltage may be set independently
from the load of the motor. This results in a separate speed-torque characteristic with definite noload speed and shunt characteristic for each armature voltage setting: the decrease in speed between
no-load and nominal load is only relatively slight. The dc shunt wound motor.
Where:
VA
=
VT - V R
VR
=
IA . RA
VA
=
Voltage across armature circuit.
VR
=
Voltage drop across the resistance.
IA
=
Current in the armature circuit
RA
=
Armature series resistance.
VT
=
Terminal voltage of the DC source.
The smaller load caused the lower voltage drop across series resistor and smaller speed changing
effect. The opposite is also true, the greater load and the greater voltage drop across the series
resistor, which reduces in motor speed.
Consumed power ( PI )
The consumed power PI is calculated from the measured values of VT, IA , and IE as following
formula:
=
VT (IA + IE)
PO
=
M.ω
PO
=
Delivered power in watt
M
=
Torque in NM.
ω
=
Angle velocity
=
1/ s
=
2 Π n / 60
PI
Delivered power (PO)
Where:
47
Part 4
Alternating current
generator
By
Suthipong Thanasansakorn
1/6/2000
Southeast Asia Fishery development center
Training Department
Thailand
48
GENERATOR
Introduction:
The brush-less ac generatoris the output current of an armature type ac exciter mounted
on the shaft of the main generator to excite the field system of the main generator through a
rotary rectifier, thus generating ac power.
Types of generator
(1) The types of generator are available for the kind of current:
DC generator such as Shunt, compound, series generators
AC generator as Shingle phase, three-phase, multi phase generators
(2) Generators are classified as follows depending upon the kind of prime mover, or power
turning them: Water-wheel generator, Steam turbine generator, Motor-generator, Diesel
generator (light fuel oil, heavy fuel oil), Gasoline generator, Wind-wheel generator, Atomic
power generator.
As AC sources for civil engineering construction work, small gasoline generators are used
in smaller work, and diesel generators in larger work. In general, diesel generators are use as
auxiliary or emergency power sources. (AC generators in particular). Formerly, DC generators
were used for marine power generation and in special power plants, but recently DC generators
owing to expensive costs for wiring, protective equipment, etc have replaced these. Many DC
generators are used for welding machines, and AC generators are also in use. Depending upon
on the applications, they are divided into the two generator types: with a diesel engine and with
a gasoline engine.
Principle of alternating current
Alternating current alternately varies its magnitude and direction with time.
If a conductor is placed between the north and south poles as illustrated on the next
page, and turned clockwise, electricity is generated with respect to the position to the
position of the conductor as shown in the following illustration.
49
Illustrate the direction of current alternately
changes
When both ends A and B of the conductor are on the axis X – X’, they cross the flux at
a right angle to cut the flux. When both ends are on axis Y – Y’, they do not cut the flux at all.
Thus, when ends A and B are rotated clockwise, the direction of the current alternately changes
and the magnitude also varies as illustrated.
Such electromotive force is called “Voltage” The alternating of electric power and a
machine producing alternating electromotive force is called an “AC generator”.
Types of AC generator
The types of Ac generator are revolving-field type, revolving-armature type, and
inductor type. The generator illustrated in the following figure is called revolving-armature type
because the armature revolves. To induce electromotive force, a coil is only required to cut the
flux. Therefore, when a field is revolved with a coil fixed as in Fig. (b), an electromotive force is
induced. Such a generator is called revolving-field type. The revolving-armature type is mainly
used for generators with a smaller capacity, but it is difficult to insulate the armature winding of
this type of generator, so the revolving field type is used instead.
50
Revolving speed of and frequency
If an armature or a field makes one revolution (360ð) per second, a wave form of
alternating current is drawn.50 revolutions per second called “50 cycles” (Hz)60 revolutions per
second called “60 cycles” (Hz) The number of cycles per second is called frequency
Generator frequencies
The electromotive force moves via one cycle per turn in a two-pole alternating current
generator, but via two cycles per turn in a four-pole generator. That is, electromotive force of
generators with P number of poles will go through cycles per revolution. If Ns(synchronous
speed) is the number of revolutions per minute, the frequency of the electromotive force will be
given by the following formula:
Frequency =
=
Revolution per second × Number of pole pairs
Revolutions per minute Number of poles
×
60
2
f =
Ns P
(Ns × p)
× =
60 2
120
Ns =
(120 × f)
P
Where:
F
Ns
=
=
P
=
Frequency (Hz)
Number of revolutions
at synchronous speed
Number of poles
RPM of the different frequencies and number of poles
Number of poles
2P
4P
6P
8P
10P
12P
60 Hz
3,600
1,800
1,200
900
720
600
50 Hz
3,000
1,500
1,000
750
600
500
Cycles
rpm: unit of revolutions per minute
51
Capacity and current of generator
Single – phase
Capacity (kVA) = Voltage (E) × Current (I) ×
1
1000
= VI /1000
(kW) = Voltage (E) × Current (I) × P0wer factor (cosθ ) ×
KW
=
VI cosθ /1000
Current (I)
=
Capacity (kW)
× 1000
Voltage(E)
1
1000
Three – phase system
Capacity (kVA) = 3 × Voltage (E) × Current (I) ×
1
1000
= (1.732 VI) / 1000
= 3 × Voltage (E) × Current (I) × Power factor (cosθ ) ×
(kW)
1
1000
=(1.732 VI cos θ) /1000
Generally, the power factor of a three – phase generator is taken for 0.8, then
(kW) = (kVA) × 0.8
Relationship between engine output and the generator capacity
Generator capacity ≅
Engine output
0.746 × generatoe eff × (Engine output(ps) ×
Power factor
1
)
k = KW
capacity (KVA) × Power factor ⎞⎟
= PS
⎟
0.746 × Generator eff.
⎠
⎛ Generator
≅ K ⎜⎜
⎝
Where:
K
=
Allowance factor is the mechanical output power of the
engine it is usually taken as follow:
K
=
1.2 for diesel engine and
1.4 is for gasoline engine.
Generally, the power factor (cosθ) is 1.0 for a single-phase generator and 0.8 for a three–phase
generator.
Example If a three-phase generator having an efficiency of 85% and power factor is
o.8 set to a 100 PS diesel engine, how much kVA of output can be obtained?
Where:
100 PS =
Engine output x K
52
Solution.
Generator capacity ≅
=
0.746 × generatoe eff × (Engine output(ps) ×
Power factor
1
)
k = KW
0.746 × 0.85 × (100 ×1/1.2)
0.8
= 66.05 kW
= 82.56 KVA
Consequently an output of 82.56 kVA can be obtained.
Brush-less generator
Generators with out brushes are called brush-less generators. They use a permanent
magnet for excitation, rotary rectifier, etc.
Fig. 1A the equivalent diagram of brush-less generator
The rotor is wound with the field coil of the main generator and armature coil of the
main generator and the field coil of the AC exciter. Excited current rectified through the rectifier
is supplied by the reactor in the generator output circuit to the field coil of the AC exciter. In
addition, the armature coil of the AC exciter wound on the rotor is magnetized, and another
excited current rectifier by the revolving rectifier is supplied to the field coil of the main
generator. The rotor revolves and thus the armature coil of the main generator on the stator
generates power.
Generator construction
The generator is composed mainly of a stator frame, stator core, stator winding, field
core, field winding, shaft, bearing, ac exciter, rotary rectifier, and so forth. Cooling is take by the
fan fitted to the rotor the rotor fixed on the driven side through the ventilating port on the noncoupling side, and passes over the surfaces of the field core and coil ends and through the air
53
ducts arranged in the stator core, field core. Eeffectively absorbing the heat generated by them.
The heated air is let out of the generator via the ventilating port on the coupling side.
The ac exciter over changes or is incorporated in the generator on the non-coupling
side. The rotary rectifier is also mounted on the same side to supply an exciting current to the
field winding of the generator.
Fig. 1B the overall generator construction
Stator part
The stator frame is constructed of welded mild steel plates, being so designed as to
have sufficient mechanical strength and to withstand electric shock. The stator core is provided
with silicon steel plate, which is good in magnetic characteristics. And coated with an insulating
varnish for prevention of eddy current is punched, and the punched plated elements are piled
along the inner circumference of the stator frame from one side and equipped with air duct at
each of regular pile intervals. The core thus formed is forced in fastened by stator clappers made
of steel plate.
The stator winding is formed of electric wire of insulation class “ B “ or “ F “, and placed
in the slots which have been formed in the inner periphery of the stator core and protected with
an insulating material of class B or F. The winding thus placed is fastened to the stator core by
special wedges and then subjected to sufficient varnish impregnation and drying. The outward
appearance of the stator is shown in Fig. 2
Fig. 2 The stator part
54
Rotor Part
The field core is made up from the laminations of material having a exceedingly high
coercive force for ease of voltage self-establishment. The laminated core is fitted into the shaft
(or the spider) and clamped at both ends by means of rotor clambers, and serves also to protect
the winding. The field core has such cross-sectional configuration as illustrated in fig.3
Fig.3 illustrated the laminated rotor core of a generator.
The slots in which the field coils are to be placed are formed in a number of slot groups,
which corresponds to the number of poles. The pole center exists between slot groups.
The damper bars are provided only in generators for parallel running. The damper bar
as illustrated in fig.2
The field winding is formed of electric wire of insulation class B or F and placed in the
slots furnished with a class B or F insulating material. The coils placed in the slots are fixed
firmly to the field core by special wedge and subjected to sufficient varnish impregnation and
drying.
The coil ends are bound by piano wire or special tape so as not to move out by
centrifugal force. The external appearance of the rotor is shown in Fig. 3 the shaft is made of
an excellent forge steel material in careful consideration of mechanical strength.
For the
generator to be directly coupled to a diesel engine, special attention has been paid to prevention
of shaft breakage due to torsion vibration.
Fig. 4 External appearance of the rotor
55
Bearings
The bearings are either ball or sleeve bearings. The ball bearings are lubricated with
grease. The sleeve bearing is each split in two for convenience of disassembly and reassembles.
Supported by the lower portions of end brackets or the lower portion of pedestals and cases the
bearings have sufficient strength to withstand external forces, axial load and vibration.
Lubrication system
Three lubrication systems are employed.
a) Grease lubrication
b) Oil ring lubrication
c) Force lubrication
Ventilation
In the generator of the enclosed and self-ventilation type, the fan fitted to the rotor on
the non-driven side cause cooling air to be drawn in through the port on the non-coupling side
and to pass over the surfaces of the stator core and the coil ends. The air effectively receives
heat generated by these components and then flows out through the vent port on the coupling
side.
The ventilation ports are provided with sufficient protecting for protecting of the human
body and against the entry foreign solid objects.
AC. EXCITER
The ac exciter is of the rotating armature type and located on the non-coupling side of
the generator, overhanging or mounted within the frame of the generator proper. The exciter is
composed mainly of a stator frame, field core, field winding, armature winding.
Stator
The stator frame is constructed of welded mild steel plates, being so designed as to
have satisfactory rigidity and strength.
The field core is made by placing laminations of a material having excellent magnetic
characteristics in the stator frame and is fastened by clambers. In the field core, slots in which
the field winding is to be placed are formed in number of slot groups, which corresponds to the
number of poles. The pole center exists between slot groups. A damper winding is provided for
enhancing the instantaneous characteristics and the damping effect.
56
The field winding is formed of electric wire of insulation class B or F and placed in the
slots formed along the periphery of the field core and furnished with an insulating material of the
employ classed. The winding put in the slots is subjected to sufficient varnish impregnation and
drying and fastened to the field core by wedges.
Rotary rectifier
The rotary rectifier is fitted to the generator shaft end for ease of inspection.
Two
conductive disks are attached to a rotary rectifier boss through an insulating plate.
The
conductive disk carries silicon rectifiers for each phase, a silicon carbide varistor (silistor) for
surge prevention and other parts.
These components serve for three-phase full wave
rectification of ac output of the ac exciter to supply a dc exciter current to the field winding of
the generator.
Space heater
The generator windings may absorb moisture while the generator is at rest. In order to
prevent this moisture absorption, it is necessary to keep the temperature within the resting
generator a little higher than the ambient temperature. A space heater is provided to satisfy this
requirement.
The space heater is located at the lower portion of the generator frame so that the heat
air circulates in the generator to serve effectively to protect the windings and the related parts
from moisture. Therefore, when it is intended to keep the generator at rest is sure to turn on
the power switch for the space heater. When starting the generator is sure to cut off the power
supply to the space heater. Fig. 5 shown the mounted space heater
Fig.5 Show the outward appearance of the space heater
Air Filter (special specification)
A drip-proof protected type generator, the generator is equipped with an air filter, it is
located at the cooling air suction port so as to minimize the entry of dust, oil vapor and other
foreign matter into the generator, so that the generator interior can be kept cleaner than
otherwise.
57
Air cooler (special specification)
In the generator of the totally enclosed, air cooling type, an air cooler is mounted on top
of the main body of the generator.
a) As a rule seawater is used for cooling. The maximum temperature of seawater
(the
temperature at the inlet of the air cooler) is 32 °C.
b) The cooler employs double tubing.
The inner into which seawater enters is a
seamless copper alloy tube having cooling fins brazed around the periphery.
The advantage of employing the double tubing is that, if the cooling tube damage owing
to corrosion by sea water, leaking water passes through the space between the inner and outer
tubes and flows out of the cooler so that there is no danger of water intrusion into the
generator. Also, the leaking water discharged out of the cooler flows through a drainpipe and
detected by a water leakage detector.
c) In the cooler, about 5% of the whole cooling tubing is intended for auxiliary use and
equipped with blind plugs. In case a normal cooling tube is damaged, an auxiliary tube can be
used, with the damaged normal tube clogged with blind plugs.
d) The tolerance of the cooling capacity is determined to meet the case in which the
cooling efficiency of the cooling tubes including the auxiliary tubes lowers by about 20% of the
designed overall efficiency owing to dirt attached to the cooling tubes.
e) The tube plate is made of naval brass, and the water chamber is formed of cast iron.
A rubber coating is applied on the inside surface of the water chamber, and also zinc
or mild steel is used for prevention of electrolytic corrosion.
Exciting operation
The understanding of the operation principle is very important for proper handling of the
generator and quick troubleshooting. Refer to the basic circuit arrangement show in Fig. 1A
The output of the rotary armature type ac exciter coupled to the generator rotor shaft serves to
excite the field system of the main generator through the rotary rectifier fitted to the end of the
generator shaft. The ac. exciter has two separate field windings, that is, the first field winding F1
and second controlled field winding F2
The second field winding is a control winding for stabilizing the generator voltage, and
the AVR output currents through the second field winding to excite the same winding so as to
keep fine regulation of the generator voltage.
The reactor and the current transformer are set to pass exciting currents for overcompensation as compared with the exciting current required both to maintain the desired
generator terminal voltage and to compensate for the voltage drop due to the load current
armature reaction. As a result, the automatic voltage regulator (AVR) supplies its output current
differentially to the control winding to offset part of the excitation of the first field winding,
thereby keeping the generator terminal voltage constant.
58
Silicon rectifier and protecting device
The silicon rectifier elements as shown in Fig.6
Role of rectification, the current passes
the rectifier elements only in one direction or half circle of the sine wave and no current in the
opposite direction. The silicon rectifier has a peak voltage and would be damaged by applies of
over-voltage which exceeds the peak reverse voltage. Therefore, the silicon rectifier must be
used together with a protecting device against over-voltage.
The silicon rectifier is connected to the silicon rectifier on the dc side which is used for
protecting the over-voltage when the generator is short-circuited the silicon input voltage might
become an over-voltage nearly 10 times as high as the rating. The silistor is the varsity made of
silicon carbide, when the voltage applied to the silistor decreases, its resistance increase non
linearly, and vice versa so that its serve effectively for over-voltage absorption
Ohm’s tester
Fig.6 illustrated the measurement procedure of silicon
Operation of the generator
When operating the generator, practice the following items completely in order to
ensure safety and to elongate the lifetime of the generator and machine.
Check foe any matter, iron fragments loose screws etc. should this check be neglected,
there might occur serious damage to the generator during operating conditions.
Preparation for running
a) Installed condition:
-Check if the coupling has been done at the proper position.
-Check if the clamping bolts are fully tightened.
b) Terminal section:
-Inspect the lead wires for any flaw or crack in their coatings.
-Check for sure that terminal treatment and correct terminal codes and phases.
-Check for looses bolts and nuts in the terminal section.
c)
Insulation resistance
Measure the insulation resistance between each winding of the generator and
ground with 500 V megger. It is desirable that the measure resistance is above
one MΩ.
59
-In order to avoid applying the megger voltage to the AVR is sure to disconnect
or short the terminals.
d) Air cooler
-Check the cooling tube for deposition of scales.
-Check if oil and dust is attached to the cooling tube fins.
-Check for water accumulation in the water leak sensor.
-Examine the degree of wear of the zinc or mild steel used for prevents of
electrolytic corrosion.
e) Bearing section
-In case of oil ring lubrication system makes that the lubricating oil is supplied
up to the red mark of oil gauge.
During running
When operating a new generator, do so under no load at low speed. Meanwhile, inspect
components as to temperatures, vibration, sound, etc.
a) Sleeve bearing oil ring rotation
b) Abnormal sound in the bearing section
c) Abnormal vibration
d) Temperature rises in the bearing section.
f)
Rapid temperature rises.
Monitoring of the generator panel
At the time of testing at factory, adjustments are made in relation to power factor change,
rotating speed change, load variation, etc., so that, when the rated rotating speed is
approached, the voltage will build up within ±1.0% of the rate voltage. There fore, there is no
need to control the voltage regulator.
Others
a) Temperature rise in the components (refer to the test record)
b) Check to confirm that the readings of the meters on the switchboard are always
below the values specified on the name plates especially, output voltage and current.
c) Check the bearings are properly lubricated.
Parallel running
For parallel running of generators, when both generators are matched in frequency and
phase and brought into parallel running, any frequency difference and phase difference are
completely eliminated because a synchronizing force acts on both generator as soon as they are
connected in parallel.
If there is any voltage difference, a reactive crosscurrent will flow.
Therefore, unless there is any means for controlling the wattles crosscurrent, no stable parallel
run can be affected.
For this reason, a crosscurrent compensation system employing a
differential current transformer for each generator serves for stable operation.
60
a) Independent operation. During independent operation of one generator, the secondary
winding B of the differential current transformer (DCT) is short by ACB auxiliary contacts of the
other generator, so that the condition is equivalent to the assumed one in which the secondary
winding of the cross compensating current transformer (DCT) is shorted. Therefore, during the
single running, this circuit has no influence upon the AVR.
b) Operate with respect to reactive crosscurrent during parallel running. During parallel
running, the ACB auxiliary contacts of both generators are opened.
So, if a reactive cross
current (Ic) flows between both generators, current tends to flows, corresponding to the cross
current component, through the secondary windings of DCT1 and DCT2 show in Fig. 7. The
circuit of the DCT secondary windings B is equivalent to the circuit shown in Fig. 8
If we consider that the DCTs are constant current sources, since their secondary
windings B are connects in a crossing manner, the circuit is equivalent to a circuit in which two
constant current source of the same capacity are connected in series in opposite directions as
shown in Fig. 8. In this case, therefore, the DCT secondary current by reactive cross current
does not flow through the secondary windings, and acts on the AVRs through the secondary
winding A.
That is, with respect to reactive crosscurrent, the DCT secondary windings B can be
regarded as not connected, and across the crosscurrent compensation resistor in the AVR there
occurs the same voltage as in the case of no DCT provided. The crosscurrent compensating
effect is approximately the same as with no DCT.
Fig. 7 Cross current in the parallel generators
c) Load current during parallel running.
Fig. 8 the equivalent circuits
With regard to load current, the circuit of the
DCT secondary windings B in Fig.9 is equivalent to the circuit shown in Fig. 10 and further to the
circuit shown in Fig. 10 in which two constant current sources are connected. Then the DCT
secondary currents with respect to the load current become identical and the condition is
equivalent to the assumed one in which the DCT secondary windings B are shorted.
Fig. 10 equivalent during parallel running
Fig. 9 load current during parallel running
61
Accordingly, the DCT secondary current due to the load current exerts no influence upon
the AVR.
In the case of current compensation circuit with no DCT, when the generators are for
example, under full load at a lagging power factor of 80%, the dropping characteristic is about
3.5 % as shows by curve B in Fig.11. Meanwhile, a crosscurrent compensating circuit including
DCTs shows almost the same characteristic as during independent running, as represented by
curve A in Fig. 11
Fig. 11 Load current characteristic of the generator.
As described above, the crosscurrent compensating circuit-incorporating DCTs serves,
when the loads are balanced. To discriminate between reactive crosscurrent (90° lagging
current) and lagging current by load and exhibits the drooping characteristic only with respect to
the reactive current (90° lagging current) so, the voltage regulation during parallel running is
markedly improved as compares with the case when no DCT is used. On the generator load
sharing are unbalanced, if we assume that the first generator operating under full load (lagging
power factor of 80 %). And the second generator operating under no load, are brought into
parallel running.
Fig. 12 The direction of current flow between two generators
62
Both generators have difference DCT secondary currents due to load current, so that
there cause no such operation as described before. In this case, a half of the secondary current
of CCT1 of the first generator flows through the winding A of DCT1 to the load resistance R1 in
the AVR. While the remaining half through the winding B of DCT1 to the winding B of DCT2, and
through the winding A of DCT2 to the load resistance R2 in the AVR of the secondary generator.
At the same time, the impedance of the secondary winding of DCT2, as viewed from the primary
winding side of DCT2, is very large, so that the condition is equivalent to the assumed one in
which the primary of DCT2, is opened. Thus, the circuit of DCT1, and DCT2 is equivalent to the
circuit A or B
In Fig.12 that is the first generator has its voltage lowered by the flow of lagging
current. While the voltage of the second generator is increased since the current I
DCT 2 A
flowing
through the winding a Of DCT2 via the winding B of DCT1 acts as current 180° out of phase with
the current I
DCT 1 A
flowing through the winding A of DCT1. So that there occurs the same
operations as if the second generator were supplied with a leading current.
All winding A and B of DCT1 and DCT2 are identical in the number of winding turns, and
AVR load resistance R1 and
R2
have almost identical values. So, if we assume that DCT is an
ideal current transformer, current I
DCT 1A
is substantially the same as current
I
DCT 2A
and the
voltage drop percentage of the first generator is almost the same as the voltage rise percentage
of the secondary generator. This voltage difference cause a current flow from second to first
generator, and through such operation with respect to cross current as described, both
generators are stabilized at an intermediate voltage. Accordingly, also for unbalanced loads of
lagging power factor, the voltage is approximately equal to the voltage setting of both
generators.
Requirement for parallel running
1, In order to operate a generator in parallel with another generator in operation, it is
necessary for both generators to satisfy the following conditions:
a) There be identical in frequency.
b) Both generators are equal in voltage magnitude.
d) There are corresponding in phase.
2, Conditions required of prime movers.
a) Both of prime movers have a uniform angular velocity
b) Too adequated in speed regulation.
In case the above-mentioned conditions are not satisfied, the following undesirable
phenomena and situations will take place.
a) In case there is no matching in phase, much cross current will flow, and
closing for parallel running will be impossible.
63
b) Any difference in voltage magnitude will cause a flow of cross current
between the generators, so that the generators with higher and lower voltages will have lagging
and leading currents respectively, and thus reactive power shedding will be impossible.
c) If the generator load characteristics do not include a drooping characteristic
with respect to reactive power, stable shedding of reactive power is impossible, and this may
give rise hunting or step-out.
d) In case the angular speed is not uniform, load shedding can not be
performed. The prime mover is provided with an adequately sized flywheel for angular velocity
uniformity.
e) Without governor speed characteristics do not include stability or quick
response, just as in the case of (E) load shedding is unstable or hunting or step-out may result.
Synchronizing
1, Carry out governor adjustment on the prime mover side until the frequency of the
generator to be connected in parallel with the forerunning generator is approached.
2, While watching the synchronizing lamps, or synchro scope, finely adjust the governor,
and close the circuit breaker at the point of synchronism.
a) As to the synchronizing lamps, the time when one of the lamps has gone off
and the two others have become brightest is the phase coincidence point.
b) As to the synchro scope, the time when the needle has come to the top center is the phase
coincidence point.
c) It is a proper method to start the closing operation with the closing time of
the circuit breaker taken into account, and to close the circuit breaker just at the point of
synchronism
(The needle of the synchro scope has come to the top center).
Load shedding.
After completion of synchronizing, while paying attention to frequency, operate to
governor on the forerunning generator side in the speed lowering direction and operate the
governor on the after-running generator side in the speed raises direction. By so doing make a
gradual load shift for load shedding.
Release from parallel operation.
When disconnecting one of generators running in parallel from the bus, take the
following steps:
1, operate the governor of the driver for the generator to be release in speed
lowering direction and operate the governor of the driver for the generator on the bus side in
the speed raising direction for load shifting.
64
2, When the load on the generator to be release become zero, trip the circuit
breaker for the same generator.
Temperature rises.
The monitoring of the temperature of devices is an effective means for finding faults.
Check the temperature rise in components by referring to the table allowable temperature
rise limits as:
Measuring
Mechanical parts
method
Insulation
Insulation
class B
class F
°C
°C
Armature winding
By thermometer
60
75
( stator winding )
By resistance
70
90
Field winding of cylindrical rotor
By resistance
80
100
By thermometer
70
90
Core or other mechanical parts
adjacent to insulation winding
Note:
1. Where the standard ambient temperature is 45 °C or 40 °C instead of 50 °C, the
difference between them should be added to the temperature rise limits specified in the table
2.
When LR, GL and NV standards are applied, the standard ambient temperatures
shown in the table are for the standard ambient temperature of 45°C.
3. For the generator with an air cooler, allowable temperature rise limits should be
obtained by adding (50 °C
temperature of cooling seawater of the limits specified in the given
table. Generally, the temperature of cooling seawater is 30 °C, so that 20°C should be added to
the values in the table.
Insulation resistance
Insulation resistance is an important index for judging the performance of an electric
machine to be good or not.
Insulation material changes with time under influence of, independently or in
combination, heat, moisture, vibration, mechanical damage, dust, chemical change by acid or
alkali, salt content, air, and so forth.
a) Insulation resistance provides excellent data for judging insulating conditions
So, measure it periodically during shunt-down, and record also the ambient temperature, relative
humidity, etc.
b) For insulation resistance measurement, use 500 V. DC mega ohms.
c) Precaution for insulation resistance measurement:
65
Component (AVR etc.) which include semiconductors such as transistors and thyristors must be
subjects to circuit disconnection or shorting for prevention of application of abnormal voltage,
prior to insulation resistance measurement.
d) Judgment of measured resistance.
The resistance measured by 500 V
megger is acceptable if it is above the value calculated by the following formula.
Insulation resistance
=
3 x Measure voltage of main line (volts)
_________________________________________________
Rated output (KW or KVA) +1000
The best way for judging the insulation good or not is to compare the newly measured
resistance with the value obtained by previous measurement. The variation from the preceding
measurement result is significant so that insulation resistance measurement should always be
continually carried out with recording.
e) The minimum allowable insulation resistance varies depending upon the
machine model, size, etc. and is therefore difficult to determine at a particular value. However,
as a simple measure for routine maintenance, one MΩ is considered to be a minimum safe
value.
Inspection of exciting equipment and AVR
A. For grounding and conductor connections check once about every six months. When
maintenance work is to be done, make sure that no voltage is applied to the exciting system
before starting the work.
When subjecting the exciting system to insulation resistance
measurement or dielectric strength test, do so after shorting the terminals of the silicon rectifier
and AVR or disconnecting them.
B. How to check the silicon rectifier
a) The silicon rectifier may become faulty if an over-voltage is applied or if an
over-current flows. When the silicon rectifier becomes faulty, almost no generator voltage will
be produced.
The silicon rectifier is composed of six elements which have such outward
appearance as shown in Fig. 13
b) For simply judging the silicon rectifier to be good or not, measure resistance using a
Fig. 13
Heavy
tester by the procedure illustrated
in Fig.
14 duty silicon rectifier type
(1) Disconnect all wires connected to the silicon rectifier stack.
(2) Using the resistance measuring range of the tester, measure the forward and reverse
resistance of each rectifier element.
66
The forward resistance is acceptable if it is below 10Ω, while the reverse
resistance is acceptable if above 100 kΩ.
Any rectifier element that is conductive in both
forward and reverse directions is defective.
If a faulty element has been found by the above measurement, replace it with a
good element.
Ohm’s tester
Fig.14 illustrated the measurement procedure of silicon rectifier.
Rotary rectifier
The rotary rectifier exhibits stable performance with excellent mechanical and electric
characteristics. So, correct operation and maintenance will allow the rotary rectifier to be used
without hindrance for a long time.
a) The rotary rectifier is incorporated in a three-phase full-wave rectifying circuit as
shown in Fig.15 As shown a varistor is connected for sure protection of the silicon rectifiers.
Fig.15 the equivalent rotary silicon rectifier with an over-voltage protector
b) It is important to keep the rotary rectifier and the surroundings clean just like the
stator and rotor windings of the main generator and exciter. Carry out cleaning periodically just
as for the winding. Also at the same time as cleaning check for any grounded terminal or lead
wire and any loose bolt or nut around the rotary rectifier.
67
c) Implement list of trouble shooting guide line.
symptom
Abnormal voltage
decrease
Cause
1. Short-circuited elements of the silicon
Remedy
Replace the good elements.
rectifier of the exciting system
2. Reactor disconnection.
3. Current transformer secondary short
circuit.
4. Short-circuited thyristor in the AVR.
Replace
thyristors
AVR.Replace
or
the
the
printed
circuit board
5. Printed circuit board out of order with
resultant control failure.
6. Abnormal rise in the field temperature
of the main generator.
Abnormal voltage
rise
1. Print circuit board out of order, or
thyristor gate circuit disconnection.
Decrease the load on the
generator.
Replace a circuit board or
The AVR.
2. Detecting transformer disconnected.
3. Disconnection or shorting of the circuit
to the control winding.
4. Broken wire or faulty contact in the
Replace the rheostats.
rheostat for voltage regulation
5. Abnormal in rotating speed.
6. Disconnection or short circuit of the
power transformer secondary winding.
Make a regulation or replace
the rheostat.
1. Faulty contact in the rheostat for
voltage regulation.
2. AVR. Damping circuit out of order.
Incessant voltage
Replace The AVR.
change or hunting
68
Part 5
Optimization of electrical power
By
Suthipong Thanasansakorn
1/6/2000
Southeast Asia Fishery development center
Training Department
Thailand
69
Key to electrical energy reduction
By understanding power basic is one can reduce the electrical energy. The total requirement
is comprised of two components, as in the power triangle as illustrate in fig.1
This diagram had shown the resistive portion or kilowatt (KW). It is ninety degree out of
phase with the reactive portion (KVAR). The reactive power is necessary to build up the magnetic
field of the inductive device such as motors and coils. But otherwise it is non-usable. The resistive
portion is also known as the active power, which is directly converted to useful work.
The hypotenuse of the power triangle is referred to as the kilovolt ampere or apparent power
(KVA). The angle between Kw and KVA is the power factor angle.
KW
=
KVA cos.θ
KVA
=
KW / cos.θ
KVAR
=
KVA sine θ
Relationship between power, voltage and current
For the balance three phase load.
Power in watt
=
3VL IL cos θ
=
3VL IL cos
1000
Kilowatt
For single phase load
Power in watt
=
VL IL cos.θ
=
VL IL cos θ
1000
70
Kilowatt
Motor loads.
Each electrical load in the system has own an inherent power factor. The motor loads usually
specified by horsepower such as 2 HP., 3 HP.. These may be convert to apparent power(KVA) or
active power(KW) by use of:
KVA
=
HP × 0.746
effM × PF
KW
=
HP × 0.746
effM
effM
=
Motor efficiency.
PF
=
Motor power factor.
HP
=
Motor horsepower
And
Kilowatt.
Where:
Example
A plant of load is comprise of:
1. 40 kW of lighting operated at unity power factor.
2. 10 sets of 20 HP motors running at full load.
3. 5 sets of 40 HP motors running at full load.
Where:
20 HP. Motors have form factor
And
effM
=
0.862
Power factor (cos. θ)
=
0.835
effM
=
90.9
Power factor (cos. θ)
=
29°
40 HP. Motors
What is the total power factor resulting from these loads?
Solution.
1.
Find the consumption of 20 HP motors, 10 sets.
KWTotal
=
( HP ×eff0.746 )× 10
M
=
( 200×.8620.746 )× 10
71
2.
=
173.08
KW
θ
=
33°
tan θ
=
0.65
KVAR
=
KW tan θ
=
173.08 x 0.65
=
112.5
KVAR
The consumption of 40 horsepower, 5 sets.
( HP ×eff0.746 )× 5
=
KWTotal
M
=
( 40 ×0.900.746 )× 5
=
166 KW
tan θ
=
0.55
KVAR
=
KW tan θ
=
166 x 0.55
=
91 KVAR.
=
40+ 173 + 166 KW
=
379 KW
=
112.5 +91 KVAR
=
203.5 KVAR
Sum of active power(KW)
Sum of reactive power
Active power (Kw)
40
123
166
θ
112.5
Reactive power (KVAR)
Apparent power (KVA)
91
And the hypotenuse of power triangle as known as apparent power (KVA)
72
How to improve the plant power factor
1. Reducing an inefficient loading. The motors must running to full load condition its have a
significantly better in power factor.
2.Provide the external capacitor in parallel with motors or at the distribution equipments.
3. Used of efficiency motors.
4. Using of synchronous motor instead of induction motors.
How Capacitors improve power factor
and recovered electrical power.
Referent
to the common role of generator it is produced an electric power with the
relationship between magnetic field and length of conductor and the speed of rotation. Voltage which
produce has supplied to load with comprised of different power consumption as know as active
power, reactive power are the most commonly.
Recently, the electric applications are composed of active and reactive power as known as
resistive and inductive loads. These loads have effected to the electric plant system cause to voltage
drop. The plant has to control and kept system stable by increase or charge with the reactive power
as know in the practical as increase excited current to the generator. Also speed of the prime mover
has drop due to heavy load it must be increase to certain level for maintain both of voltage and
frequency.
Capacitor,
capacitor is one of electric loads.
It is different from others compare with
resistive and inductive loads. Resistive load consume only an active power, for inductive load it
consume both of active and reactive power. But the capacitive load consumes a few of active power
and gives better in capacitive power that is opposite with the inductive load as illustrated in the figure
below.
Capacitive power
Active power
Reactive power
73
As the figure illustrated above mention, sum of portion is become to leading or lagging power
factor is depnding on the value of capacitive and reactive power. In case of capacitive loads is
become more bigger than reactive load. It will cause the total power factor of the system become
leading. But usualy most of consumptions are reactive load that is lagging in power factor.
What
is happen in case of lagging power factor. The hypotenuse of the power triangle
portion as known as an apparent power (KVA) that becomes large. It is necessary to reduce the
amount of load consumption by reduce the reactive power. But it is impossible to control the system
with out reactive load. Then the system has to provided a compensate power as known as capacitive
power to adjust or reduce the reactive consumption it will cause in better power angle. In practical it
is usually maintain to coverage power factor between 0.8 to 0.85
For industry that is consumes a lot of electric power so that it is necessary to reduce the
reactive power and need in better power factor control. Because the electrical authority has installed
with the special kilowatt meter to record the demand of all electric consumption such as kWh, KVA,
KVAR and power factor.
Sum of power portion is equivalent as show as illustrated below.
Active power (P)
θ2
Apparent power after
power factor adjusted
(S2)
Reactive power
before adjusted
(Q1)
θ1
Reactive power
after adjusted (Q2)
Capacitive power additional (QC)
Apparent power before power
factor adjustment by external
capacitor.
(S1)
Example
The 2 kilowatts load has connected to220 volts 50 Hz source operated with 0.70
power factor. The system need to corrected the power factor being to 0.90 find:
1. Reactive power before power factor adjustment.
2. Reactive power after power factor adjustment.
3. Capacitive power and capacitance to correct the power factor
Solution
=
cos θ1
=
0.70
cos-1
=
45.6°
PF2
=
cosθ2
PF1
74
cos
-1
=
0.90
=
26°
From the power triangle, the reactive power before adjust can be compute by
(Q1)
=
P tanθ1
=
2 Kw x tan 45.6°
=
2040 VAR
=
2.04 KVAR
=
P tanθ2
=
2 Kw x tan 26°
=
975 VAR
=
0.975 KVAR
=
Q1 – Q2
=
2.04 – 0.975 KVAR
=
1.065 KVAR
Reactive power after
power factor adjust (Q2)
Capacitive power for
power factor adjustment (Qc)
The capacitive value is compute as the following formula
Cap.
Capacitor
Apparent power (S1)
Apparent power (S2)
=
Qc /(V2 x ω)
=
Qc /(V2 x 2 x 3.14 x 50Hz)
=
1065 / (2202 x 2x3.14 x 50)
=
70 x 10-6
=
70 μF
=
Kw / PF1
=
2.0 Kw / 0.7
=
2.857 KVA
=
Kw / PF2
=
2.0 Kw / 0.9
=
2.222 KVA
75
Apparent power is reduce
=
2.857 – 2.222 KVA
=
0.635
KVA
Lets consider the current consumption compare with the system before and after correction
of power factor by the following power relation.
Kw
=
V x I x PF.
I1
=
Kw / (V x PF1)
=
2.0 /(220 x 0.7)
=
12.98 Amp.
=
Kw / (V x PF2)
=
2.0 / (220 x 0.9)
=
10.10 Amp.
=
12.98 – 10.10 Amp.
=
2.88
I2
Then, the system
current is reduced
Amp.
Conclusion
The system is able to reduced reactive power consumption from 2.04 KVAR into 0.975 KVAR.
and recovered the electric power consumption 0.635 KVA by installed a compensated capacitor 1.065
KVAR 70 Micro-farad. In which no capacitor on the system, The improvement of power factor is by
increasing the exciter current is used, but this method the plant system has to charged the extra
reactive power as calculated about 1.065 KVAR into generator instead of capacitive power from the
capacitors.
76
Improvement of power factor on diesel generator
The principal is to reduce engine consumption, one of others method is by load demand
control. Since the demand power of electric application as illustrate by power triangle and its factors.
The compensated of electrical consume is called “power factor correction” power capacitors is
introduced.
Most of generators in the fishing industry like on board are installed with direct coupling with
diesel engine. This system has called “diesel generators” or auxiliary engine. Diesel engine must
adequate in power to drive the generator with constant in speed and reliability in mechanical power.
Fishing vessels are used diesel engine to driving the generator. When the generator capacity
is determined by the total of load requirement, but the generator driving horsepower is varies
according to the electric demand power at that moment. The engine horsepower requirement is
determine by the simply method as following formula.
Generator capacity (KVA) x Power factor x gen. Eff.
=
Engine horsepower (BHP)
Engine horsepower loss (BHP)=
0.746 x engine eff.
Generator capacity (KVA) x gen.eff. (θ2 - θ1)
0.746 x engine efficiency
Where:
θ1
=
Power factor before correction
θ2
=
Power factor after correction
0.746watt
=
1 Horsepower (SI unit)
Example a fishing vessel has 40 kVA diesel generator which supplies to inductive loads its cause the
electrical power factor lagging 0.7 if adjust the power factor become 0.9. What is happened to the
system? The generator efficiency is 100%.
Solution on generator effect
Generator capacity
=
40
KVA
With 0.7 power factor before correction
Kw1
77
=
40 KVA x 0.7 x 1.0
=
28
Kilowatt
With 0.9 power factor after correction
Kw2
=
40 KVA x 0.9 x 1.0
=
36 Kilowatt
By above given formula. It is seem that after power factor correction from 0.7 into 0.9 the
generator has recovered the electrical power become larger 8 kilowatts and there is capability to
supplies electrical power up to 36 kilowatt
Solution for the engine capacity
Engine horsepower (HP)
=
40 KVA x 0.9 x 1.0/ (0.746 x 0.8)
=
60.32 HP
In case of low efficient power factor
Power loss by reactive power
∴ The diesel engine power has losses
=
(θ2 - θ1) x KVA / (0.746 x geneff )
=
(0.9 – 0.7) x 40 KVA / (0.746 x 0.8)
=
(0.2 x 40) / (0.746 x 0.8)
=
13.40 HP
7. How to reduce the fuel consumption?
As mention above is one process of electrical utilize by power factor correction, which apply
to diesel generator. The capacitors are used to supplies or compensate the capacitive power instead
of increase the exciter current or increase the engine speed. This method without increase in fuel
injection, it’s the ways to minimize of fuel consumption and improve the system efficiency.
In case of increase the exciter current it is surly the system has better in voltage and power
factor, but there is cause to losses reactive power on the system. Because exciter currents is a parts
of generated voltages of the generator its self. And cause to the revolution of driven engine is
reduced due to induction forces of magnetic flux density which increase. These magnetic fluxes are
against the revolving of the engine.
It is automatically when the engine speed is slow down; the voltage and frequency are
become decreasing. And it is necessary to increase the engine speed by automatic or man control to
make the system in proper service condition by increasing the fuel injection into the engine to
increase the engine power and speed. From an example as above the engine power has losses about
13.40 horsepower.
8. How to identify the engine has how much save?
In this case has to consider together with the engine performance curved on the specific fuel
consumption and horsepower tested. Recently the most save fuel consumption is about 160 g/hp/hr.
The amount of fuel consumption saves will be the amount of specific consumption in g/hp/hr multiply
with recovered horsepower and operation hours.
78
From an example the recovered horsepower is 13.40 HP, in case of 24 hr operated.
Fuel consumption save =
160 g/hp/hr x 13.40 x 24
=
51.45 kg.
=
51.45 kg. / Specific weight
=
51.45 / 0.85
=
60.52
.... Litters
Litters/day
9. The diesel engine horsepower on generator operation.
Since the principal of generator is need in strong enough prime movers for driving itself, which
has constant in speed regulation, and power given. The prime mover must be better in speed
regulation on the various loads condition such as in case of light or heavy load operation, the engine
must not over speed or speed dropped operation. That is to maintain and keep stable in speed of the
synchronous (N) operation
N=
Where:
120F
P
F
=
Frequency (Hz)
P
=
Magnet pole of generator
So, that the electrical automatic constant speed governor, or constant speed governor is used
in this process to control the speed of the engine.
79
Guide line on engine power
Brake horsepower. This power is developed from the cylinder movements through
crankshaft, which is reduced by friction losses of the movements as like cylinder, piston, camshaft,
bearing and the power required for cooling equipment.
The net power is called brake horsepower this power is used to work. Such as driving the
generator, propeller, hydraulic pump, water pump etc.,
This power is measured by brake horsepower machine called dynamometer by means of
torque (Kg.m) along with the measure of engine speed (rpm) and calculates
BHP =
2 × 3 . 14 TN
75 × 60
=
TN
716.2
Shaft horsepower. This power measured at the shaft end. It is means that shaft
horsepower is depend upon the transmission coefficient (ηT) In case of direct coupling like diesel
generator, brake horsepower is equal to shaft horsepower. Shaft horsepower can be improved or
reduced by means of clutch and transmission devices.
In which has no dynamometer available the generator is apply instead of dynamometer. By
produced an output voltages currents and supply to electric loads or dummy load in order to
measured the electric consumption in kilowatts and convert to shaft horsepower as giving by the
following formula.
SHP
=
BHP x ηT
In case of direct coupling, transmission efficiency (ηT) equal to 100%
ηT
=
1
BHP
=
SHP
=
100%
Torque along the shaft (Tsh)
=
75 × 60 × SHP
2 × 3 . 14 × Eng eff × N
Engine power of the generator is different from main engine on the vessels, due to constant
in speed operation, which there is change in load condition. That is horsepower is the relationship
between torque on the shaft and the operation speed (shaft speed). In case of generator, the
80
demand powers of load consumption cause to produced the friction or torque to against the
movements of the engine as illustrated in the following formula.
BHP = SHP
=
KVA x PF x 1.34
In practical the efficiency (η) has proportional with the output of the machine. So that,
efficiency is important matter on the selection of the machine to meet the utilized of power with long
life services. In practical the mechanical efficiency of diesel engine is stronger than gasoline engine its
value is between 70-80% and 60-70%in order. That is the engine has capability about 70-80% or
60-70% of the maximum power on continuously operating situation.
Therefore, the engine power must bigger than loads power about 120% or 140 % according
to the engine selection such as diesel or gasoline engine.
Torque along the shaft (Tsh) during generator operation.
(Tsh)
=
=
∴BHP at n revolution
=
=
75 × 60 × SHP
2 × 3 . 14 × N
75 × 60 × (KVA × PF × 1.34)
2 × 3.14 × N
TN
716.2
75 × 60 × (KVA × PF × 1.34) × N
2 × 3.14 × Eng eff × N
716.2
Example
Compute the engine power to drive the generator on the capacity of 40 KVA, power factor
0.9, 1500 rpm., generator efficiency is 100% and engine efficiency is 80%.
Solution
Tsh)
=
=
=
BHP
=
75 × 60 × SHP
2 × 3 . 14 × N
75 × 60 × (KVA × PF × 1.34)
2 × 3.14 × N
23.044
Kg.m
75 × 60 × (KVA × PF × 1.34) × N
2 × 3.14 × Eng eff × N
716.2
81
=
=
∴
75 × 60 × ( 40 × 0.9 × 1.34) × 1500
2 × 3.14 × 0.8 × 1500
716.2
60.33
hp
Need the engine has constant speed at 1500 rpm on continuously power 60.33
Horsepower to drive the generator of 40 kilowatts with 0.9 power factor. For more effective of the
utilized of the engine the maximum torque should be cover between 1450-1550 rpm. of the engine
speed.
Lighting optimization
To reduced the electric lighting power of the fishing light method. By understanding the basic
of lighting. There is several ways to improve the efficiency of the lighting system will become utilized.
Foot-candle
Foot-candle is measure illuminations of one standard candlepower on the luminous intensity
surrounding a candle with one foot away. 1 foot-candle = 10.764 Lumen/square meter (Lux)
r =1’
10.764 lm/
m2 (lux)
Standard candle
Luminous flux (F)
Luminous flux is the quantity of light unit as mention in lumen that is throughout surrounding
the source (lamp or candle) in one second.
Since 1 foot-candle is the luminous flux quantity surrounding on the spherical area of 1-foot
radius equal to 4Π lumen/second.
F
=
4Π x 1 Lumen.
=
12.56
82
Lumen.
Luminous intensity (I)
Luminous intensity is the candlepower (CP) throughout surround of the lamp such as 50, 100,
200,400 candlepower.
Efficiency of lamp (Eff.)
An efficiency of lamp is the proportional between luminous flux and electrical demand power
in watt. That is given in lumen per watt.
Efficiency of lamp
=
luminous flux / watt
(Lm/watt)
Example
How much the luminous flux and efficiency of 100 candlepower with 20 watts lamp?
Solution
Where: I illustrated the capacity of candlepower.
F
=
4Π I
=
4 x 3.14 x 100
=
1256
=
1256 / 20
=
62.8
Lumen
And
Eff.
Lumen/watt.
Illumination (E)
Illumination is the quantity light through to the certain area it is usually symbol by “E” and it
is taken unit in lumen/square meter, lumen/ square feet and lumen/square meter. Lumen/meter2 is
usually called Lux.
The relationship of illumination of above mention is illustrated as the equation as following.
E
=
F/A
E
=
Illumination (lumen/meter2 or lux)
F
=
Luminous flux (Lumen)
A
=
An area of the light throughout.
Where:
There are two common lighting methods are used.
1.Lumen method
83
2.Point by point method
The lumen method is used the principal of an equal foot-candle level throughout the task
which must be maintain in the surrounding area. This method is used frequently by the lighting
designer it is very simplest method. However this method waste the electrical energy
Point by point method this method has calculated the lighting requirement throughout the
task and not equal in foot-candle lighting in the surrounding area. This method utilizes in electrical
power consumption.
Lumen method
N=
F1 × A
Lm × L1 × L 2 × Cu
N=
F1 × A
Lm × MF × Cu
OR
Where:
N
=
The number of lamp require.
F
=
The requirement of luminous flux level at the task. (lumen)
A
=
Area of lighting area in square meter
Lm
=
The lumen output per lamp. A lumen is a measure of lamp
intensity unit. Its value usually found in the manufacturers
catalogue.
M.F.
Cu
=
Maintenance factor.
=
L2 / L1
=
The coefficient of utilization. It represent the ratio of the
output (lumen) generated by the lamp reaching to the
working plane. The coefficient of utilization makes allowance
for light absorbed or reflected by walls, ceiling and the
fixture itself. Its value is compute by following formula.
L1
=
The lamp depreciation factor. It takes into account that the
lamp lumen depreciation with time. Its value is found in
catalogue.
L2
=
The luminaries (fixture) dirt depreciation factor. It take into
Account the effect of dirt on the luminaries and varies with
type of luminaries of the atmosphere it is operated.
84
Point by point method
There are three commonly used lighting formulas associated with this method. The
incandescent lamp or mercury vapor laminar is used as a point source. These equations given bellow
omit inter reflections, The total measure illumination will be greater than calculated value. Inter
reflection can be taken into account by referent to a lighting hand book or electrical engineering
handbook.
F2 =
0.35 × CP
D
F3 =
CP × cos θ
D2
F3 =
CP × sin θ
D2
D
Where:
F2
=
The lamination produced at the point on plane area directly
under the source.
F3
=
The illumination on horizontal plan area
F4
=
The illumination on the vertical plan area
CP
=
The candle power or luminous flux of the source in the
particular direction its value is found in the manufacturer
catalogue.
D
=
The distance in feet to the point of the lamination
θ
=
The angle between D and the direct component
85
Beam lumen method
The objective of floodlighting is continuously the luminous instead of sun light after sunset to
the abstract the outdoor and indoor objects such as complex center, garden, sport complex. Also the
beam lumen method has used in the special area that need the luminous flux without glare effected
as in case of navigation lamp on wheelhouse, personal lighting on the vehicles.
Beam lumen is the luminous flux floodlighting from the lamp and the relationship of beam
efficiency.
Total beam lumen
=
luminous flux of the lamp x beam efficiency
Co-efficient of beam utilization (CBU) is the proportional between the luminous flux
throughout to the object and the total beam lumen that is illustrated as equation as follow:
CBU
=
La / TBL
CBU
=
Co-efficient of beam utilization
La
=
Luminous flux on the specific surface
TBL
=
Total beam lumen
Where:
So that the luminous flux on the specific surface are able to adjust between 60-100% of the
total beam lumen. Since it is depend upon the surface of the object. A large surface of the object will
cause high percentage dropped of luminous flux on the object. And then beam lumen adjustment is
very important, in case of over beam angle it is cause to low in co-efficient of beam utilization.
a
b
Figure as illustrates above “b” object has received a luminous flux better than an object “a” due to
large in surface.
Lamp requirement of beam lumen method is compute by following formula.
N
=
A x F / ( BL x CBU x MF )
86
Where:
N
=
Number of lamp require
A
=
Area of lighting surface (m2)
F
=
Luminous flux on the task (Lumen)
BL
=
Beam lumen (Lumen)
CBU
=
Co-efficient of utilization factor
MF
=
Maintenance factor
Lighting control system
The best way to reduce lighting loads is to assure that lights are turned off when not in use
or to dim light during different periods. The lighting controls are usually required for the following.
1. Turned light on at the start of office hours.
2. Turned light off at the close of office hours.
3. Turned light off base on external sunlight condition.
4. Turned light on in specified area of duty.
5. Turned light on at the burglar alarm is activated.
The above function above are accomplished by time clock switch, photo electric relays, solid state
dimmer, A low voltage switch control is used control the light and recently solid state dimmer are
available for high inert discharge lamp as well as incandescent lighting system.
Analysis of the lighting method for power saves.
The two method for compute the lighting requirements to meet the objective of electric
power saves are concludes following.
1. Laminations (fixture) should be chose base on a high efficiency of utilization for the
application.
2. Point by point method should be used at the working area with the task lighting levels.
3. Efficient lamp depreciation with high lumen per wattage consumption should be used.
4. Installation of the laminations should be based on an environmental, in the dirty
environmental, lamps should prevent dust and moisture build-up.
5. A good surface reflector should be equipping with the lamp for improve depreciation
efficiency.
6. To reduce the electrical power consumption beam lumen method should be added
together with the others method, such as in the special point or working places that is
need light intensity, beam lumen to be used.
87
Part 6
ALTERNATING CURRENT
MOTOR CONTROL
By
Suthipong Thanasarnsakorn
2002
Southeast Asian Fisheries development
Center
Training department
Thailand
88
Alternating Current Motor Control
If an AC motor is started on full voltage, it will draw from two to six time of its normal
running current. Because the motor is constructed to withstand the shock of starting, no harm
will be causes; it is generally desirable to take some measure to reduce the starting current;
For the small motor, or where the load can stand the shock of starting and no
objectionable line disturbance are created, a hand operated or an automatic starting switch
can be used for control of the motor. This type of switch connects the motor directly across
the line is called an across the line starter or full voltage starter.
In the case of large motor, where the starting torque must develop gradually, or where the
high initial current will affect the line voltage, it is necessary to insert in the line some device
which will reduce the starting current. This device may be a resistance unit or
autotransformer. Controllers which use this method of starting a motor is called reduce
voltage starters. Controllers are also to protect the motor from overheat and overloading, to
provide speed control, to provide for reversing the motor, and to provide under voltage
protection.
The following popular types of conductors will be described: push-button switch
starter for small motors, magnetic across the line Star, Wye-delta starters, drum starters,
part-winding starters, two speed controllers, plugging and controllers.
Fig. 1 Pushbutton switch starter connected to a single-phase motor.
89
Push-button Switch Starter for Fractional Horsepower Motors
This is a simple type of switch connects the motor directly to the line. Two pushbutton are located on the switch, one for starting and the other for stopping the motors.
Pressing the start button case the contacts inside the switch to make and connect the motor
across the line. Pressing the stop button cause the contacts to break apart and open the
circuit to the motor. This type is show in Fig. 1
The usual type of push-button switch starter is equipped with a thermal overload
device connect in series with the line. It opens the circuit to the motor an overload current
persists for a short period of time.
Most of switch starters can be used for single, two or three-phase motors. Fig. 1
shows a diagram of a push-button starter connected to a single-phase motor and Fig. 2
shows such a starter connected to three-phase motors. In Fig. 1- 1, when the start button is
pressed it close the contacts of L1 and L2 and connect the motor across the line. If an
overload occurs, the thermal relay will trip the releasing mechanism and cause the contacts
to open, thereby stopping the motors. To reset the tripping mechanism, it is usually to press
the stop button. If the motor is running normally and it is necessary to stop it, the contacts
are released by pressing the stop button. Fig. 3 is an illustration of a manual starter.
Fig. 2 Pushbutton switch starter connected to a three-phase motor.
90
Fig.3 Type of a protection switch or manual starter
Magnetic Full-wave Starter
A starter, which connects a motor directly across the line, is called a full- wave
starter. If this starter is operated magnetically, it is called a magnetic full-wave starter. A
magnetic starter designed to operate a three-phase motor is shown in Fig. 4 Some of the
wiring symbols in this and other diagrams are shown in Fig. 5 Fig.4 has three normally open
main contacts which closed connect the motor directly to the line. It also has a magnetic
holding coil, which closes the main contacts upon being energized, and also closes a normally
open auxiliary and departure the contact of the normally closed auxiliary. The normally open
auxiliary used to maintaining contact to the line through the holding coil. The main and
auxiliary contacts are generally joined by insulating connecting bar so that all contacts will
close or open when the holding coil becomes energized. It is obvious that just sending a
small current through the coil can operate any size of magnetic switch. Starters are often
equipping with single or dual-voltage coil for operating on either high or low voltage. Coil is
made in two section in-series for high voltage, parallel for low voltage.
It should be note that two overload relays are shown in Fig.4. Most three-phase
starters are made with provision for two overload elements as standard equipment, as
illustrated.
91
Fig.4 A magnetic across the line starter connected to a three-phase motor with
two overload elements
Fig. 5 wiring diagram symbols
92
Fig. 6 wiring diagram symbols
3-phase supply
Fig. 7 A magnetic starter for a
three-phase motor
93
An advantage of the magnetic starter over a manual starter is that merely pressing a
pushbutton, which may be located some distance from both the starter and the motor, may
operate it. This tends to convenience and safety in starting and stopping a motor, especially if
it is high voltage or if it must be controlled from one or more remote points.
Overload Relays.
Nearly all-magnetic starters are equipped with an overload device to protect the
motor from excessive current. Two types of overload relays are used on magnetic starters,
and these are either magnetic or thermal in operation. The thermal overload relay may be
either the bimetallic or solder-pot type.
A thermal relay is illustrated in Fig. 8 (a) and (b). This bimetallic type of relay
consists of a small heater coil or strip which is connected in series with the line and which
generates heat by viture of the current flowing through it; the amount of the heat generated
deepens on the current flow in the line. Mounted adjacent to or directly inside, the coil is a
strip formed of two metals. This is fixed at one end. The others end being free to move. The
two metals have different degree of expansion, and the strip will bend when heated. The free
end normally keeps the contacts of the control circuit opened. When an overload occurs, the
heater heats the thermostatic so that it will bend separate the two contacts, thereby opening
the holding-coil circuit and stopping the motor. The bimetallic type of overload relay is usually
designed with a feature which permits automatic resetting, although it is also design for
manual resetting. Some overload relays are ambient-compensated to provide maximum
protection where the temperature surrounding the relay differs from the temperature
surrounding the motor.
Fig. 8a bimetallic overload relay
Fig. 8b. Bimetallic overload relay
94
Pushbutton Stations
Magnetic starters are controlled by means of pushbutton stations. The most common
station has start and stop, as shown in fig. 9 when the start button is pressed, two normally
open contacts are closed and when the stop button is pressed, two normally close contacts
are opened. Spring action returns the button to their original position when finger pressure is
removed. To operate a magnetic switch by a start-stop station, it necessary to connect the
holding coil to the station contacts so that when the start button is press, the coil will become
energized; and when the stop button is pressed the holding coil circuit is opened.
Fig. 9 Start Stop stations
A diagram of a typical full-voltage magnetic equipped with two thermal overload
relays and connected to a start-stop station is shown in Fig. 10 In the diagram to follow,
heavy lines indicate the motor circuits, and light lines show control circuits. The operation of
this starter is as follow:
When the start button of Fig.10 is pressed it completes. The circuit from L1 to the
normally closed contacts of the stop button through coil M and close contacts of the overload
relays to L2. Thus the coil is energized and it closes contacts M and connects the motor
across the line. A maintaining circuit is completed at point 3 to keep the holding coil
energized after the finger is removed from the start button. Pressing the stop button opens
the coil circuit and causes all contacts to open. If prolonged overload should occur during the
operation of the motor, the motor relay contacts will open and de- energized the holding coil.
If an overload condition has caused the relay to trip it will be necessary to reset the relay
contact by hand before the motor can be restarted.
95
Fig. 10 A simplified diagram of magnetic across the line starter.
Fig. 11 showed a line diagram of the
control circuit.
Fig. 12 line diagram of a magnetic
across the line starter
96
The manufacturers make magnetic full-voltage starters. A typical controller is shown
in Fig. 13, Fig.14, and Fig.15 shown controllers with a step-down transformer in the control
circuit. This permits operating the control circuit at a lower voltage than the line voltage, and
usually done for safety reasons.
If a control circuit transformer is used, the primary should be connected to the line
terminals of the starter. These diagrams one end of the secondary is grounded, and also one
side of control coil M is connected to the grounded.
Fig.13 Three-phase starter with step down transformer in control circuit
97
Fig.14 Three-phase starter with control circuit transformer and secondary fuse.
Fig.15 Three phase starter with control circuit
transformer and pilot light
98
Combination Starters
A combination starter consists of a magnetic starter and disconnects switch mounted
in the same enclosure. These starters are supplied with either a fused disconnect switch or
circuit breaker. The fuse or circuit breaker provides short-circuit protection by disconnecting
the line. A combination starter with circuit breaker will prevent a phasing by simultaneously
opening all lines when a fault occurs in any phase. This type of starter can be quickly reset,
when the fault has been cleared.
Fig.16 illustrates a circuit breaker and fused
combination starter.
Pushbutton Station Connections
A number of control circuits will be illustrated involving various combinations of
pushbutton stations. All of these diagrams employs one type of magnetic switch butt others
can be used. Fig.17 illustrates a magnetic switch, which is operated from either of two
stations. The pushbuttons are shown in two positions. Fig.18 shows a straight-line diagram of
the control circuit of two start-stop stations. Fig.19 gives the control circuit of three start-stop
stations. In these diagrams the start buttons are connected in parallel and the stop buttons
are connected in series. This must be done, regardless of the number of stations.
99
The maintaining contact is always connected across the start button. All stop buttons
are connected in series with the holding coil to earth, therefore the motor can be stopped any
position in case of emergency.
Fig.17 the magnetic switch controlled by two stop-start stations
Fig.18 the control circuits for three start-stop stations
100
Fig.19 illustrates a circuit breaker, fuse and two startstop push button combination starter.
Jogging
Magnetic switches can be jogged or inched by this method the motor is made to run
only while the finger is pressing the jog button. As soon as pressure is removed, the motor
stops.
Jogging may be accomplished by using:
1. A station with a selector pushbutton
2. A station with a selector switch.
3. A station with standard pushbutton and a jog relay.
101
Fig.20 illustrates a start-jog-stop station by selector
switch
Fig.20 shows a control circuit of full-voltage magnetic starter concerned to a startjog-stop station having a selector pushbutton. This button is constructed with a sleeve that
may be turned to either a jog or run position. With the sleeve turned to the run position, the
start and stop buttons functions as in ordinary start-stop station. With the sleeve in the jog
position the circuit to the holding contacts is broken and the motor will run only when the jog
button is held down.
The operation of the control circuit of Fig.20 is as follow. With the selector sleeve on
run, pressing the start button completed a circuit from L1 through the contact of overload
relay, stop button and the close contacts of the jogging selector button, the start contacts,
the holding coil to L2. This energizes the holding coil, causing contacts M to make and
connect the motor across the line. The maintaining auxiliary contact keeps the holding coil in
the circuit after the finger is removed from the start button. Pressing the stop button opens
the coil circuit. With the selector sleeve on jog, the current cannot flow to the start button
because the front contacts are in open position. Depressing the jog selector button completes
a circuit through the overload relay, stop button, the jog contacts of the selector button, the
holding coil, to L2. The holding coil will energize only when the button is pressed.
Fig 21 illustrate a jog or run is start by pushbutton
102
Fig.21 is show jog stations, which use a selector switch. The start button is used to
jog or run the motor, depending on the position of the switch in each case with the button.
In the jog position the holding auxiliary contact is broken.
Fig. 1 - 22 Illustrate a jog or run is start by pushbutton.
When the start button is pressed, the relay coil is energized, thus the closing the
relay contacts, CR; CR close the circuit for the holding coil, causing auxiliary contacts M to
close. This completes the maintaining circuit for the holding coil M, when the start button is
released. In the meantime all the main contacts are made, closing the circuit for the motor.
If the jog button is pressed while the motor is at the stand still, a circuit is formed through
the holding coils only as long as the button is pressed
Start-Stop Station with a Pilot Light
Sometimes it is advisable to have a pilot light on the pushbutton station to indicate if
the motor is running. The lamp usually is mounted on the station and is connected across the
holding coil. Such as connection is shown in Fig. 1- 23 and 1- 24 Fig 1-25 shows a control
circuit with pilot light on when motor is stopped. Normally closed contacts are needed on this
starter. With the motor running these contacts are open. Contacts are closed when the motor
is stopped and pilot light goes on. A start-stop station with a pilot light is pictured in Fig.1- 23
103
Fig. 1 - 23 Push Button Station with Pilot light connected to a 3 Phase starter.
Fig. 1 - 24 A Simple control circuit of a Start-Stop Station with a Pilot light
Fig. 1 - 25 Pilot light indicates when motor is not running.
Full-voltage Reversing Starter
The magnetic starters shown thus far are designed to operate the motor in one direction,
either clockwise or counter clockwise. If it is necessary to reverse the motors, its connections
must be changed.
104
Some applications, such as conveyors, hoists, machine tools, elevators, and other
require a motor starter that can reverse the motor when a button is pressed. Thus two of the
line leads can be interchanged to reverse a three-phase motor by means of a magnetic
reversing switch. A reversing starter of this type is shown in the Fig.1 - 26. The circuit is
giving in Fig. 1 -27 and 1 - 28.
Note that it is necessary to use a Forward-Reverse-Stop station with three buttons
and that two operating coils are used, one for forward rotation and the other for reverse
rotation.
Two sets of main and auxiliary contacts are used. One set closes when forward
operation is desired, the other set close for reverse rotation. These contacts are connects in
such a manner that two line wires feeding the motor is interchanged when the reverse
contacts close.
In operation pressing the forward button completes a circuit from L1 the stop button,
the forward button, the forward coil, and the overload contacts to L2. This energizes the coil,
which closes the contacts for forward operation of the motor. Auxiliary contacts F also close,
maintaining the current through coil F when the button is released. Pressing the stop button
opens the circuit through the forward coil that releases all contacts. Pressure on the reverse
button energizes the reverse coil that closes the reverse contacts. Terminals T1 and T3 are
now interchanged and the motor reverses.
Fig. 1 - 26 an AC. Full- Voltage
magnetic reversing controller
105
Fig. 1 - 27 A reversing magnetic starter operated by a Forward-Reverse- Stop Station
Usually, reversing starter are equipped with a mechanical interlock in the form of a
bar will prevent the reverse contacts from making while the forward contacts are closed. This
bar is pivoted in the center, and when the forward contractor goes in it move the bar into a
position where it is impossible for the reverse contacts to make. This starter does not have
electrical interlock to prevent the forward and reverse coils from being energized
simultaneously. All of these starters are equipped with overload relays generally of the
thermal-relay type. Remember, however that many starters use three relays for three-phase
motors.
Sometime more than one forward-reverse-stop station is used to control a magnetic
reversing switch. Fig. 1 - 28 shows connection diagrams of two such stations in difference
positions
Besides having mechanical interlock, most reversing starter is electrically interlock. In
this system, additional normally closed auxiliary contacts are used to prevent the forward and
reverse contractors from being energized at the same time. The holding circuit of each main
contractor coil is wired through the normally closed auxiliary contacts of the opposing
contractors, thus providing the electrical interlock.
106
Fig.1 - 28 Connection For two Forward-Reverse and
Stop stations reversing magnetic switch.
In operation, pressing the forward button closes a circuit from L1 through the stop
button, the forward button, the reverse normally closed auxiliary contacts, the forward limit
switch (if used), the forward coil and the overload contacts to L2. The maintaining contacts
for the forward coil keep it energized when pressure is removed from the button. At the same
time, the normally closed forward auxiliary contacts are opened, preventing a complete circuit
through the reverse coil.
107
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement