Comparisons of Torque Performance in Surface-Mounted PM Vernier Machines With Different Stator Tooth Topologies

Permanent magnet vernier (PMV) machines, as one of the flux modulation machines, have gained considerable attention due to inherent high torque density and low torque ripple. This paper mainly investigates the torque performance of PMV machines with different stator tooth topologies. First, it begins with the theoretical analysis of surface-mounted PMV machines, which has a generic number of flux modulation poles (FMPs), PM poles, and armature poles. Then, the expression of flux linkage, back-electromotive force (EMF) and electromagnetic torque are derived successively, where the equivalent winding factor is proposed and defined to evaluate the flux modulation of stator auxiliary teeth and multi-harmonics coupling effect. Next, the back EMF, torque, power factor, flux-weakening capability, and PM demagnetization of PMV machines with different number of stator teeth are analyzed and compared. The final results of theoretical analysis and finite-element analysis can match well. Meanwhile, the non-uniform distribution of FMPs for increasing the average torque is first proposed and analyzed. A parameter K titled as FMP ratio is defined to evaluate the relationship among the angles among FMPs, the influence of the variation of K on the back EMF and torque is deeply explored and analyzed. Besides, the optimal range of K is carefully validated, which will help to optimize the design parameters more conveniently in the future.

I. Introduction

In recent years, permanent magnet (PM) direct-drive machines have gained considerable attention due to competitive torque density, high transmission efficiency, and low vibration noise [1], [2]. However, the higher torque demand always leads to larger machine size and inevitable material consumption [3]. At present, the flux-modulation machines with larger torque density have emerged based on the so-called flux-modulation effect.