slip and rotor frequency of induction motor
We know that rotor rapidly accelerates in the direction of rotating field. In practice, the rotor can never reach the speed of stator flux. If it did, there would be no relative speed between the stator field and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor. The friction and windage would immediately cause the rotor to slow down. Hence, the rotor speed (N) is always less than the suitor field speed (Ns). This difference in speed depends upon load on the motor. The difference between the synchronous speed Ns of the rotating stator field and the actual rotor speed N is called slip. It is usually expressed as a percentage of synchronous speed i.e.,
(i) The quantity Ns - N is sometimes called slip speed.
(ii) When the rotor is stationary (i.e., N = 0), slip, s = 1 or 100 %.
(iii) In an induction motor, the change in slip from no-load to full-load is hardly 0.1% to 3% so that it is essentially a constant-speed motor.
Rotor Current Frequency:
The frequency of a voltage or current induced due to the relative speed between a vending and a magnetic field is given by the general formula;
where N = Relative speed between magnetic field and the winding
P = Number of poles
For a rotor speed N, the relative speed between the rotating flux and the rotor is Ns - N. Consequently, the rotor current frequency f' is given by;
i.e., Rotor current frequency = Fractional slip x Supply frequency
(i) When the rotor is at standstill or stationary (i.e., s = 1), the frequency of rotor current is the same as that of supply frequency (f' = sf = 1´ f = f).
(ii) As the rotor picks up speed, the relative speed between the rotating flux and the rotor decreases. Consequently, the slip s and hence rotor current frequency decreases.