Slip Frequency Excitation for Variable Speed Wound-Field Synchronous Motor

This paper discusses a new wound field variable speed synchronous motor topology that has the exciter coupled to the same shaft as the main motor. The mechanism of excitation is similar to that of a brushless alternator, but since the motor has a varying speed, the relative speed of the exciter rotor flux with respect to its stator is kept constant. This is facilitated by having a slip frequency supply to the exciter stator. In order to validate field production, the main motor is supplied with a slow-ramping three-phase voltage, which causes the main rotor to lock with the field and run as a synchronous motor. The design of such a motor with a precise control mechanism reduces cost, enhances the overall speed-range, and ensures safe field shutdown in the event of loss of normal control, preventing overvoltage due to a fast spinning motor. Most of the currently used commercial motors employ permanent magnet rotors, but the availability of raw materials used for production of this particular subsystem is grossly inadequate in our country, resulting in large-scale imports of the same. As an effort towards import substitution, apart from the objective of enhancing operational efficiency, this study has made a beginning towards substituting permanent magnets with an electromagnet. In this context, this paper details the outcome of using slip frequency excitation coupled with scalar control on the performance of the motor.