Induction Motor MCQs Part 17

Contents

- 1 An induction motor is favored over a DC motor since it
- 2 The slip of three phase induction motor at no-load is
- 3 A 3-phase slip-ring induction motor develops starting torque at slip of 0.04 with slip-rings short-circuited
- 4 When rotor of three phase induction motor is blocked, the value of slip is
- 5 A 3-phase has 4 pole star-connected stator winding and runs on 230 V, 60 Hz supply

## An induction motor is favored over a DC motor since it

An induction motor is favored over a DC motor since it:

- Provides fine speed control
- Provides high starting torque
- Is simple and rugged in construction
- None of these

Correct answer: 3. Is simple and rugged in construction

## 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

## A 3-phase slip-ring induction motor develops starting torque at slip of 0.04 with slip-rings short-circuited

A 3-phase slip-ring induction motor develops starting torque at slip of 0.04 with slip-rings short-circuited. If rotor resistance is increased to 6 times by inserting external resistance in its rotor circuit, then at rated at rated torque, the slip would be:

- 0.12
- 0.24
- 0.52
- 0.68

Correct answer: 2. 0.24

Solution:

The rated torque of 3-phase induction motor

T = KsR2/(R2² + s² X2²)

——————-

At normal speed s is so small that sX2 is neglible as compared to R2.

Therefore,

T∝s/R2

T/T’ = (0.04/R2) * (6R2/ s)

1 = 0.24/s or s = 0.24

## When rotor of three phase induction motor is blocked, the value of slip is

When rotor of three phase induction motor is blocked, the value of slip is:

- 0.1
- 0.5
- 1
- Zero

Correct answer: 3. 1

Explanation: When rotor is blocked N = 0 and s = Ns/Ns = 1

## A 3-phase has 4 pole star-connected stator winding and runs on 230 V, 60 Hz supply

A 3-phase has 4 pole star-connected stator winding and runs on 230 V, 60 Hz supply. The rotor resistance is 0.2 Ω/phase. The standstill reactance is 0.8 Ω/phase. The full load slip and ratio of stator to rotor turns are 4% and 2 respectively. The speed at maximum torque is:

- 1220 RPM
- 1280 RPM
- 1310 RPM
- 1350 RPM

Correct answer: 4. 1350 RPM

Solution:

Slip at maximum torque (s) = R2/X2 = 0.2/0.8 = 0.25

Speed at maximum torque (N) = (1-s) * Ns

————————–

Ns = 120f/P = 120 * 60 / 4 = 1800

————————-

N = (1 – s) * Ns = ( 1 – 0.25) * 1800 = 1350 RPM