Motors and Generators Flashcards
(108 cards)
1
Q
If a reactor coolant pump (RCP) rotor seizes, RCP motor current will __________; and if the rotor
shears, RCP motor speed will __________.
A
increase, increase
2
Q
A nuclear power plant is operating at steady-state 80 percent power when a reactor coolant pump
(RCP) shaft seizes. Which one of the following indications would not accompany the seized shaft?
A
Decreased flow rate in the remaining reactor coolant loop(s).
3
Q
A nuclear power plant is operating at steady-state 100 percent power when a reactor coolant pump
(RCP) malfunction occurs. Thirty seconds after the malfunction, which one of the following can be
used by an operator to determine whether the malfunction is a locked RCP rotor or a sheared RCP
rotor? (Assume no operator action is taken.)
A
RCP ammeter indications
4
Q
During a reactor coolant pump (RCP) locked rotor event, RCP motor current will…
A
increase due to the increased rotor torque.
5
Q
A motor-driven cooling water pump is operating normally. How will pump motor current respond if
the pump experiences a locked rotor?
A
Increases immediately to many times running current, then decreases to zero upon breaker trip.
6
Q
A cooling water pump is being driven by an AC induction motor. Which one of the following
describes how and why pump motor current will change if the pump shaft seizes?
A
Increases due to decreased counter electromotive force.
7
Q
A motor-driven centrifugal pump exhibits indications of pump failure while being started in an idle
cooling water system. Assuming the pump motor breaker does not trip, which one of the following
pairs of indications would be observed if the failure is a locked pump shaft?
A
Excessive duration of peak starting current with zero system flow rate.
8
Q
When a motor-driven centrifugal pump was started, the motor ammeter reading immediately increased
to, and stabilized at, many times the normal operating value. Which one of the following describes a
possible cause for the ammeter response?
A
The pump shaft seized upon start and did not rotate.
9
Q
If the generator bearings on a motor-generator set begin to overheat from excessive friction, which one
of the following will occur?
A
Motor windings will begin to heat up
10
Q
A thermal overload device for a large motor protects the motor from…
A
sustained overcurrent by opening the motor breaker or motor line contacts.
11
Q
Which one of the following will provide the initial motor protection against electrical damage caused
by gradual bearing failure?
A
Thermal overload device
12
Q
Which one of the following will result from prolonged operation of an AC induction motor with
excessively high stator temperatures?
A
Decreased electrical resistance to ground due to breakdown of winding insulation.
13
Q
Continuous operation of a motor at rated load with a loss of required cooling to the motor windings
will eventually result in…
A
breakdown of the motor insulation and electrical grounds.
14
Q
Thermal overload devices will provide the first electrical protection for a pump motor in the
event of…
A
gradual motor bearing damage.
15
Q
Which one of the following trip signals will trip the breaker for an operating motor that experiences a
seized rotor?
A
Time-delayed overcurrent
16
Q
A large AC motor has a maximum ambient temperature rating of 40°C. Which one of the following
will occur if the motor is continuously operated at rated load with an ambient temperature of 50°C?
A
Accelerated breakdown of the motor winding insulation, leading to a short circuit within the motor
windings.
17
Q
A main generator that is connected to an infinite power grid has the following initial indications:
100 MW
0 MVAR
2,900 amps
20 KV
If main generator field current is reduced slightly, amps will __________; and MW will __________.
A
increase; remain the same
18
Q
Excessive current will be drawn by an AC induction motor that is operating…
A
with short-circuited stator windings
19
Q
A main generator that is connected to an infinite power grid has the following indications:
500 MW
300 MVAR (out)
2,800 amps
If main generator field current is reduced slightly, amps will __________; and MW will __________.
A
decrease; remain the same
20
Q
A main generator is connected to an infinite power grid. If the voltage supplied to the generator field
is slowly and continuously decreased, the generator will experience high current due to… (Assume no
generator protective actuations occur.)
A
excessive generator MVAR in.
21
Q
An AC generator is supplying an isolated electrical system with a power factor of 1.0. If generator
voltage is held constant while real load (KW) increases, the current supplied by the generator will
increase in direct proportion to the __________ of the change in real load. (Assume the generator
power factor remains constant at 1.0.)
A
amount
22
Q
A main generator that is connected to an infinite power grid has the following indications:
600 MW
100 MVAR (in)
13,800 amps
25 KV
If main generator excitation current is increased slightly, amps will initially __________; and MW
will initially __________.
A
decrease; remain the same
23
Q
A main generator that is connected to an infinite power grid has the following indications:
600 MW
100 MVAR (in)
13,800 amps
25 KV
If main generator excitation current is decreased slightly, amps will __________; and MVAR will
__________.
A
increase; increase
24
Q
A main generator is connected to an infinite power grid. Which one of the following conditions will
exist if the generator is operating underexcited?
A
Negative MVAR (VARs in) with a leading power factor
25
A diesel generator (DG) is supplying both KW and KVAR to an electrical bus that is connected to an
infinite power grid. Assuming DG and bus voltage do not change, if the DG voltage regulator
setpoint is increased slightly, DG KW will __________; and DG amps will __________.
remain the same; increase
26
A diesel generator (DG) is supplying an electrical bus that is connected to an infinite power grid.
Assuming DG terminal voltage and bus frequency do not change, if the DG governor setpoint is
increased from 60.0 Hz to 60.1 Hz, DG KVAR load will __________; and DG amps will __________.
remain the same; increase
27
A main generator that is connected to an infinite power grid has the following indications:
600 MW
100 MVAR (out)
13,800 amps
25 KV
If main generator field current is decreased, amps will initially __________; and MVAR will initially
__________.
decrease; decrease
28
A diesel generator (DG) is supplying both KW and KVAR to an electrical bus that is connected to an
infinite power grid. Assuming bus voltage does not change, if the DG voltage regulator setpoint is
decreased slightly, DG KW will __________; and DG amps will __________.
remain the same; decrease
29
A main generator that is connected to an infinite power grid has the following indications:
100 MW
0 MVAR
2,900 amps
20 KV
If main generator excitation is increased, amps will __________; and MW will __________.
increase; remain the same
30
A main generator is supplying power to an infinite power grid. If the generator field current is slowly
and continuously increased, the generator will experience high current due to: (Assume no generator
protective actuations occur.)
. excessive generator MVAR out.
31
Two identical 1,000 MW generators are operating in parallel supplying the same isolated electrical
bus. The generator output breakers provide identical protection for the generators. Generator A and
B output indications are as follows:
Generator A Generator B
22 KV 22 KV
60.2 Hertz 60.2 Hertz
800 MW 800 MW
50 MVAR (out) 25 MVAR (in)
A malfunction causes the voltage regulator for generator B to slowly and continuously increase the
terminal voltage for generator B. If no operator action is taken, generator B output current will...
initially decrease, and then increase until the output breaker for generator B trips on overcurrent.
32
Refer to the partial drawing of two identical radial-flow centrifugal pumps in a cooling water system
(see figure below). Each pump is driven by an identical three-phase AC induction motor.
The cooling water system is being returned to service following maintenance on the pumps. Pump A
was started five minutes ago to initiate flow in the cooling water system. Pump B is about to be
started.
When pump B is started, which one of the following would cause the motor ammeter for pump B to
remain off-scale high for a longer time than usual before stabilizing at a lower running current?
Pump B was initially rotating in the reverse direction
33
A main turbine-generator is operating in parallel with an infinite power grid. If the turbine control
valves (or throttle valves) slowly fail open, the generator will experience high current primarily due
to... (Assume no generator protective actuations occur.)
excessive generator MW.
34
A main generator is operating and connected to an infinite power grid. Elevated main generator
winding temperature requires a reduction in reactive load from 200 MVAR (out) to 150 MVAR (out).
To accomplish the reactive load reduction, the operator must __________ the generator field current;
when generator reactive load equals 150 MVAR (out) the generator power factor will be __________
than the initial power factor.
decrease; larger
35
A main generator that is connected to an infinite power grid has the following indications:
22 KV
60 Hertz
575 MW
100 MVAR (in)
Which one of the following contains a combination of manual adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will initially result in an
increase in main generator amps?
Voltage Speed
Setpoint Setpoint
Decrease Increase
36
A nuclear power plant startup is in progress. The main generator has just been connected to the
power grid with the following generator indications:
20 KV
288 amps
10 MW
0 MVAR
The operator suspects the main generator is operating under reverse power conditions and attempts to
increase generator load (MW) normally. If the main generator is operating under reverse power
conditions when the operator attempts to increase generator load, generator MW will initially
__________; and generator amps will initially __________.
decrease; decrease
37
A main generator is connected to an infinite power grid with the following generator output
parameters:
22 KV
60 Hertz
575 MW
100 MVAR (in)
Which one of the following contains a combination of manual adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will initially result in a
decrease in main generator amps?
Voltage Speed
Setpoint Setpoint
Increase Decrease
38
A main generator is connected to an infinite power grid with the following initial generator
parameters:
22 KV
60 Hertz
600 MW
100 MVAR (out)
Which one of the following contains a combination of manual adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will initially result in an
increase in main generator amps?
Voltage Speed
Setpoint Setpoint
Increase Increase
39
A main generator is connected to an infinite power grid. Which one of the following pairs of main
generator output parameters places the generator in the closest proximity to slipping a pole?
800 MW; 600 MVAR (in)
40
A main generator is connected to an infinite power grid with the following initial generator
parameters:
22 KV
60 Hertz
600 MW
100 MVAR (out)
Which one of the following contains a combination of manual adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will initially result in a
decrease in main generator amps?
Voltage Speed
Setpoint Setpoint
Decrease Decrease
41
During a surveillance test, a 4,000 KW diesel generator (DG) and a 1,000 MW main generator (MG) at
a nuclear power plant are connected to the same power grid.
The following stable generator output conditions exist:
Diesel Generator Main Generator
700 KW 800 MW
200 KVAR (out) 100 MVAR (out)
A malfunction then occurs, causing the voltage regulator for the MG to slowly and continuously
increase the MG field current. If no operator action is taken, the DG output current will __________
until a breaker trip separates the generators.
initially decrease, and then increase
42
A main generator is connected to an infinite power grid with the following generator output
parameters:
22 KV
60 Hertz
575 MW
100 MVAR (in)
Which one of the following contains a combination of minor adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will cause the main generator
to operate at a power factor closer to 1.0? (Assume the generator power factor remains less than 1.0.)
Voltage Speed
Setpoint Setpoint
Increase Increase
43
A main turbine-generator is connected to an infinite power grid with the following generator output
parameters:
25 KV
20,000 amps
830 MW
248 MVAR (out)
Which one of the following will significantly increase main generator output amperage without a
significant change in main generator MW output? (Assume the generator power factor remains less
than 1.0.)
Increasing the main generator voltage regulator setpoint
44
A main generator is connected to an infinite power grid with the following generator output
parameters:
22 KV
60 Hertz
575 MW
100 MVAR (out)
Which one of the following contains a combination of manual adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will result in main generator
operation at a power factor closer to 1.0? (Assume the generator power factor remains less than 1.0.)
Voltage Speed
Setpoint Setpoint
Decrease Increase
45
During a surveillance test, a 4,000 KW diesel generator (DG) and a 1,000 MW main generator (MG) at
a nuclear power plant are connected to a power grid.
The following stable generator output conditions initially exist:
Diesel Generator Main Generator
700 KW 800 MW
200 KVAR (out) 100 MVAR (out)
A malfunction then occurs, causing the voltage regulator for the MG to slowly and continuously
decrease the MG field current. If no operator action is taken, the DG output current will __________
until a breaker trip separates the generators.
increase continuously
46
```
A main generator is connected to an infinite power grid with the following generator output
parameters:
100 MW
0 MVAR
2,625 amps
22 KV
If the main generator field current is decreased, main generator amps will initially __________; and
MW will initially __________.
```
increase; remain the same
47
A 4,000 KW diesel generator (DG) is supplying 2,000 KW to a 4.16 KV emergency bus. The DG
governor is in the isochronous mode (no speed droop). The emergency bus is about to be
synchronized with, and then connected to, an infinite offsite power grid by closing the emergency bus
normal power feeder breaker.
The following stable emergency bus and normal power conditions currently exist:
Emergency Bus Normal Power
(from DG) (from Offsite)
4.16 KV 4.16 KV
60.0 Hz 60.1 Hz
When the emergency bus normal power feeder breaker is closed, the DG will… (Assume no
additional operator action.)
transfer KW load to the offsite power grid until the DG is completely unloaded.
48
A radial-flow centrifugal cooling water pump is being powered by a 480 VAC induction motor. If the
motor input voltage slowly decreases from 480 VAC to 450 VAC, the pump flow rate will
__________; and the motor current will __________. (Assume the motor does not stall.)
decrease; increase
49
A main generator is connected to an infinite power grid with the following generator output
parameters:
22 KV
60 Hertz
575 MW
100 MVAR (out)
Which one of the following contains a combination of minor adjustments to the main generator
voltage regulator and speed control setpoints such that each adjustment will cause the main generator
to operate at a power factor farther from 1.0? (Assume the generator power factor remains less than
1.0.)
Voltage Speed
Setpoint Setpoint
Increase Decrease
50
A main generator is connected to an infinite power grid with the following generator output
parameters:
22 KV
60 Hertz
975 MW
200 MVAR (out)
Main generator stator winding temperature is abnormally high. Which one of the following contains
a combination of manual adjustments to the main generator speed control and voltage regulator
setpoints such that each adjustment will reduce the main generator stator winding temperature?
(Assume power factor remains less than 1.0.)
Voltage Speed
Setpoint Setpoint
Decrease Decrease
51
A 4,000 KW rated diesel generator (DG) is supplying 2,000 KW to a 4.16 KV emergency bus. The
DG governor is in the isochronous mode (no speed droop). The emergency bus is about to be
synchronized with, and then connected to, an infinite offsite power grid by closing the emergency bus
normal power feeder breaker.
The following stable emergency bus and normal power conditions currently exist:
Emergency Bus Normal Power
(from DG) (from Offsite)
4.16 KV 4.16 KV
60.1 Hz 59.9 Hz
When the emergency bus normal power feeder breaker is closed, the DG will… (Assume no
additional operator action is taken.)
acquire KW load from the offsite power grid and ultimately exceed its KW load rating.
52
A fault on the offsite AC electrical distribution system caused a sustained 30 percent voltage reduction
on all phases of the onsite three-phase AC electrical distribution system. As a result, several
operating three-phase AC induction motors in the plant experienced automatic breaker trips.
Which one of the following could be responsible for the automatic breaker trips?
Excessive motor current leading to breaker trips from thermal overload
53
A shutdown nuclear power plant is operating normally when an electrical fault causes a sustained 20
percent voltage reduction on all phases of the onsite three-phase AC electrical distribution system.
Assume that all previously-operating three-phase AC induction motors continue operating, and the
mechanical load on each motor remains the same.
As a result of the voltage reduction, the operating three-phase AC induction motors will draw
__________ current; and will experience __________ stator temperatures.
more; higher
54
If the speed of a centrifugal pump is increased to double pump flow rate, pump motor current will...
increase eight-fold.
55
A motor-driven centrifugal pump is operating with the following parameters:
Pump speed = 1,800 rpm
Pump head = 100 psid
Motor current = 10 amps
What will be the approximate value of pump head if pump speed is increased such that the motor
draws 640 amps?
1,600 psid
56
A motor-driven centrifugal pump is operating with a flow rate of 3,000 gpm and a current requirement
of 200 amps. If the pump speed is reduced such that the flow rate is 2,000 gpm, what is the final
current requirement at the new lower speed? (Assume a constant motor voltage.)
59 amps
57
A motor-driven centrifugal pump is operating with the following parameters:
Speed = 1,800 rpm
Motor current = 40 amps
Pump head = 20 psi
Pump flow rate = 400 gpm
What will be the approximate values of pump head and motor current if pump speed is increased to
2,000 rpm?
25 psi, 55 amps
58
A motor-driven centrifugal pump is operating at 600 rpm with the following parameters:
Motor current = 10 amps
Pump head = 50 psi
Pump flow rate = 200 gpm
What will be the approximate value of pump head if the flow is increased such that the motor draws
640 amps?
800 psi
59
A motor-driven centrifugal pump is operating with a throttled discharge valve in an open system. If
the pump discharge valve is fully opened to increase system flow rate, which one of the following will
increase?
Motor amps
60
A centrifugal pump is operating with the following parameters:
Speed = 3,600 rpm
Motor current = 100 amps
Pump head = 50 psi
Pump flow rate = 400 gpm
What will be the approximate values of pump head and motor current if pump speed is decreased to
2,000 rpm?
15.4 psi, 17.1 amps
61
A two-speed centrifugal pump is driven by an AC motor with the following initial conditions:
Pump speed = 400 rpm
Motor current = 40 amps
Pump head = 60 psid
What will be the approximate value of pump head if pump speed is increased to 1,600 rpm?
960 psid
62
A motor-driven centrifugal pump is operating with the following parameters:
Speed = 1,200 rpm
Motor current = 40 amps
Pump head = 20 psid
Pump flow rate = 400 gpm
What will be the approximate values of pump head and motor current if pump speed is increased to
1,600 rpm?
36 psid, 95 amps
63
A motor-driven centrifugal pump is operating with the following parameters:
Speed = 1,200 rpm
Motor current = 40 amps
Pump head = 20 psi
Pump flow rate = 400 gpm
What will be the approximate values of pump head and motor current if pump speed is increased to
1,800 rpm?
45 psi, 135 amps
64
A motor-driven centrifugal pump is operating with the following parameters:
Speed = 1,800 rpm
Motor current = 40 amps
Pump head = 20 psid
Pump flow rate = 400 gpm
What will be the approximate values of pump head and motor current if pump speed is decreased to
1,200 rpm?
9 psid, 12 amps
65
A motor-driven centrifugal pump is operating with a flow rate of 3,000 gpm and a motor current of
150 amps. If the pump speed is reduced such that the flow rate is 2,000 gpm, what is the final motor
current at the new lower speed?
44 amps
66
A motor-driven centrifugal pump is operating at 600 rpm with the following parameters:
Motor current = 100 amps
Pump head = 50 psid
Pump flow rate = 880 gpm
What will be the approximate value of pump head if pump speed is increased to 1,200 rpm?
200 psid
67
A multi-speed centrifugal pump is operating at 3,600 rpm with a flow rate of 3,000 gpm. Which one
of the following approximates the new flow rate if the speed is decreased to 3,000 rpm?
2,500 gpm
68
A multi-speed centrifugal pump is operating with a flow rate of 1,800 gpm at a speed of 3,600 rpm.
Which one of the following approximates the new flow rate if the pump speed is decreased to 2,400
rpm?
1,200 gpm
69
A multi-speed motor-driven centrifugal pump is operating with the following parameters:
Motor current = 27 amps
Pump head = 50 psid
Pump flow rate = 880 gpm
Which one of the following will be the approximate new value of pump head if pump speed is
increased such that the motor draws 64 amps?
89 psid
70
Which one of the following describes the relationship between the current drawn by an AC induction
motor and the amount of heat generated in the motor windings?
Heat generation is proportional to the square of the current.
71
A motor-driven centrifugal pump is operating at 600 rpm with the following parameters:
Motor current = 100 amps
Pump head = 50 psid
Pump flow rate = 880 gpm
What will be the approximate value of pump head if pump speed is increased such that the motor
draws 640 amps?
173 psid
72
A rotary positive displacement pump (PDP) is being used to supply water to a piping system. The
PDP is driven by an AC induction motor. The initial parameters are:
System pressure = 500 psig
PDP flow rate = 50 gpm
PDP motor current = 40 amps
After several hours, the PDP motor speed is increased such that the new PDP flow rate is 100 gpm. If
system pressure does not change, what is the approximate value of the PDP motor current at the 100
gpm flow rate?
80 amps
73
Refer to the pump performance curves for a centrifugal cooling water pump (see figure below). The
pump is being driven by a single-speed AC induction motor. Pump flow rate is being controlled by a
throttled discharge flow control valve.
The following initial pump conditions exist:
Pump motor current = 50 amps
Pump flow rate = 400 gpm
What will be the approximate value of pump motor current if the flow control valve is repositioned
such that pump flow rate is 800 gpm?
Less than 100 amps
74
Consider two identical single-speed AC induction motors, one of which is connected to a radial-flow
centrifugal pump and the other to a reciprocating-type positive displacement pump (PDP). Both
pumps are taking suction at the same elevation from a vented water storage tank.
Each pump has a maximum design backpressure of 800 psig, and each is operating with the following
initial conditions:
Flow rate = 200 gpm
Backpressure = 400 psig
Motor current = 100 amps
If the backpressure for each pump increases to 600 psig, the centrifugal pump will have a __________
flow rate than the PDP; and the centrifugal pump will have a __________ motor current than the PDP.
lower; lower
75
Refer to the pump performance curves for a centrifugal cooling water pump (see figure below). The
pump is being driven by a single-speed AC induction motor. Pump flow rate is being controlled by a
throttled discharge flow control valve.
The following initial pump conditions exist:
Motor current = 100 amps
Pump flow rate = 800 gpm
What will be the approximate value of pump motor current if the flow control valve is repositioned
such that pump flow rate decreases to 400 gpm?
Greater than 75 amps
76
An AC induction motor is connected to a radial-flow centrifugal pump in a cooling water system.
When the pump is started, the time period required to reach a stable running current will be shorter if
the pump discharge valve is fully __________; and the stable running current will be lower if the
pump discharge valve is fully __________.
closed; closed
77
A centrifugal pump is driven by a single-speed AC induction motor. Pump flow rate is controlled by
a throttled discharge flow control valve.
The following initial pump conditions exist:
Pump motor current = 50 amps
Pump flow rate = 400 gpm
What will the resulting pump motor current be if the flow control valve is repositioned such that pump
flow rate increases to 800 gpm?
Cannot be determined without additional information.
78
An axial flow ventilation fan is being driven by an AC motor. The fan is operating at its maximum
rated flow rate. How will the fan motor current initially change if the flow rate through the fan is
decreased by partially closing a discharge damper?
The motor current will increase, but not in accordance with the centrifugal pump laws.
79
Consider two identical single-speed AC induction motors, one of which is connected to a radial-flow
centrifugal pump and the other to a rotary-type positive displacement pump (PDP). Both pumps are
taking suction from the bottom of a vented water storage tank.
Each pump is operating with the following initial conditions:
Flow rate = 200 gpm
Backpressure = 600 psig
Motor current = 100 amps
If the backpressure for each pump decreases to 400 psig, the centrifugal pump will have a __________
flow rate than the PDP; and the centrifugal pump will have a __________ motor current than the PDP.
higher; higher
80
Refer to the pump performance curves for a centrifugal cooling water pump (see figure below). The
pump is being driven by a single-speed AC induction motor. Pump flow rate is being controlled by a
throttled discharge flow control valve.
The following initial pump conditions exist:
Motor current = 10 amps
Pump flow rate = 200 gpm
What will be the approximate value of pump motor current if the flow control valve is repositioned
such that pump flow rate increases to 800 gpm?
15 amps
81
A motor-driven radial-flow centrifugal pump is operating to provide makeup water from a constant
head source to a vented storage tank that is 30 feet tall. The pump is located at the base of the tank
and discharges directly into the bottom of the tank. As the tank water level increases from 20 to 25
feet, the pump discharge pressure will __________; and the pump motor current will __________.
increase; decrease
82
An air-cooled AC induction motor is initially operating at steady-state conditions, producing a work
output of 50 hp. A reduction in cooling air flow rate to the motor causes the average stator winding
temperature to increase by 20ºF. To maintain a 50 hp work output at the higher stator winding
temperature, the voltage applied to the motor must be __________ because the stator winding
resistance has __________.
increased; increased
83
The rate of heat production in the stator windings of an AC induction motor is __________
proportional to the __________ of the stator current.
directly, square
84
An axial flow ventilation fan is being driven by an AC motor. The fan is operating at 90 percent of
rated flow rate with its discharge damper partially closed. How will the fan motor current change if
its discharge damper is fully opened?
The motor current will decrease, but not in accordance with the centrifugal pump laws.
85
Initially, an AC induction motor is operating with the following steady-state conditions:
Motor current = 25 amps
Average stator winding temperature = 140°F
Ambient temperature = 90°F
Assume the stator winding electrical resistance, motor heat transfer properties, and ambient
temperature do not change. If a change in motor load causes the motor current to increase to 50 amps,
which one of the following will be the new steady-state average stator winding temperature?
290°F
86
Refer to the partial drawing of two identical centrifugal pumps in a cooling water system (see figure
below). Each pump is driven by an identical three-phase AC induction motor.
The cooling water system is being returned to service following maintenance on the pumps. Pump A
was started five minutes ago to initiate flow in the cooling water system. Pump B is about to be
started.
When pump B is started, which one of the following will cause pump B to experience high starting
current for a shorter time than usual before stabilizing at a lower running current?
The motor coupling for pump B was removed and not reinstalled.
87
The starting current in a typical AC induction motor is usually much higher than the full-load running
current because...
rotor speed during start is too low to generate significant counter electromotive force in the stator
88
The average starting current for a typical AC induction motor is approximately...
five to seven times its normal running current
89
Which one of the following describes the motor current indications that would be observed during the
start of a large AC motor connected to a load?
Amps immediately increase to many times the normal operating value and then decrease to the
normal operating value.
90
If the discharge valve of a large motor-driven centrifugal pump is kept closed during a normal pump
start, the current indication for the AC induction motor will rise to...
several times the full-load current value, and then decrease to the no-load current value.
91
Which one of the following is a characteristic of a typical AC induction motor that causes starting
current to be greater than running current?
The rotor magnetic field induces an opposing voltage in the stator that is proportional to rotor
speed.
92
The starting current in an AC motor is significantly higher than the full-load running current because...
little counter electromotive force is induced in the stator windings during motor start.
93
Starting current in an AC induction motor is typically __________ times the full-load running current.
5 to 7
94
Which one of the following describes the motor current during the start of a typical motor-driven
radial-flow centrifugal pump with a closed discharge valve?
Current immediately increases to many times the full-load value and then rapidly decreases to the
no-load value after several seconds and then stabilizes.
95
Which one of the following describes the motor current indications that would be observed during the
start of a large motor-driven radial-flow centrifugal pump with a closed discharge valve?
Current immediately increases to many times the no-load value and then rapidly decreases to the
no-load value after several seconds.
96
Two identical AC induction motors are connected to identical radial-flow centrifugal pumps being
used to provide cooling water flow in separate systems in a nuclear power plant. Each motor is rated
at 1,000 hp. The discharge valve for pump A is fully open and the discharge valve for pump B is fully
shut. Each pump is currently off.
If the pumps are started under these conditions, the longer time period required to stabilize motor
current will be experienced by the motor for pump _____; and the higher stable motor current will be
experienced by the motor for pump _____.
A; A
97
Which one of the following describes when the highest stator current will be experienced by an AC
induction motor?
Immediately after energizing the motor
98
Two identical AC induction motors are connected to identical radial-flow centrifugal pumps in
identical but separate cooling water systems. Each motor is rated at 200 hp. The discharge valve for
pump A is fully shut and the discharge valve for pump B is fully open. Each pump is currently off.
If the pumps are started under these conditions, the longer time period required to stabilize motor
current will be experienced by the motor for pump _____; and the higher stable motor current will be
experienced by the motor for pump _____.
B; B
99
Two identical AC induction motors are connected to identical radial-flow centrifugal pumps in
identical but separate cooling water systems. Each motor is rated at 200 hp. The discharge valve for
pump A is fully shut and the discharge valve for pump B is fully open. Each pump is currently off.
If the pumps are started under these conditions, the shorter time period required to reach a stable
running current will be experienced by the motor for pump _____; and the higher stable running
current will be experienced by the motor for pump _____.
A; B
100
To minimize the duration of high starting current, an AC induction motor should be started
__________ to __________ the stator counter electromotive force.
unloaded; quickly establish
101
Two identical AC induction motors are connected to identical radial-flow centrifugal pumps in
identical but separate cooling water systems. Each motor is rated at 200 hp. The discharge valve for
pump A is fully open and the discharge valve for pump B is fully closed. Each pump is currently off.
If the pumps are started under these conditions, the shorter time period required to reach a stable
running current will be experienced by the motor for pump _____; and the higher stable running
current will be experienced by the motor for pump _____.
B; A
102
What is the primary reason for limiting the number of starts for an electric motor in a given period of
time?
Prevent overheating of the windings due to high starting currents.
103
The frequency of starts for large AC motors should be limited to prevent excessive...
heat buildup within the motor.
104
The number of starts for an electric motor in a given period of time should be limited because
overheating of the __________ can occur due to the __________ counter electromotive force
produced at low rotor speeds.
windings; low
105
The frequency of start/stop cycles for an electrical motor is limited to prevent...
overheating the motor windings
106
Frequent starts of large motors will result in overheating of the motor windings due to high current
flow caused by...
low electrical resistance of the motor windings.
107
Which one of the following is the primary reason for limiting the number of motor starts in a given
time period?
Prevents overheating of the motor windings
108
A large centrifugal pump is driven by a 200 horsepower AC induction motor. The motor breaker
control circuit contains the following protection devices: instantaneous overcurrent relay, motor
thermal overload relay, control power fuses, and an anti-pumping device.
The pump had been manually started and stopped several times during a 5 minute period when the
motor breaker tripped. Which one of the following is the most likely cause of the breaker trip?
Motor thermal overload.
