synchronous motors MCQs – Page 2

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

synchronous motors Mcqs
51. An over excited synchronous motor is used for____________?
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A. variable speed loads

B. low torque loads

C. power factor corrections

D. fluctuating loads

52. While starting a synchronous motor by induction motor action, field winding is usually_____________?
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A. kept open-circuited

B. connected to D.C. supply

C. short-circuited by low resistance

D. none of the above

53. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as_________?
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A. V

B. V2

C. 1/V

D. V312

54. The shaft of synchronous motor is made of____________?
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A. alnico

B. mild steel

C. stainless steel

D. chrome steel

55. When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way
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A. augments it directly

B. none of the above

C. directly opposes it

D. cross-magnetises it

56. A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because?
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A. mechanical load on the rotor remains constant

B. synchronous motor has large airgap

C. stator supply is not required to produce magnetic field

D. synchronous motor has no slip

57. The efficiency of a properly designed synchronous motor will usually fall in range___________?
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A. 99 to 99.5%

B. 75 to 80%

C. 85 to 95%

D. 60 to 70%

58. The working of a synchronous motor is similar to_____________?
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A. turbine

B. transmission of mechanical power by shaft

C. distribution transformer

D. none of the above

E. gear train arrangement

59. The armature current of the synchronous motor has higher values for_____________?
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A. high excitation only

B. low excitation only

C. both A. and (b)

D. none of the above

60. The back e.m.f. in the stator of a synchronous motor depends on____________?
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A. flux density

B. none of the above

C. rotor speed

D. number of poles

E. rotor excitation
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

81. The back e.m.f. of a synchronous motor depends on______________?
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A. all of the above

B. load

C. speed

D. load angle
82. A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
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A. the motor continue to run in the same direction

B. the motor will run in the reverse direction

C. none of the above

D. the motor will stop

E. the winding of the motor will burn

83. By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?
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A. By changing the supply frequency

B. By interchanging any two phases

C. By changing the applied voltage

D. By changing the load

84. While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerating torque is produced by_____________?
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A. induction motor torque in field winding

B. induction motor torque in damper winding

C. eddy current and hysteresis torque in pole faces

D. reHetance motor torque due to saliency of the rotor

E. all of the above methods
85. Synchronous motors are_____________?
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A. none of the above

B. essentially self-starting

C. self-starting

D. not-self starting
86. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop_________________?
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A. inductor motor torque

B. induction generator torque

C. synchronous motor torque

D. d.c. motor toque

E. none of the above

87. In a synchronous motor, the maximum power developed depends on all of the following except:
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A. supply voltage

B. maximum value of coupling angle

C. rotor excitation

D. direction of rotation
88. The power developed by a synchronous motor will be maximum when the load angle is____________?
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A. 120

B. 90

C. zero

D. 45

89. Which of the following motors is non-self starting ?
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A. D.C. series motor

B. Wound round induction motor

C. synchronous motor

D. Squirrel cage induction motor

90. To limit the operating temperature an electrical machine should have proper____________?
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A. current rating

B. power factor

C. voltage rating

D. speed

91. . In a synchronous motor which loss varies with load ?
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A. Copper loss

B. Bearing friction loss

C. Windage loss

D. Core loss

92. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?
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A. High field current

B. Low field current

C. High core losses

D. Low short circuit ratio

93. Which of the following losses is not dissipated by the stator core surface in a synchronous motor?
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A. Iron losses in the stator

B. Windage losses

C. None of the above

D. Eddy current losses in the conductors

E. Copper losses in the slot portion of the conductors

94. If the field of a synchronous motor is underexcited, the power factor will be_____________?
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A. zero

B. leading

C. lagging

D. unity

95. Which of the following losses, in a synchronous motor, does not vary with load?
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A. None of the above

B. Any of the above

C. Windage loss

D. Copper losses

96. A synchronous motor will always stop when:______________?
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A. load in motor varies

B. supply voltage frequency changes

C. excitation winding gets disconnected

D. supply voltage fluctuates

97. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
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A. Er (net resultant voltage) in armature is increased

B. none of the above

C. power factor is decreased

D. Eb (back e.m.f.) becomes less than V (applied voltage)

E. speed of the motor is reduced

98. The torque angle, in a synchronous motor, is the angle between_______________?
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A. magnetising current and back e.m.f.

B. the rotating stator flux and rotor poles

C. the supply voltage and the back e.m.f.

D. none of the above

99. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected______________?
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A. it runs as a reluctance motor at the same speed

B. it runs as a reluctance motor at a lower speed

C. none of the above

D. the motor stops
100. When load on an over-excited or under excited synchronous*motor is increased, rate of change of its armature current as compared with that of power factor is___________?
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A. more

B. equal

C. twice

D. less