Cogging Torque Suppression in Flux-Switching Permanent Magnet Machines by Superposition of Single Rotor Tooth

Due to specific doubly salient structure and high air-gap flux density, flux switching permanent magnet (FSPM) machines typically suffer from severe cogging torque, which results in undesired torque and speed ripples as well as acoustic noise and vibration, especially at low speeds. In this paper, a new modeling approach is proposed to investigate the cogging torque in FSPM machines, and consequently, novel rotor-teeth-shifting based methods are proposed to reduce cogging torque. Firstly, based on the superposition of each single rotor tooth, an analytical expression of cogging torque in FSPM machines is derived and confirmed by finite element analysis (FEA). Then, three novel techniques, i.e., successively shifting rotor teeth, alternately shifting rotor teeth, and stepped skewing rotor blocks, are proposed for the cogging torque minimization of FSPM machines. The generally analytical expressions of these four methods are deduced, through which the analytically optimal shifting angles can be determined. Finally the effectiveness of these techniques are verified by both FEA and experiments. It turns out to be that these four approaches are so general that they can readily be extended to other FSPM machines.

I. Introduction

With the rapid developments of permanent magnet (PM) materials, brushless machines equipped with PMs are developing dramatically due to the high power (torque) density, high efficiency, excellent dynamic response and high power factor. One such example is found in the flux-switching permanent magnet (FSPM) machines firstly introduced in 1955 [1]. Compared with conventional rotor-PM machines having magnets in rotor, FSPM machines exhibit the merits of convenient heat dissipation, robust rotor structure, large torque capability, and high fault-tolerant capability [2]. From the previous literatures, the most common method for design and analysis of FSPM machines is finite element method (FEM) [3]–[5], though nonlinear variable-network magnetic circuits are also proposed [6]–[10]. Besides, a lot of work focus on the optimizations of the topologies and dimensions of the FSPM machines, e.g., rotor pole width, winding configuration, and combinations of stator slots and rotor poles [11]–[13].