A three-phase 20 HP. 208 V, 60 Hz, 6 - pole, star - connected induction motor delivers 15 kW at 5% slip. Calculate the frequency of the rotor current.

The synchronous speed is given by

Where,

f is the supply frequency in Hz or C/s

P is the number of poles

The induction motor rotates at a speed (Nr) close but less than the synchronous speed.

Slip of an induction motor is given by,

Where,

Ns is the synchronous speed

and Nr is the rotor speed

Rotor current frequency (fr) = (s)(f)

The slip is negative when the rotor speed is more than the synchronous speed of the rotor field and is in the same direction.

Calculation:

Given, frequency (f) = 60 Hz

Slip (s) = 5 % = 0.05

Rotor current frequency (fr) = 0.05 x 60

fr = 3 Hz