What is a slip and frequency of rotor current in stand still and running condition?

When we connect three phase applied to stator winding then rotating flux will set up due to this flux rotor start rotating but it will never catch up the synchronous speed of flux Ns=120 f/p. if it did so then no relative speed between flux and rotor then no EMF will induces, no current and so no torque to maintain rotation. So motor will always rotate less than synchronous speed of magnetic flux Ns = 120 f/p.

The difference between synchronous speed and actual speed Nr is known as slip.

     Slip = Ns - Nr. 

Unit revolution per second.

Express percentage of synchronous speed 

% slip S = ( (Ns -  Nr) /NS) * 100 %

One more term we call it as slip speed

Slip speed = sNs = Ns- Nr 

when rotor is stationary then frequency of rotor current is same as supply frequency. But when rotor start rotating then frequency depend upon relative speed between the two rotating magnetic flux and rotor. Let's say at any sleep Speed the frequency of rotor current is f' .

Ns - Nr = 120f'/ p   -------  (1)

Ns = 120f / p -------------------(2)

Divide equation 1 by question 2 we get

Ns-Nr/ Ns = f'/ f

 Sf = f'

So frequency of rotor current under running condition is

 f' = sf.