Polyphase Induction Motors MCQ Questions & Answers

21

A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen? It will . . . . . . . . .?

A) Stall after sometime B) Stall immediately C) Continue to run at lower speed without damage D) Get heated and subsequently get damaged

Discuss

Answer & Solution

Answer: Option D

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22

The circle diagram for an induction motor cannot be used to determine

A) Efficiency B) Power factor C) Frequency D) Output

Discuss

Answer & Solution

Answer: Option C

Solution:

The circle diagram for an induction motor is a useful tool for analyzing various performance parameters.

It can be used to determine:

Efficiency – By analyzing the power losses and output power on the diagram.

Power factor – By examining the power factor on the diagram based on the reactive and real power components.

Output – By evaluating the output power and losses.

However, the circle diagram does not provide direct information about the frequency of the induction motor. The frequency is typically determined separately through other means or directly from the motor's specifications.

Therefore, Option C is the correct answer as it indicates what cannot be determined using the circle diagram.

23

Short-circuit test on an induction motor cannot be used to determine

A) Windage losses B) Copper losses C) Transformation ratio D) Power scale of circle diagram

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Answer & Solution

Answer: Option A

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24

The torque of a rotor in an induction motor under running condition is maximum

A) At the unit value of slip B) At the zero value of slip C) At the value of the slip which makes rotor reactance per phase equal to the resistance per phase D) At the value of the slip which makes the rotor reactance half of the rotor

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Answer & Solution

Answer: Option C

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25

What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero?

A) The slip of the motor will be 5% B) The rotor will not run C) The rotor will run at very high speed D) The torque produced will be very large

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Answer & Solution

Answer: Option B

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26

In three-phase induction motors sometimes copper bars are placed deep in the rotor to

A) Improve starting torque B) Reduce copper losses C) Improve efficiency D) Improve power factor

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Answer & Solution

Answer: Option A

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27

The speed characteristics of an induction motor closely resemble the speed-load characteristics of which of the following machines

A) D.C. series motor B) D.C. shunt motor C) Universal motor D) None of the above

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Answer & Solution

Answer: Option B

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28

Step-less 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

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Answer & Solution

Answer: Option B

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29

The starting torque of a 3 phase squirrel cage induction motor is

A) Twice the full load torque B) 1.5 times the full load torque C) Equal to full load torque D) Any of the above

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Answer & Solution

Answer: Option B

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30

When the equivalent circuit diagram of double 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

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Answer & Solution

Answer: Option B

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Polyphase Induction Motors MCQ Questions & Answers | Electrical Engineering