Polyphase Induction Motors MCQ Questions & Answers

81

The low power factor of induction motor is due to

A) Rotor leakage reactance B) Stator reactance C) The reactive lagging magnetizing current necessary to generate the magnetic flux D) All of the above

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Answer & Solution

Answer: Option D

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82

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

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Answer & Solution

Answer: Option B

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83

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

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Answer & Solution

Answer: Option A

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84

Slip of an induction motor is negative when

A) Magnetic field and rotor rotate in opposite direction B) Rotor speed is less than the synchronous speed of the field and are in the same direction C) Rotor speed is more than the synchronous speed of the field and are in the same direction D) None of the above

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Answer & Solution

Answer: Option C

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85

The torque developed by a 3-phase induction motor least depends on

A) Rotor current B) Rotor power factor C) Rotor e.m.f. D) Shaft diameter

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Answer & Solution

Answer: Option D

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86

The good power factor of an induction motor can be achieved if the average flux density in the air gap is

A) Absent B) Small C) Large D) Infinity

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Answer & Solution

Answer: Option C

Solution:

Power factor in an induction motor is influenced by the flux density in the air gap.

A good power factor can be achieved when the average flux density in the air gap is large.

This is because a larger flux density reduces the magnetizing current required, leading to a higher power factor.

If the flux density is too small, more magnetizing current is needed, which lowers the power factor.

87

The rotor of an induction motor runs at

A) Synchronous speed B) Below synchronous speed C) Above synchronous speed D) Any of the above

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Answer & Solution

Answer: Option B

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88

In N_s is the synchronous speed and s the slip, then actual running speed of an induction motor will be

A) N_s B) s.N_s C) (1 - s)N_s D) (N_s - 1)s

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Answer & Solution

Answer: Option C

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89

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

A) Bearings B) Shaft C) Stator core D) None of the above

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Answer & Solution

Answer: Option C

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90

Insertion of resistance in the rotor of an induction motor to develop a given torque

A) Decreases the rotor current B) Increases the rotor current C) Rotor current becomes zero D) Rotor current remains same

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Answer & Solution

Answer: Option A

Solution:

Inserting resistance into the rotor circuit of an induction motor increases the total impedance of the rotor circuit.

This increased impedance causes the rotor current to decrease for a given torque since current is inversely proportional to impedance.

The purpose of adding resistance is to control the torque and current characteristics during starting or at low speeds.

Therefore, inserting resistance decreases the rotor current, making Option A the correct answer.

Polyphase Induction Motors MCQ Questions & Answers | Electrical Engineering