Effect of Slip on The Rotor Circuit
Effect of Slip on the Rotor Circuit:
When the rotor is stationary, s = 1. Under these conditions, the per phase rotor e.m.f. E2 has a frequency equal to that of supply frequency f. At any slip s, the relative speed between stator field and the rotor is decreased. Consequently, the rotor e.m.f. and frequency are reduced proportionally to sEs and sf respectively. At the same time, per phase rotor reactance X2, being frequency dependent, is reduced to sX2.
Consider a 6-pole, 3-phase, 50 Hz induction motor. It has synchronous speed Ns = 120 f/P = 120 ´ 50/6 = 1000 r.p.m. At standsill, the relative speed between stator flux and rotor is 1000 r.p.m. and rotor e.m.f./phase = E2(say). If the fullload speed of the motor is 960 r.p.m., then,
(i) The relative speed between stator flux and the rotor is now only 40 r.p.m. Consequently, rotor e.m.f./phase is reduced to:
(ii) The frequency is also reduced in the same ratio to:
(iii) The per phase rotor reactance X2 is likewise reduced to:
Thus at any slip s,
Rotor e.m.f./phase = sE2
Rotor reactance/phase = sX2
Rotor frequency = sf
where E2,X2 and f are the corresponding values at standstill.