polyphase induction motors MCQs – Page 4

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

polyphase induction motors Mcqs

61. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor _____________?
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A. By injecting e.m.f. in the rotor circuit

B. Rotor rheostat control

C. By changing the number of stator poles

D. By operating two motors in cascade

62. The good power factor of an induction motor can be achieved if the average flux density in the air gap is___________?
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A. absent

B. large

C. small

D. infinity

63. The shape of the torque/slip curve of induction motor is__________?
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A. rectangular parabola

B. straigth line

C. parabola

D. hyperbola

64. The starting torque of a 3-phase squirrel cage induction motor is____________?
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A. twice the full load torque

B. 1.5 times the full load torque

C. equal to full load torque

65. The starting torque of a three phase induction motor can be increased by__________?
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A. increasing slip

B. increasing current

C. both A. and (b)

D. none of the above

66. Size of a high speed motor as compared to low speed motor for the same H.P. will be__________?
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A. any of the above

B. smaller

C. bigger

D. same

67. A 3-phase slip ring motor has____________?
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A. double cage rotor

B. wound rotor

C. any of the above

D. short-circuited rotor

68. Short-circuit test on an induction motor cannot be used to determine___________?
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A. copper losses

B. transformation ratio

C. windage losses

D. power scale of circle diagram

69. In a squirrel cage motor the induced e.m.f. is___________?
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A. dependent on the shaft loading

B. dependent on the number of slots

C. slip times the stand still e.m.f. induced in the rotor

D. none of the above

70. Which motor is preferred for use in mines where explosive gases exist ?
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A. Induction motor

B. Air motor

C. Synchronous motor

D. D.C. shunt motor

71. The speed of a squirrel-cage induction motor can be controlled by all of the following except_____________?
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A. changing winding resistance

B. changing number of poles

C. reducing supply voltage

D. changing supply frequency

72. Which is of the following data is required to draw the circle diagram for an induction motor ?
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A. Block rotor test, no-load test and stator resistance test

B. Block rotor test and no-load test

C. Block rotor test only

D. No load test only

73. The circle diagram for an induction motor cannot be used to determine____________?
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A. efficiency

B. power factor

C. output

D. frequency

74. For which motor the speed can be controlled from rotor side ?
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A. Squirrel-cage induction motor

B. Slip-ring induction motor

C. Both A. and (b)

D. None of the above

75. The efficiency of an induction motor can be expected to be nearly____________?
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A. 80 to 90%

B. 60 to 90%

C. 99%

D. 95 to 98%

76. Insertion of reactance in the rotor circuit_______?
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A. increases starting torque but maxi-mum torque decreases

B. increases starting torque as well as maximum torque

C. reduces starting torque as well as maximum torque

D. increases starting torque but maxi-mum torque remains unchanged

77. The torque of an induction motor is___________?
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A. proportional to the square of the slip

B. directly proportional to slip

C. inversely proportional to slip

D. none of the above

78. The frame of an induction motor is usually made of____________?
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A. cast iron

B. aluminium

C. silicon steel

D. bronze

79. The cogging of an induction motor can be avoided by____________?
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A. auto-transformer starter

B. having number of rotor slots more or less than the number of stator slots (not equal)

C. using DOL starter

D. proper ventilation

80. The torque developed by a 3-phase induction motor least depends on__________?
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A. shaft diameter

B. rotor power factor

C. rotor current

D. rotor e.m.f.

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