Polyphase Induction Motors: Multiple Choice Questions

32+ Multiple Choice Questions and Answers related to Polyphase Induction Motors in Electrical and Electronics Engineering.

 

Polyphase Induction Motors

 

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

View Answer

B. rotor conductors are short-circuited through end rings

 

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

View Answer

C. self-starting with low torque

 

The starting torque of a squirrel-cage induction motor is ?

A. low
B. negligible
C. same as full-load torque
D. slightly more than full-load torque

View Answer

A. low

 

In an induction motor no-load the slip is generally ?

A. less than 1%
B. 1.5%
C. 2%
D. 4%

View Answer

A. less than 1%

 

The frame of an induction motor is usually made of ?

A. silicon steel
B. cast iron
C. aluminium
D. bronze

View Answer

B. cast iron

 

In a star-delta starter of an induction motor ?

A. resistance is inserted in the stator
B. reduced voltage is applied to the stator
C. resistance is inserted in the rotor
D. applied voltage perl stator phase is 57.7% of the line voltage

View Answer

D. applied voltage perl stator phase is 57.7% of the line voltage

 

Polyphase Induction Motors

 

Insertion of resistance in the stator of an induction motor ?

A. increases the load torque
B. decreases the starting torque
C. increases the starting torque
D. none of the above

View Answer

B. decreases the starting torque

 

The starting torque of a three phase induction motor can be increased by ?

A. increasing slip
B. increasing current
C. both A. and (b)
D. none of the above

View Answer

C. both A. and (b)

 

In an induction motor if the air gap is increased than which of the following statement is correct ?

A. the power factor will be low
B. windage losses will be more
C. bearing friction will reduce
D. copper loss will reduce In an induction motor

View Answer

A. the power factor will be low

 

Lubricant used for ball bearing is usually ?

A. graphite
B. grease
C. mineral oil
D. molasses

View Answer

B. grease

 

The shape of the torque slip curve of induction motor is ?

A. parabola
B. hyperbola
C. rectangular parabola
D. straight line

View Answer

C. rectangular parabola

 

In a three-phase induction motor ?

A. power factor at starting is high as compared to that while running
B. power factor at starting is low as compared to that while running
C. power factor at starting in the same as that while running
D. None of the above

View Answer

B. power factor at starting is low as compared to that while running

 

Polyphase Induction Motors

 

Which one is correct about a three-phase induction motor ?

A. iron losses in stator will be negligible as compared to that in rotor
B. iron losses in motor will be neg¬ligible as compared to that in rotor
C. iron losses in stator will be less than that in rotor
D. iron losses in stator will be more than that in rotor

View Answer

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

 

Slip ring motor is recommended where ?

A. speed control is required
B. frequent starting, stopping and reversing is required
C. high starting torque is needed
D. all above features are required

View Answer

D. all above features are required

 

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

View Answer

D. harmonics developed in the motor

 

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

View Answer

A. low horsepower motors

 

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

View Answer

C. harmonics develped in the motor

 

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

View Answer

D. all of the above

 

Polyphase Induction Motors

 

An induction motor is identical to ?

A. D.C. compound motor
B. D.C. series motor
C. synchronous motor
D. asynchronous motor

View Answer

D. asynchronous motor

 

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

View Answer

A. the motor will run in reverse direction

 

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

View Answer

B. slip ring induction motors only

 

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

View Answer

B. Slip-ring induction motor

 

Which of the following component is usually fabricated out of silicon steel ?

A. Bearings
B. Shaft
C. Statorcore
D. None of the above

View Answer

C. Statorcore

 

In case of the induction motors the torque is ?

A. inversely proportional to (Vslip)
B. directly proportional to (slip)2
C. inversely proportional to slip
D. directly proportional to slip

View Answer

D. directly proportional to slip

 

Polyphase Induction Motors

 

An induction motor with 1000 r.p.m. speed will have ?

A. 8 poles
B. 6 poles
C. 4 poles
D. 2 poles

View Answer

B. 6 poles

 

A double squirrel-cage induction motor has ?

A. two rotors moving in oppsite direction
B. two parallel windings in stator
C. two parallel windings in rotor
D. two series windings in stator

View Answer

B. two parallel windings in stator

 

The number of slip rings on a squirrel cage induction motor is usually ?

A. two
B. three
C. four
D. none

View Answer

D. none

 

The efficiency of an induction motor can be expected to be nearly ?

A. 60 to 90%
B. 80 to 90%
C. 95 to 98%
D. 99%

View Answer

B. 80 to 90%

 

In case the air gap in an induction motor is increased ?

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

View Answer

B. the power factor will decrease

 

Slip rings are usually made of ?

A. copper
B. carbon
C. phospor bronze
D. aluminium

View Answer

C. phospor bronze

 

Polyphase Induction Motors

 

In case the air gap in an induction motor is increased ?

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

View Answer

B. the power factor will decrease

 

The shaft of an induction motor is made of ?

A. high speed steel
B. stainless steel
C. carbon steel
D. cast iron

View Answer

C. carbon steel

 

The torque of an induction motor is ?

A. directly proportional to slip
B. inversely proportional to slip
C. proportional to the square of the slip
D. none of the above

View Answer

D. none of the above

 

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