Page 714 - Basic Electrical Engineering

The motor speed is 1440 rpm, which should be slightly less than the
synchronous speed. Logically, the number of poles of the motor must be 4.

Percentageslip,

Example 8.7 A 400 V, 50 Hz, three-phase induction motor is rotating at
960 rpm on full load. Calculate the following for the motor:

Number of poles; full-load slip; frequency of rotor-induced EMF; speed of
the rotor magnetic field with respect to the rotor.

Also show that both the stator field and the rotor field are stationary with
respect to each other.

Solution:
f = 50 Hz

P = 2, 4, 6, etc.

Rotor speed is somewhat less than the synchronous speed N . Logically, here
s
N can only be 1500 rpm, when N = 1500 rpm, P = 4.
s
s

Full load slip,

Frequency of rotor induced EMF f = S × f = 0.04 × 50
r
= 2 Hz

Speed of rotor field with respect to rotor, N is

The rotor rotates at a speed of 1440 rpm. This means the speed of the rotor
with respect to stator, which is stationary, is 1440 rpm.