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B. Reduced Voltage Motor Starting Application Manual Liquid Cooled Generator Sets
159
7 APPENDIX
B. Reduced Voltage Motor Starting
Although voltage dip often causes various problems, a controlled reduction in voltage at the motor terminals
can be beneficial when it is used to reduce the starting kVA of a motor in applications where the reduced
motor torque is acceptable. Reducing motor starting kVA can reduce the size of the required generator set,
lessen the voltage dip, and provide a softer start for the motor loads. Make sure, however, that the motor will
develop sufficient torque to accelerate the load under reduced voltage conditions. Also, any starter that
makes a transition between start and run can cause an inrush condition nearly as severe as acrossthe
line starting unless the motor is at or near synchronous speed at transition. This may cause unaccept-
able voltage dip and potentially starter dropout.
A Comparison of Motor Starting Methods
Table 71 compares the effects of full voltage, autotransformer, and resistor starting on a 50 horsepower,
Design B, Code G motor. As can be seen, autotransformer starting requires less motor starting capacity
from the generator set. Resistor starting actually requires more kW (engine power) than acrosstheline
starting.
TYPE OF STARTER
AUTOTRANSFORMER
RESISTOR
FULL VOLTAGE
% of applied voltage
(tap)
65
50
100
% of full voltage
(multiplier)
*
0.42
0.50
1.0
Starting kVA with
reduced voltage
starter
295
**
x 0.42 = 123.9
295
**
x 0.50 = 147.5
295
**
x 1.0 = 295
Starting kW with
reduced voltage
starter (kVA x PF)
123.9 x 0.36***= 43.4
147.5 x 0.8****= 118
295x 0.36***= 106.9
Run kVA
46
46
46
Run kW
41
41
41
*
See Table 34
**
See Table 35 and multiply horsepower of 50 by the factor of 5.9 for Code Letter G.
***
See Table 36
****
See SPF for Resistor in Table 34
Table 71.
Reduced Voltage Motor Starting Comparison
© 2004 Cummins Power Generation All copies are uncontrolled Application Manual Liquid Cooled Generator Sets
160
7 APPENDIX
Full Voltage Motor Starting
Starting: Full voltage, acrosstheline starting is typical unless it is necessary to reduce motor starting kVA
because of the limited capacity of the generator set or to limit voltage dip during motor starting. There is no
limit to the HP, size, voltage, or type of motor.
Application Notes: This method is most common because of its simplicity, reliability, and initial cost. Note
on the kVA and torque curves that starting kVA remains fairly constant until the motor almost reaches full
speed. Also note that kW peaks at about 300 percent of rated kW near 80 percent of synchronous speed.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
KVA
TORQUE
SPEED (% SYNCHRONOUS)
kV
A
&
T
ORQUE (% F
.
L.)
MOTOR
1
2
3
START: CLOSE 123
RUN: NO CHANGE
MOTOR STARTING DIAGRAM
LINE
Autotransformer Motor Starting, Open Transition
Starting: The autotransformer is in the circuit only during starting to reduce voltage to the motor. The open-
ing of the circuit during transition can cause severe transients, which may even be able to cause nuisance
tripping of circuit breakers.
Application Notes: Open transition switching of reduced voltage starters should be avoided in generator set
applications, especially when the motors are not brought up to full speed at the time of transition. The rea-
son for this is that the motor slows down and gets out of synchronization during the switching transition. The
result is similar to paralleling generator sets out of phase. The kVA drawn immediately after switching can
exceed starting kVA. Also note that the starting power factor is lower when an autotransformer is used.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
KVA
SPEED (% SYNCHRONOUS)
kV
A
&
T
ORQUE (% F
.
L.)
TORQUE
MOTOR
1
2
3
4
5
6
7
8
START: CLOSE 23567
RUN: OPEN 23567; CLOSE 148
MOTOR STARTING DIAGRAM
LINE Application Manual Liquid Cooled Generator Sets
161
7 APPENDIX
Autotransformer Motor Starting, Closed Transition
Starting: The circuit is not interrupted during starting. During transfer, part of the autotransformer winding
remains in the circuit as a series reactor with the motor windings.
Application Notes: Closed transition is preferred over open transition because of less electrical distur-
bance. The switching, however, is more expensive and complex. It is the most commonly used reduced
voltage starting method for large motors with low load torque requirements, such as sewage lift pumps and
chillers. The principle advantage is more torque per current than with other reduced voltage starting meth-
ods. Operation can be automatic and/or remote. Also note that the starting power factor is lower when an
autotransformer is used.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
KVA
TORQUE
SPEED (% SYNCHRONOUS)
kV
A
&
T
ORQUE (% F
.
L.)
MOTOR
START: CLOSE 67234
TRANSFER: OPEN 67
RUN: CLOSE 15
MOTOR STARTING DIAGRAM
1
2
3
5
6
7
4
LINE
Reactor Motor Starting, Closed Transition
Starting: Reactor starting has the advantage of simplicity and closed transition, but results in lower starting
torque per kVA than with autotransformer starting. Relative torque, however, improves as the motor accel-
erates.
Application Notes: Reactor starting is generally not used except for large, highvoltage or highcurrent
motors. The reactors must be sized for HP and voltage and may have limited availability. Typically, reactor
starting costs more than autotransformer starting for smaller motors, but is simpler and less expensive for
larger motors. Starting power factor is exceptionally low. Reactor starting allows a smooth start with almost
no observable disturbance on transition and is well suited for applications such as centrifugal pumps or
fans.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
KVA
SPEED (% SYNCHRONOUS)
kV
A
&
T
ORQUE (% F
.
L.)
TORQUE
MOTOR
START: CLOSE 123
RUN: CLOSE 456
MOTOR STARTING DIAGRAM
1
2
3
5
6
4
LINE Application Manual Liquid Cooled Generator Sets
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7 APPENDIX
Resistor Motor Starting, Closed Transition
Starting: Resistor starting is occasionally used for smaller motors where several steps of starting are
required and no opening of motor circuits between steps is allowed.
Application Notes: Also available as a stepless transition starter which provides a smoother start. Resistor
starting is usually the least expensive with small motors. Accelerates loads faster because the voltage
increases with a decrease in current. Has a higher starting power factor.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
TORQUE
SPEED (% SYNCHRONOUS)
kV
A
&
T
ORQUE (% F
.
L.)
KVA
MOTOR
START: CLOSE 123
SECOND STEP: CLOSE 456
THIRD STEP: CLOSE 789
MOTOR STARTING DIAGRAM
1
2
3
5
6
4
7
8
9
LINE
StarDelta Motor Starting, Open Transition
Starting: StarDelta starting requires no autotransformer, reactor, or resistor. The motor starts starcon-
nected and runs deltaconnected.
Application Notes: This starting method is becoming more popular where low starting torques are accept-
able. It has the following disadvantages:
1. Open transition. Closed transition is available at extra cost.
2. Low torque.
3. No advantage when the motor is powered by a generator set unless the motor reaches synchronous
speed before switching. In applications where the motor does not reach synchronous speed, the generator
set must be sized to meet the surge.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
KVA
SPEED (% SYNCHRONOUS)
kV
A
&
T
ORQUE (% F
.
L.)
TORQUE
MOTOR
1
2
3
START: CLOSE 123456
RUN: OPEN 456; CLOSE 789
MOTOR STARTING DIAGRAM
4
5 6
7
8
9
LINE Application Manual Liquid Cooled Generator Sets
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7 APPENDIX
Part Winding Motor Starting, Closed Transition
Starting: Part winding starting is less expensive because it requires no autotransformer, reactor, or resistor
and uses simple switching. Available in two or more starting steps depending on size, speed, and voltage of
motor.
Application Notes: Automatically provides closed transition. First, one winding is connected to the line;
after a time interval, the second winding is paralleled with the first. Starting torque is low and is fixed by the
motor manufacturer. The purpose of part winding is not to reduce starting current but to provide starting
current in smaller increments. There is no advantage to this method if the motor is powered by a generator
set unless the motor can reach synchronous speed before transition to the line.
100
200
300
400
500
600
20
40
60
80
100
TYPICAL TORQUE AND KVA CURVES FOR
SQUIRREL CAGE INDUCTION MOTORS
KVA
SPEED (% SYNCHRONOUS)
kV
A
&