Analysis and design of a dual-rotor axial-flux vernier permanent magnet machine

This paper proposes a dual-rotor, toroidal-winding, axial-flux vernier permanent magnet (VPM) machine. By the combination of toroidal windings with the rotor-stator-rotor topology, the end winding length of the machine is significantly reduced when compared with the regular VPM machine. Based on the airgap permeance function, the back-EMF and torque expressions are derived, through which the nature of this 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 torque performance is then analytically investigated. Both the quasi-3-dimensional (quasi-3D) finite element analysis (FEA) and 3D FEA are applied to verify the theoretical analysis. With the current density of 4.2 A/mm², the torque density of the proposed machine can reach 32.6 kNm/m3. A prototype has been designed and is in manufacturing process. Experimental validation will be presented in the future.