Analysis of a Dual-Rotor, Toroidal-Winding, Axial-Flux Vernier Permanent Magnet Machine

In this paper, a vernier permanent magnet (VPM) machine with dual-rotor toroidal-winding axial-flux topology is analyzed. Through the combination of toroidal windings with the rotor-stator-rotor topology, the end winding length of the proposed machine is significantly reduced compared with that of the regular VPM machine. Based on the airgap permeance function, the back-EMF and torque expressions are derived and the nature of the machine is revealed. The influence of pole ratio (ratio of rotor pole pair number to stator pole pair number) and main geometric parameters, such as slot opening, magnet thickness, etc., on electromagnetic performance is then analytically investigated in detail. Both the quasi-three-dimensional (quasi-3D) finite element analysis (FEA) and 3-D FEA are applied to verify the derived equations. Finally, a prototype of the proposed machine has been manufactured and experimentally evaluated. The test results agree well with both theoretical and FEA analyses. With the current density of 5.8 A/mm ² and slot fill factor of 0.35, the torque density of the proposed machine prototype can reach 31.9 kNm/m ³.

I. Introduction

With the merits of high torque density and compact mechanical structure, vernier permanent magnet (VPM) machines are attracting more and more research interests [1]– [4]. Due to the so-called magnetic gearing effect [1], the rotor of a VPM machine rotates at a much lower speed than that of the stator magnetic field, which makes this kind of machine a promising alternative for low-speed, high-torque, direct-drive applications such as wind power and ship propulsion.