Induction Motor MCQs Part 8. Multiple choice questions on Induction motor with brief explanatory answers and solutions.

Contents

- 1 If one of the lines of a 3 phase induction motor is interchanged, next to that
- 2 The magnetizing current drawn by 3-phase induction motor is about what percentage of full-load stator current
- 3 Under running conditions, the maximum torque of 3-phase induction motor will happen at that value of slip at which rotor resistance/phase is X times the rotor reactance/phase. Here X implies
- 4 What is value of supply frequency of a 4-pole induction motor that has a synchronous speed of 1500 RPM
- 5 If N and Ns are speeds of rotor and rotating fields of a 3-phase induction motor respectively. Then ratio rotor input/rotor output is expressed in the form
- 6 Speed of a squirrel cage induction motor can be changed only by
- 7 A 3-phase induction motor having synchronous speed of 1000 RPM develops 10 mechanical HP at an RPM of 950. If stator loss is 500 W then, What is stator input
- 8 Rotor copper losses of a 3-phase induction motor running at some decimal slip s are mathematically expressed in the form
- 9 What is frequency of induced EMF in a 4-pole, 60 Hz induction motor operates at 5% slip

## If one of the lines of a 3 phase induction motor is interchanged, next to that

If one of the lines of a 3 phase induction motor is interchanged, next to that:

- Flux speed increases
- Flux speed decreases
- Flux speed remains keeping same direction
- Flux speed remains same but direction is reversed

Correct answer: 4. Flux speed remains same but direction is reversed

## The magnetizing current drawn by 3-phase induction motor is about what percentage of full-load stator current

The magnetizing current drawn by 3-phase induction motor is about what percentage of full-load stator current:

- 5%
- 10 to15%
- 15 to 30%
- 30 to 50%

Correct answer: 4. 30 to 50%

The conditions of an induction motor at no-load closely resemble to those of a transformer whose secondary is

Correct answer: 2. Open-circuited

## Under running conditions, the maximum torque of 3-phase induction motor will happen at that value of slip at which rotor resistance/phase is X times the rotor reactance/phase. Here X implies

Under running conditions, the maximum torque of 3-phase induction motor will happen at that value of slip at which rotor resistance/phase is X times the rotor reactance/phase. Here X implies:

- 1 time (equal)
- 5 times
- 10 times
- 15 times

Correct answer: 1. Time (equal)

## What is value of supply frequency of a 4-pole induction motor that has a synchronous speed of 1500 RPM

What is value of supply frequency of a 4-pole induction motor that has a synchronous speed of 1500 RPM:

- 0 Hz
- 50 Hz
- 60 Hz
- 100 Hz

Correct answer: 2. 50 Hz

From formula: Speed = 120*f/P; f = Speed * P/120 = 1500*4/120 = 50 Hz

## If N and Ns are speeds of rotor and rotating fields of a 3-phase induction motor respectively. Then ratio rotor input/rotor output is expressed in the form

If N and Ns are speeds of rotor and rotating fields of a 3-phase induction motor respectively. Then ratio rotor input/rotor output is expressed in the form:

- Ns/N
- N/Ns
- N-Ns
- Ns-N

Correct answer: 1. Ns/N

## Speed of a squirrel cage induction motor can be changed only by

Speed of a squirrel cage induction motor can be changed only by:

- Rheostatic control
- Cascade control
- Changing poles
- None of the above

Correct answer: 3. Changing poles

## A 3-phase induction motor having synchronous speed of 1000 RPM develops 10 mechanical HP at an RPM of 950. If stator loss is 500 W then, What is stator input

A 3-phase induction motor having synchronous speed of 1000 RPM develops 10 mechanical HP at an RPM of 950. If stator loss is 500 W then, What is stator input:

- 4.5 kW
- 8.35 kW
- 12.75 kW
- 15.25 kW

Correct answer: 2. 8.35 kW

Solution:

Stator loss in kW = 500 W = 0.5 kW

Slip (s) = (Ns â N)/Ns = (1000 â 950)/1000 = 0.05

Rotor input = (Mechanical HP * 0.746)/(1-s) = 7.85 kW

Stator input = Rotor input + Stator loss = 7.85 kW + 0.5 kW = 8.35 kW

## Rotor copper losses of a 3-phase induction motor running at some decimal slip s are mathematically expressed in the form

Rotor copper losses of a 3-phase induction motor running at some decimal slip s are mathematically expressed in the form:

- s* rotor input
- 1-s * rotor input
- 1+s * rotor input
- None of these

Correct answer: 1. s * rotor input

## What is frequency of induced EMF in a 4-pole, 60 Hz induction motor operates at 5% slip

What is frequency of induced EMF in a 4-pole, 60 Hz induction motor operates at 5% slip:

- 30Hz
- 60 Hz
- 3 Hz
- None of these

Correct answer: 3. 3 Hz

Solution: Induced rotor frequency (fâ) = s* supply frequency (f)

fâ = 0.05 * 60 = 3 Hz