Alternating current Motors

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Exam: 4032RR – Alternating-current Motors

1. To double the rated speed of an eight-pole squirrel-cage motor, the common method is to reconnect the
motor as a
A. sixteen-pole motor.
B. six-pole motor.
C. two-pole motor.
D. four-pole motor.

2. The total load of an industrial plant is 4000 kW, and the load operates at a power factor of 0.85 lag. If
an 800-hp synchronous motor with 93.5% rated efficiency and operating at unity power factor is added,
what is the new value of power factor, in percent?
A. 88.2%
B. 76.0%
C. 89.5%
D. 98.6%

3. During the starting of a synchronous motor, the common practice is to
A. place a resistance across the field winding..
B. leave the field open-circuited..
C. apply d-c field excitation immediately.
D. open the circuit of the damper winding.

4. The purpose of a double-squirrel-cage secondary in an induction motor is to by
A. reduce primary heating effects.
B. produce two rotating magnetic fields.
C. create special current characteristics.
D. develop minimum torque at starting speeds.

5. Which of the following is true of the operation of a synchronous motor?
A. It is started by applying direct current.
B. It develops full slip at synchronous speed.
C. It is started by the amortisseur winding.
D. It has an open-circuit field winding.

6. The starter of a phase-wound induction motor consists of a
A. series resistance in the primary.
B. compensator in the secondary.
C. rheostat connected to the secondary.
D. reactor connected to the primary.

7. A synchronous motor operating at unity power factor on a 550-V circuit has a rated field current of 12 A
and a pull-out torque of 160%. If the line voltage drops to 500 V and the field current decreases to 10 A,
what is the approximate maximum load, in percent, that the machine can carry for 1 min?
A. 121.21%
B. 75.75%
C. 145.45%
D. 174.48%

8. If a certain motor develops 10 ft-lb torque with an operating voltage of 110 V and is capable of carrying
the resulting current at a new voltage of 220 V, what is the new value of torque, in foot-pounds?
A. 5.0
B. 2.5
C. 40.0
D. 20.0

9. When a-c voltage is first applied to a large squirrel-cage induction motor and the rotor has not yet begun
to rotate, there is
A. full pull-out torque.
B. zero locked-rotor current.
C. maximum initial slip.
D. minimum starting resistance.

10. To produce a rotating magnetic field for starting a single-phase induction motor, the common method is
to use
A. three field resistors different in value.
B. an autotransformer device.
C. a short-circuited slip ring assembly.
D. displaced voltages in two windings.

11. If a synchronous motor rated at 80% leading power factor operates at 50% load, what is the armature
current, expressed in percent of rated value?
A. 38.0%
B. 47.0%
C. 54.0%
D. 66.0%

12. The preferred method for applying the field excitation voltage uses a field relay which operates on the
principle of
A. thermal application.
B. time application.
C. speed application.
D. angle application.

13. The type of induction motor enclosure preferred for installation in a moist and corrosive atmosphere is
the
A. splashproof motor.
B. open dripproof motor.
C. totally enclosed fan-cooled motor.
D. weather-protected type I motor.

14. The practice of interchanging two of the three leads of a running polyphase induction motor is referred
to as
A. accelerating.
B. slipping.
C. plugging.
D. synchronizing.

15. If a four-pole 60-Hz induction motor has a 6% slip in normal operation, the actual speed is
A. 1728 r/min.
B. 108 r /min.
C. 1800 r/min.
D. 1692 r/min.

16. After a synchronous motor has reached the synchronizing speed, the best time to apply the field current
is when the
A. rotor poles are traveling faster than the stator poles.
B. induced field current is increasing to a maximum value.
C. north rotor poles are directly opposite the south stator poles.
D. south stator poles are opposite the south rotor poles.

17. In the normal operation of an induction motor, the rotor turns at less than synchronous speed in order
to
A. create less heating of the squirrel-cage bars.
B. produce sufficient voltage to develop the required torque.
C. develop circulating currents in the stator frame.
D. overcome windage and friction losses.

18. If a 5-hp three-phase induction motor operates on a 230-V system with an efficiency of 93.5% and a
power factor of 80% at normal operation, what is the current, in amperes, approximately?
A. 12.5
B. 21.7
C. 10.0
D. 11.7

19. To drive a 1000-hp load at unity power factor and constant speed, the recommended type of a-c motor
is the
A. single-phase induction type.
B. polyphase synchronous type.
C. single-phase synchronous type.
D. polyphase induction type.

20. The type of motor often used for ship propulsion applications is the
A. synchronous motor.
B. induction motor.
C. shaded-pole motor.
D. repulsion motor.