SOLUTION

Speed control of Induction motor by injecting Slip Frequency E.M.F. into Rotor Circuit

In this method, a voltage is injected into the rotor circuit. The frequency of the rotor circuit is a slip frequency and hence the voltage to be injected must be at a slip frequency.

It is possible that the injected voltage may oppose the rotor-induced e.m f. or may assist the rotor-induced e.m.f.

• If it is in phase opposition, effective rotor resistance increases.
• If it is in the phase of rotor-induced e.m.f., effective rotor resistance decreases.

Since the speed of the rotating magnetic field in the space is equal to the slip sliced, the emf induced to the secondary (stator) also corresponds to the slip frequency. The voltage injected into the stator winding will always be at the slip frequency when the brushes are connected to the stator windings. It is possible to adjust the phase of the voltage injected into the secondary winding, that is, the stator either in-phase or antiphase (out of phase) with the stator-induced voltage.