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Polyphase Induction Motors General Knowledge Quiz | Polyphase Induction Motors General Knowledge Objective Type Questions & Answers

(21) Which of the following component is usually fabricated out of silicon steel ?
[A] Bearings
[B] Shaft
[C] Statorcore
[D] None of the above
Answer: Statorcore
(22) The ‘crawling” in an induction motor is caused by
[A] high loads
[B] low voltage supply
[C] improper design of machine
[D] harmonics developed in the motor
Answer: harmonics developed in the motor

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(23) The speed of a squirrel-cage induction motor can be controlled by all of the following except
[A] changing supply frequency
[B] changing number of poles
[C] changing winding resistance
[D] reducing supply voltage
Answer: changing winding resistance
(24) DOL starting of induction motors is usually restricted to
[A] low horsepower motors
[B] variable speed motors
[C] high horsepower motors
[D] high speed motors
Answer: low horsepower motors
(25) In a three-phase induction motor, the number of poles in the rotor winding is always
[A] zero
[B] more than the number of poles in stator
[C] less than number of poles in stator
[D] equal to number of poles in stator
Answer: equal to number of poles in stator
(26) In three-phase squirrel-cage induction motors
[A] rotor conductor ends are short-circuited through slip rings
[B] rotor conductors are short-circuited through end rings
[C] rotor conductors are kept open
[D] rotor conductors are connected to insulation
Answer: rotor conductors are short-circuited through end rings
(27) The maximum torque in an induction motor depends on
[A] frequency
[B] rotor inductive reactance
[C] square of supply voltage
[D] all of the above
Answer: all of the above
(28) An induction motor is
[A] self-starting with zero torque
[B] self-starting with high torque
[C] self-starting with low torque
[D] non-self starting
Answer: self-starting with low torque
(29) If any two phases for an induction motor are interchanged
[A] the motor will run in reverse direction
[B] the motor will run at reduced speed
[C] the motor will not run
[D] the motor will burn
Answer: the motor will run in reverse direction
(30) For which motor the speed can be controlled from rotor side ?
[A] Squirrel-cage induction motor
[B] Slip-ring induction motor
[C] Both (a) and (b)
[D] None of the above
Answer: Slip-ring induction motor
31 In the circle diagram for induction motor, the diameter of the circle represents
[A] slip
[B] rotor current
[C] running torque
[D] line voltage
Answer: rotor current
32 Rotor rheostat control method of speed control is used for
[A] squirrel-cage induction motors only
[B] slip ring induction motors only
[C] both (a) and (b)
[D] none of the above
Answer: slip ring induction motors only
33 Stepless speed control of induction motor is possible by which of the following methods ?
[A] e.m.f. injection in rotor eueuit
[B] Changing the number of poles
[C] Cascade operation
[D] None of the above
Answer: Changing the number of poles
34 It is advisable to avoid line-starting of induction motor and use starter because
[A] motor takes five to seven times its full load current
[B] it will pick-up very high speed and may go out of step
[C] it will run in reverse direction
[D] starting torque is very high
Answer: motor takes five to seven times its full load current
35 When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered
[A] in series
[B] in parallel
[C] in series-parallel
[D] in parallel with stator
Answer: in parallel
36 The torque developed in the cage induction motor with autostarter is
[A] K2 x torque with direct switching
[B] k2/torque with direct switching
[C] k/torque with direct switching
[D] K x torque with direct switching
Answer: K2 x torque with direct switching
37 The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type
[A] star connected only
[B] delta connected only
[C] (a) and (b) both
[D] none of the above
Answer: (a) and (b) both
38 The crawling in the induction motor is caused by
[A] low voltage supply
[B] high loads
[C] harmonics develped in the motor
[D] improper design of the machine
Answer: harmonics develped in the motor
39 Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor ?
[A] By changing the number of stator poles
[B] Rotor rheostat control
[C] By operating two motors in cascade
[D] By injecting e.m.f. in the rotor circuit
Answer: By changing the number of stator poles
40 The injected e.m.f. in the rotor of induction motor must have
[A] zero frequency
[B] the same frequency as the slip frequency
[C] the same phase as the rotor e.m.f.
[D] high value for the satisfactory speed control
Answer: the same frequency as the slip frequency

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