It has been shown in chapter 1 that the polyphase induction motor can be considered as a polyphase transformer with variable frequency in the short-circuited secondary winding and an equivalent circuit can be derived per phase of the motor in the same general way as that of a transformer. It is apparent that each winding will have resistance and leakage inductance. The leakage reactance of the rotor will, however, be dependent on the frequency of the rotor current and, since the rotor frequency is Sf, the slip frequency, where S=(N S −N)/N S is the slip and f is the supply frequency, the rotor leakage reactance is given by SX2 where X2 is the standstill value of the rotor leakage reactance. Thus an equivalent circuit for the rotor can be drawn as shown in Fig. 5.1.