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Polyphase Induction Motors

Study 99 Electrical Engineering MCQs with correct answers and explanations.

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41Electrical EngineeringMedium

When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered________?

Ain series

Bin series-parallel

Cin parallel with stator

Din parallel

Correct answer

D. in parallel

Explanation

The correct answer is in parallel.

42Electrical EngineeringMedium

A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be________?

A0.2 Hz

B2 Hz

C200 Hz

D50 Hz

Correct answer

B. 2 Hz

Explanation

The correct answer is 2 Hz.

43Electrical EngineeringMedium

A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be________?

A0.01

B0.04

C0.02

D0.03

Correct answer

B. 0.04

Explanation

The correct answer is 0.04.

44Electrical EngineeringMedium

An induction motor can run at synchronous speed when________?

Ait is run on voltage higher than the rated voltage

Be.m.f. is injected in the rotor circuit

Cit is run on load

Dit is run in reverse direction

Correct answer

B. e.m.f. is injected in the rotor circuit

Explanation

The correct answer is e.m.f. is injected in the rotor circuit.

45Electrical EngineeringMedium

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

Aimprove power factor

Bimprove efficiency

Creduce copper losses

Dimprove starting torque

Correct answer

D. improve starting torque

Explanation

The correct answer is improve starting torque.

46Electrical EngineeringMedium

A squirrel cage induction motor is not selected when________?

Ainitial cost is the main consideration

Bhigher starting torque is the main consideration

Cmaintenance cost is to be kept low

Dall above considerations are involved

Correct answer

B. higher starting torque is the main consideration

Explanation

The correct answer is higher starting torque is the main consideration.

47Electrical EngineeringMedium

The rotor of an induction motor runs at________?

Aabove synchronous speed

Bbelow synchronous speed

Cany of the above

Dsynchronous speed

Correct answer

B. below synchronous speed

Explanation

The correct answer is below synchronous speed.

48Electrical EngineeringMedium

In case of 3-phase induction motors, plugging means________?

Apulling the motor directly on line without a starter

Blocking of rotor due to harmonics

Cinterchanging two supply phases for quick stopping

Dstarting the motor on load which is more than the rated load

Correct answer

C. interchanging two supply phases for quick stopping

Explanation

The correct answer is interchanging two supply phases for quick stopping.

49Electrical EngineeringMedium

In squirrel cage induction motors, the rotor slots are usually given slight skew in order to________?

Areduce windage losses

Breduce magnetic hum

Creduce eddy currents

Dreduce accumulation of dirt and dust

Correct answer

B. reduce magnetic hum

Explanation

The correct answer is reduce magnetic hum.

50Electrical EngineeringMedium

In a three-phase induction motor________?

Apower factor at starting in the same as that while running

Bpower factor at starting is high as compared to that while running

Cpower factor at starting is low as compared to that while running

Correct answer

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

Explanation

The correct answer is power factor at starting is low as compared to that while running.

A squirrel cage induction motor is not selected when________?

Ainitial cost is the main consideration

Bhigher starting torque is the main consideration

Cmaintenance cost is to be kept low

Dall above considerations are involved

Correct answer

B. higher starting torque is the main consideration

Explanation

The correct answer is higher starting torque is the main consideration.

In case of 3-phase induction motors, plugging means________?

Apulling the motor directly on line without a starter

Blocking of rotor due to harmonics

Cinterchanging two supply phases for quick stopping

Dstarting the motor on load which is more than the rated load

Correct answer

C. interchanging two supply phases for quick stopping

Explanation

The correct answer is interchanging two supply phases for quick stopping.

In a three-phase induction motor________?

Apower factor at starting in the same as that while running

Bpower factor at starting is high as compared to that while running

Cpower factor at starting is low as compared to that while running

Correct answer

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

Explanation

The correct answer is power factor at starting is low as compared to that while running.