Electrical Machines Questions and Answers – Applications of Polyphase Induction Motors

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Applications of Polyphase Induction Motors”.

1. Wound rotor induction motor is most appropriate for the applications requiring ______________
a) high starting torque
b) variable starting torque
c) fixed starting torque
d) all of the mentioned
View Answer

Answer: b
Explanation: A wound rotor induction motor can have variable starting torque.
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2. Relatively a squirrel-cage induction motor is advantageous over wound rotor type induction motor of identical rating due to ___________
a) less conductor material used.
b) lower leakage flux
c) ruggedness
d) all of the mentioned
View Answer

Answer: d
Explanation: A squirrel-cage induction motor is more efficient than a wound rotor induction motor.

3. Relatively a squirrel-cage induction motor is advantageous over wound rotor type induction motor of identical rating due to ___________
a) less conductor material used
b) larger leakage flux
c) large starting torque
d) all of the mentioned
View Answer

Answer: a
Explanation: Squirrel cage induction motor has lesser starting torque.
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4. We can not operate a cage induction motor on a variable frequency load due to ___________
a) no availability of varying the speed
b) availability of variable speed
c) low starting torque
d) fixed slip
View Answer

Answer: a
Explanation: We can not increase or decrease the rotor resistance of a cage induction motor, hence it is not suitable for a variable frequency.

5. Speed control is possible for _______ and not possible for __________
a) induction motor, synchronous motor
b) induction motor, differential motor
c) synchronous motor, synchronous-induction motor
d) dc motor, induction motor
View Answer

Answer: a
Explanation: Speed can be adjusted for a induction motor while it can not be altered for a synchronous motor operating at normal speed.
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6. Which of the following can be done using a synchronous motor but not by induction motor?
a) Power factor improvement
b) Supplying mechanical load
c) Power factor improvement and supply mechanical load
d) None of the mentioned
View Answer

Answer: a
Explanation: Induction motor is a singly excited machine and it always needs exciting current to set up the flux in the machine.

7. The torque in an induction motor varies as ______ and in synchronous motor as _________
a) square of voltage, proportion to voltage
b) proportion to voltage, proportion to voltage
c) proportion to voltage, square of voltage
d) square of voltage, square of voltage
View Answer

Answer: a
Explanation: Torque in induction motor varies as square of the voltage and in synchronous motor as linear to voltage.
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8. If a 3-phase 350V, 50Hz squirrel-cage induction motor is run at 55 Hz supply, then?

A. Starting torque will increase
B. Maximum torque will increase
C. Maximum torque will remain same
D. Starting torque will decrease
E. Operating speed will rise

a) C, D, E
b) A, C, E
c) A, B
d) E
View Answer

Answer: a
Explanation: Frequency is increased, so the starting torque will decrease. Speed will increase as slip will reduce.
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9. When a 3-phase 350V, 50Hz squirrel-cage induction motor is run at 35 Hz supply, then?

A. Starting torque will increase
B. Maximum torque will increase
C. Maximum torque will remain same
D. Starting torque will decrease
E. Operating speed will rise

a) A, C
b) A, B, E
c) A, B
d) C, D, E
View Answer

Answer: a
Explanation: Frequency is decreased so the slip increases. Starting torque will increase and the maximum torque will remain same.

10. Reducing the poles of a 3-phase induction motor, it _________
a) will decrease maximum power factor
b) will increase maximum power factor
c) will make no change in power factor
d) can not be depicted
View Answer

Answer: a
Explanation: If number of poles reduces, speed increases, slip decreases, reactance increases, power factor angle increases and so the power factor decreases.

11. When the poles of a 3-phase wound rotor induction motor increased, it _________
a) will decrease maximum power factor
b) will increase maximum power factor
c) will make no change in power factor
d) can not be depicted
View Answer

Answer: b
Explanation: If number of poles increases, speed decreases, slip increases, reactance decreases, power factor angle decreases and so the power factor increases.

12. A 3-phase induction motor taking a line current of 200 A, is started by direct switching. If an auto transformer of with 50% tapping is made to be used, the motor line current and supply line current will be respectively?
a) 100, 50
b) 50, 100
c) 50, 200
d) 50*1.73, 200
View Answer

Answer: a
Explanation: The motor line current will reduce by a fold of four times and the line current by half.

13. A 3-phase induction motor is running for a constant load torque at rated voltage and frequency. If both the frequency and voltage are halved following will be the related statements ignoring the stator losses.

A. The difference between actual speed and synchronous speed remains same.
B. Per unit slip remains same
C. Stator current remains same
D. Air gap flux remains same

a) A, C, D
b) A, B, C, D
c) B, C, D
d) C, D
View Answer

Answer: a
Explanation: Per unit slip varies as the reactance will change as the frequency has been halved.

14. No load test has been conducted on an induction motor for different supply voltages and input power vs voltage plot has been drawn. The curve is extrapolated to intersect the y-axis. This point is?
a) friction and windage losses
b) core losses
c) stray losses
d) all of the mentioned
View Answer

Answer: a
Explanation: The extrapolated point is friction and windage losses of the induction machine.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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