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

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

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,

\(s = \frac{{{N_s} - {N_r}}}{{{N_s}}}\)

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