polyphase induction motors MCQs – Page 5

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

polyphase induction motors Mcqs

81. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move___________?
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A. in the opposite direction as the direction of the rotating field

B. in any direction depending upon phase squence of supply

C. in the same direction as the direction of the field

82. In a three-phase induction motor_______________?
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A. iron losses in motor will be negligible as compared to that in rotor

B. iron losses in stator will be more than that in rotor

C. iron losses in stator will be negligible as compared to that in rotor

D. iron losses in stator will be less than that in rotor

83. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to__________?
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A. reduce magnetic hum

B. reduce eddy currents

C. reduce accumulation of dirt and dust

D. reduce windage losses

84. As load on an induction motor goes on increasing______________?
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A. its power factor remains constant

B. its power factor goes on decreasing

C. its power factor goes on increasing even after full load

D. its power factor goes on increasing up to full load and then it falls again

85. In case of 3-phase induction motors, plugging means__________?
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A. pulling the motor directly on line without a starter

B. interchanging two supply phases for quick stopping

C. locking of rotor due to harmonics

D. starting the motor on load which is more than the rated load

86. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
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A. The torque produced will be very large

B. The slip of the motor will be 5%

C. The rotor will run at very high speed

D. The rotor will not run

87. The torque of a rotor in an induction motor under running condition is maximum__________?
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A. at the unit value of slip

B. at the value of the slip which makes the rotor reactance half of the rotor

C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase

D. at the zero value of slip

88. In case the air gap in an induction motor is increased?
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A. the magnetising current of the rotor will decrease

B. the power factor will decrease

C. speed of motor will increase

D. the windage losses will increase

89. Lubricant used for ball bearing is usually___________?
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A. graphite

B. mineral oil

C. grease

D. molasses

90. The injected e.m.f. in the rotor of induction motor must have__________?
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A. zero frequency

B. the same phase as the rotor e.m.f.

C. high value for the satisfactory speed control

D. the same frequency as the slip frequency

91. In three-phase squirrel-cage induction motors____________?
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A. rotor conductors are connected to insulation

B. rotor conductors are kept open

C. rotor conductors are short-circuited through end rings

D. rotor conductor ends are short-circuited through slip rings

92. Slip ring motor is recommended where____________?
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A. frequent starting, stopping and reversing is required

B. high starting torque is needed

C. speed control is required

D. all above features are required

93. Which type of bearing is provided in small induction motors to support the rotor shaft ?
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A. Ball bearings

B. Cast iron bearings

C. None of the above

D. Bush bearings

94. Less maintenance troubles are experienced in case of___________?
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A. slip ring induction motor

B. squirrel cage induction motor

C. both A. and (b)

D. none of the above

95. In the circle diagram for induction motor, the diameter of the circle represents____________?
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A. rotor current

B. running torque

C. line voltage

D. slip

96. In medium sized induction motors, the slip is generally around___________?
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A. 0.04%

B. 4%

C. 0.40%

D. 14%

97. An induction motor is identical to_________?
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A. synchronous motor

B. D.C. compound motor

C. D.C. series motor

D. asynchronous motor

98. An induction motor with 1000 r.p.m. speed will have____________?
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A. 2 poles

B. 6 poles

C. 8 poles

D. 4 poles

99. The power factor of an induction motor under no-load conditions will be closer to_____________?
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A. 0.2 lagging

B. 0.5 leading

C. unity

D. 0.2 leading

100. Star-delta starting of motors is not possible in case of____________?
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A. single phase motors

B. high speed motors

C. low horse power motors

D. variable speed motors

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