A four-pole, three-phase, wound-rotor induction machine is to be used as a variable-frequency supply. The frequency of the supply connected to the stator is 60 Hz.

(a) Let the rotor be driven at 3600 r/min in either direction by an auxiliary synchronous motor. If the slip-ring voltage is 20 V, what frequencies and voltages can be available at the slip rings when the rotor is at stand still?

(b) If the slip-ring voltage is 400 V, when the rotor frequency is 120Hz, at what speed must the rotor be driven in order to give 150 Hz at the slip-ring terminals?What will the slipring voltage be in this case?