Electrical Engineering MCQs on Induction Motor Part 18

Contents

- 1 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
- 2 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
- 3 The rotor of three phase wound rotor induction motor is loaded with
- 4 The efficiency of an induction motor is approximately
- 5 As compared to rotor resistance, the rotor reactance of three phase induction motor at starting 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

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:

- 0.08 Ω
- 0.18 Ω
- 0.25 Ω
- 0.4 Ω

Correct answer: 2. 0.18 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:

- 2
- 4
- 6
- 8

Correct answer: 2. 4

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:

- Single phase winding
- Three phase winding
- Copper bars
- 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

- 95%
- 80-90%
- 50-60%
- 40-50%

Correct answer: 2. 80-90%

## As compared to rotor resistance, the rotor reactance of three phase induction motor at starting is

As compared to rotor resistance, the rotor reactance of three phase induction motor at starting is:

- Smaller
- Larger
- Just identical
- None of above

Correct answer: 2. Larger

See complete list of Electrical Engineering MCQs here