Question

Explain how excitation of the rotor is produced and supplied.

When the 3-phase stator windings are fed by a 3-phasc supply then, a magnetic flux of constant magnitude but rotating at synchronous speed, is set up.

The flux passes through the air-gap, sweeps past the rotor surface and so cuts the rotor conductors which as yet, are stationary.

Due to the relative speed between the rotating flux and the stationary conductors, an e.m.f. is induced in the latter according to Faraday's laws of electro-magneto induction.

The frequency of the induced e.m. f. is the same as the supply frequency.

Its magnitude is proportional to the relative velocity between the flux and the conductors and its direction is given by Fleming's Right-hand rule. Since the rotor bars or conductors form a closed circuit, rotor current is produced whose direction, as given by Lenz's law, is such as to oppose the very cause producing it.

In this case, the cause which produces the rotor current is the relative velocity between the rotating flux of the stator and the stationary rotor conductors. Hence, to reduce the relative speed, the rotor starts running in the same direction as that of the flux and tries to catch up with the rotating flux.