Rotor e.m.f. :
When the rotor is stationery, an induction motor is equivalent to a 3-phase transformer with secondary short-circuited. Therefore, the induced e.m.f. per phase E2 in the rotor at the instant of starting is given as:
When the rotor starts gaining speed, the relative speed of the rotor with respect to stator flux i.e. slip, is also decreased and is given by sE2. Hence of slip ‘s’, the induced e.m.f. in the rotor is s times the induced e.m.f. in the rotor at standstill.
Rotor current :