Abstract

Vernier permanent magnet (VPM) machines can be utilized for direct drive applications by virtue of their high torque density and high efficiency. The purpose of this paper is to develop a general design guideline for split-slot low-speed VPM machines, generalize the operation principle, and illustrate the relationship among the numbers of the stator slots, coil poles, permanent magnet (PM) pole pairs, thereby laying a solid foundation for the design of various kinds of VPM machines. Depending on the PM locations, three newly designed VPM machines are reported in this paper and they are referred to as 1) rotor-PM Vernier machine, 2) stator-tooth-PM Vernier machine, and 3) stator-yoke-PM Vernier machine. The back-electromotive force (back-EMF) waveforms, static torque, and air-gap field distribution are predicted using time-stepping finite element method (TS-FEM). The performances of the proposed VPM machines are compared and reported.

Introduction

WITH a growing concern about fossil fuel depletion and environmental pollution crises, electric propulsion systems are increasingly being used in motorcycles, automobiles, trains, vessels, and as generators for wind turbines. Low-speed direct-drive gearless machines are more meritorious than their high-speed counterparts with gear boxes because the former have no associated gear and no related problems such as oil maintenance, noise issue, losses, and so on. However, conventional direct-drive machines suffer from bulkiness and low efficiency. Vernier permanent magnet (VPM) machines which exploit magnetic gear effects to modulate the magnetic fields are capable of producing high torque at a relatively low speed [1], [2]. Due to their structural compactness, high torque density, and high efficiency, VPM machines are desirable for direct-drive applications [3], [4].