# Induction Motor MCQ Part 4

Induction Motor MCQ Part 4

## What is the approximate efficiency of a 3 phase, 4, 60 Hz induction motor running at an RPM 1200

What is the approximate efficiency of a 3 phase, 4, 60 Hz induction motor running at an RPM 1200:

1. 50%
2. 66.66%
3. 78.98%
4. 86%

Solution:

Solution: Ns = 120*f/P = 1800

s = (Ns – N)/Ns = 0.33

Approximate efficiency = 1 – s = 1 – 0.33 = 0.66 = 66.66%

## A voltmeter gives 180 oscillations per minute when connected to the rotor of the induction motor. Calculate the slip if the supply frequency is 60 Hz

A voltmeter gives 180 oscillations per minute when connected to the rotor of the induction motor. Calculate the slip if the supply frequency is 60 Hz:

1. 2%
2. 5%
3. 8%
4. 11%

Solution:

f’ = 180/60 = 3

s = f’/f = 3/60 = 0.05 = 5%

## Value of rotor reactance as compared to rotor resistance at starting instance of 3 Phase Induction Motor

The rotor reactance of 3 phase induction motor at the instance of starting is ___________ as compared to rotor resistance:

1. Smaller
2. Identical
3. Larger
4. None of the above

## If a 3 phase induction motor is running at certain slip s. Then rotor copper losses expressed mathematically are

If a 3 phase induction motor is running at certain slip s. Then rotor copper losses expressed mathematically are:

1. s * rotor input
2. (1 – s) * rotor input
3. (1 + s) * rotor input
4. 2s * rotor input

Correct answer: 1. s * rotor input

## The induction motor is best described in three words as

The induction motor is best described in three words as:

1. Singly excited machine
2. Doubly excited machine
3. Triply fed machine