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.

I. Introduction

With ever-increasing demand for electric vehicles, the scope for research in the field of electrical drive systems has gained utmost priority. In this context, the choice of motors and associated drives is one of the most critical research components aimed at enhancing the operational efficiency of electric vehicles. Currently, Permanent Magnet Synchronous Motors (PMSM), Brushless DC (BLDC) and Switched reluctance motors (SRM) are predominant owing to their high efficiencies. SRMs do not have permanent magnets or windings on the rotor, are relatively inexpensive, and most importantly, perform well beyond base speeds [1]. However, SRMs are difficult to control owing to their requirement of uni-polar drives and the need to limit drive current supply within the rising slope of the inductance profile, apart from producing higher noise [1] [2]. BLDC motors on the other hand, have a wider air gap than SRMs and are relatively easy to control owing to the ease of identifying commutation instants using Hall sensors. They also have high efficiencies and a high torque to weight ratio [3], but suffer from torque ripple during commutation. PMSM has other advantages like high starting torque and better efficiency with smoother operation due to the sinusoidal air gap flux and no commutation torque ripple [4]. The performance of PMSM is close to ideal, but permanent magnet machines have the drawback of limited or no flux weakening operation [5] [6] and the danger of high voltages in the event of loss of control and a drive that continues to spin. The cost and limited/controlled nature of permanent magnet materials is another disadvantage that has led to exploring other toplologies.