Question

A three-phase induction motor with eight poles runs from a three-phase supply of 50 Hz. What is the synchronous speed of the motor?
1. 3000 rpm
2. 1500 rpm
3. 1000 rpm
4. 750 rpm

Answer 1

Correct Answer - Option 4 : 750 rpm

Concept:

In a three-phase induction motor, synchronous speed is

\({N_s} = \frac{{120f}}{P}\)

Where Ns = Synchronous Speed in rpm

f = supply frequency in Hz

P = number of poles

Rotor speed, Nr = Ns (1 – s)

Calculation:

Given that, 

P = 8, f = 50 Hz

The synchronous speed of the induction motor is,

\({N_s} = \frac{{120 \times 50}}{8} = 750\;rpm\)