synchronous motors MCQs – Page 4

Practise focused synchronous motors questions with marked answers for exam preparation and subject revision.

synchronous motors Mcqs
61. A synchronous motor can be started by________?
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A. providing damper winding

B. D.C. compound motor

C. pony motor

D. any of the above
62. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is_______________?
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A. under-loaded

B. supplied with unbalanced voltage

C. over-loaded

D. none of the above

63. Synchronous motor always runs at______________?
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A. less than synchronous speed

B. none of the above

C. the synchronous speed

D. more than synchronous speed

64. Which of the following methods is used to start a synchronous motor?
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A. Damper winding

B. Star-delta starter

C. Resistance starter in the armature circuit

D. Damper winding in conjunction with star-delta starter
65. An important advantage of a synchronous motor over wound round induction motor is that
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A. none of the above

B. its speed may be controlled more easily

C. its speed is independent of supply frequency

D. its power factor may be varied at will
66. The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as_____________?
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A. slip torque

B. pull-out torque

C. synchronising torque

D. breaking torque

67. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
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A. will become more

B. will remain unchanged

C. none of the above.

D. will become less
68. The net armature voltage of a synchronous motor is equal to the_______________?
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A. arithmetic sum of Eb and V

B. arithmetic difference of Eb and V

C. vector sum of Eb and V

D. vector difference of Eh and V
69. Power factor of a synchronous motor is unity when_____________?
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A. the armature current is minimum

B. none of the above

C. the armature current is maximum

D. the armature current is zero

70. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
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A. the motor stops

B. it continues to run at the same speed

C. it runs at a very high speed

D. it runs at a slower speed

71. In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be____________?
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A. 750 r.p.m.

B. 3000 r.p.m.

C. none of the above

D. 1500 r.p.m.
72. A synchronous motor working at leading power factor can be used as____________?
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A. noise generator

B. voltage booster

C. mechanical synchronizer

D. phase advancer
73. A synchronous motor develops maximum power when load angle is____________?
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A. 90

B. 45

C. 60

D. 120

74. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to____________?
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A. increase in both armature current and power factor angle

B. increase in back e.m.f. but decrease in armature current

C. increase in both armature current and power factor which is lagging

D. increase in torque angle but decrease in back e.m.f.

75. Synchronsizing power of a synchronous machine is_____________?
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A. none of the above

B. direcly proportional to the synchronous reactance

C. equal to the synchronous reactance

D. inversely proportional to the synchronous reactance
76. Stability of a synchronous machine:
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A. remains unaffected with increase in excitation

B. decreases with increase in its excitation

C. any of the above

D. increases with increase in its excitation
77. In which of the following motors the stator and rotor magnetic field rotate at the same speed?
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A. Universal motor

B. Induction motor

C. Reluctance motor

D. Synchronous motor
78. Under which of the following conditions hunting of synchronous motor is likely to occur?
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A. Over-loading for long periods

B. Small and constant load

C. Periodic variation of load

D. Over-excitation

79. Mostly, synchronous motors are of_____________?
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A. induction type machines

B. smooth cylindrical type machines

C. salient pole type machines

D. alternator type machines

80. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately
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A. six times

B. nine times

C. one-third

D. thrice

E. twice