In Squirrel Cage Induction Motor the rotor has conductors just like damper bars and as we can see in Fig A same number of poles are induced in the rotor as the stator which is an essential condition for Torque production.
In Wound Rotor Induction Motor winding on the rotor should be such that same number of poles are induced in the rotor as the stator.
Whenever current reverses its direction a pole is induced so in wound rotor the winding should be designed such that current reversal takes place as many no times as there are poles on the stator.
As we can see in Fig B the Torque is produced on the rotor in the same direction as the rotating magnetic field because the torque tries to reduce the relative speed (Ns – Nr) between stator and rotor.
In this phasor diagram the torque is produced in such a direction that the rotor mmf tries to catch the stator mmf in order to make a angle between them zero.
