Question

What is meant by slip in an induction motor? Develop an expression for the frequency of rotor currents in it.

Answer 1

In an induction motor the rotor never runs at the synchronous speed but slightly less than it.

Slip is defined as the ratio of difference in the synchronous speed and rotor speed to the synchronous speed.

s = (Ns-Nr)/Ns  

The frequency of the rotor current will be reduced directly corresponding to slip.

We know that frequency is directly proportional to speed,

fs ∝ Ns

&

fr ∝ Ns - Nr

=> fr/fs = Ns-Nr/Ns

=>   Fr = sFs