Unity Power-Factor Controller for a Synchronous Motor

A simple electronic controller for maintaining the input power factor of a synchronous motor, close to unity, irrespective of load is described. The controller is applicable to a synchronous motor with an additional quadrature field winding. An electronic feedback circuit detects the armature current and causes variation of firing angle of a controlled bridge rectifier which provides the excitation for the quadrature field winding. The induced EMF due to the quadrature field winding cancels out the synchronous reactance voltage drop, thereby keeping the power factor close to unity without the need of changing the main field excitation. A prototype controller is constructed and experimental results are presented.