Synchronous Induction Motor
In the applications where high starting torque and constant speed are desired then synchronous induction motor can be used. It has the advantages of both synchronous motor and induction motor. The synchronous motor gives constant speed whereas induction motors can be started against full load torque.
Consider a normal slip ring induction motor having three phase winding on the rotor .The motor is connected to the exciter which gives D.C. supply to the rotor through slip rings. One phase carries full D.C. current while the other two carries half the full D.C. current as they are connected in parallel. Due to this D.C. excitation, permanent poles (N and S) formed on the rotor.
Initially it is run as a slip ring induction motor with the help of starting resistances. When the resistances are cut out the motor runs with a slip. Now the connections are changed and the exciter is connected in series with the rotor windings which will remain in the circuit permanently.
As the motor is running as induction motor initially high starting torque (up to twice full load value) can be developed. When the D.C. excitation is provided it is pulled into synchronism and starts running at constant speed. Thus synchronous induction motor provides constant speed, large starting torque, low starting current and power factor correction.