Multiple choice questions on Induction Motor Part 4 with brief explanatory answers.

Contents

- 1 The slip of three-phase induction motor at no-load is
- 2 The number of stator windings in a four-speed squirrel cage induction motor are
- 3 The rotor core loss of 3 phase induction motor at no-load is
- 4 For a three-phase induction motor, the reactance of the rotor circuit is maximum at
- 5 The stator of 3-phase induction motor produces magnetic field which by nature is
- 6 What happens to the power factor of rotor circuit of 3 phase induction motor when its slip increases
- 7 A 3-phase induction motor whose stator is delta connected is carrying full load. If one of its fuses blows out then the motor
- 8 An induction motor is
- 9 If air-gap is increased in an induction motor
- 10 In three-phase induction motor, the number of poles in the rotor winding is always

## The slip of three-phase induction motor at no-load is

The slip of three phase induction motor at no-load is:

- Practically zero
- 0.4
- 0.525
- 1

Correct answer: 1. Practically zero

## The number of stator windings in a four-speed squirrel cage induction motor are

The number of stator windings in a four-speed squirrel cage induction motor are:

- One
- Two
- Three
- Four

Correct answer: 2. Two

## The rotor core loss of 3 phase induction motor at no-load is

The rotor core loss of 3 phase induction motor at no-load is:

- Practically zero
- Small
- Large
- Half of the full load

Correct answer: 1. Practically zero

## For a three-phase induction motor, the reactance of the rotor circuit is maximum at

For a three-phase induction motor the reactance of rotor circuit is maximum at:

- No load
- Half load
- Full load
- Starting

Correct answer: 4. Starting

Explanation:

Rotor reactance = s * Rotor reactance at standstill position

Under starting condition s = 1, Hence the rotor reactance will be maximum at starting

## The stator of 3-phase induction motor produces magnetic field which by nature is

The stator of 3-phase induction motor produces magnetic field which by nature is

- Steady in nature
- Rotating
- Alternating
- None of above

Correct answer: 2. Rotating

## What happens to the power factor of rotor circuit of 3 phase induction motor when its slip increases

What happens to the power factor of rotor circuit of 3 phase induction motor when its slip increases:

- P.F increases
- P.F decreases
- P.F remains constant
- P.F is independent of slip

Correct answer: 2. P.F decreases

Explanation:

Rotor P.F = Rotor Resistance/Rotor reactance

With an increase in slip the rotor reactance increases (due to increase in frequency) while the rotor resistance is constant. Thus with increase of slip, the P.F decreases.

## A 3-phase induction motor whose stator is delta connected is carrying full load. If one of its fuses blows out then the motor

A 3-phase induction motor whose stator is delta connected is carrying full load. If one of its fuses blows out then the motor:

- Will continue running burning its two phases
- Will stop and carry heavy current causing permanent damage to its winding
- Will continue running without any harm to the winding
- Will continue running burning its one phase

Correct answer: 4. Will continue running burning its one phase

## An induction motor is

An induction motor is:

- Self-starting with low torque
- Self-starting with zero torque
- Not self-starting
- Self-starting with high torque

Correct answer: 1. Self-starting with low torque

## If air-gap is increased in an induction motor

If air-gap is increased in an induction motor:

- Bearing friction will reduce
- The power factor will be low
- Copper loss will be reduced in an induction motor
- Windage losses will be more

Correct answer: 2. The power factor will be low

## In three-phase induction motor, the number of poles in the rotor winding is always

In three-phase induction motor, the number of poles in the rotor winding is always:

- More than the number of poles in stator
- Less than number of poles in stator
- Equal to number of poles in stator
- Zero

Correct answer: 3. Equal to number of poles in stator