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

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Questions
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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