# Induction Motor MCQs Part 18

## A 3 phase, 8 pole, 60 Hz induction motor has equivalent resistor of 0.05 Ω/phase. If stalling speed is 650 RPM. The rotor reactance/phase is

A 3 phase, 8 pole, 60 Hz induction motor has equivalent resistor of 0.05 Ω/phase. If stalling speed is 650 RPM. The rotor reactance/phase is:

1. 0.08 Ω
2. 0.18 Ω
3. 0.25 Ω
4. 0.4 Ω

Solution:

Synchronous speed Ns = 120f/P

Ns = 120 * 60/8 = 900 RPM

Slip (s) = Ns – N/Ns = (900 – 650)/900 = 0.27

Since torque is max (stalling) s = R2/X2

OR X2 = R2/s

X2 = 0.05/0.27 = 0.18 Ω

## A 12 pole, 3-phase alternator is coupled to an engine that is running at 600 RPM. The alternator supplied 3-phase induction motor that has full-load speed of 1480 RPM. Number of poles of motor are

A 12 pole, 3-phase alternator is coupled to an engine that is running at 600 RPM. The alternator supplied 3-phase induction motor that has full-load speed of 1480 RPM. Number of poles of motor are:

1. 2
2. 4
3. 6
4. 8

Solution:

Frequency of voltage generated by alternator is:

f = NP/120 = 600 * 12 / 120 = 60 Hz

———————

Synchronous speed at 60 Hz Ns = f * 120/P

For Pole number = 2 The Speed = 3000

For Pole number = 4 The Speed = 1500

For Pole number = 6 The Speed = 1000

For Pole number = 8 The Speed = 750

For Pole number = 10 The Speed = 600

———————

Since the nearest speed to 1480 RPM is 1500 RPM, therefore the number of poles of motor are 4

## The rotor of three phase wound rotor induction motor is loaded with

The rotor of three phase wound rotor induction motor is loaded with:

1. Single phase winding
2. Three phase winding
3. Copper bars
4. Squirrel cage induction rotor with bars tilted at 45 degrees

Correct answer: 2. Three phase winding

## The efficiency of an induction motor is approximately

The efficiency of an induction motor is approximately

1. 95%
2. 80-90%
3. 50-60%
4. 40-50%