Analytical Methods for Optimal Rotor Step-Skewing To Minimize Cogging Torque in Permanent Magnet Motors

In permanent magnet motors, the existence of cogging torque causes torque ripple and speed fluctuations. Rotor step-skewing is the most commonly adopted method for minimizing the cogging torque in PM motors. In this paper, the general analytical expression for cogging torque with rotor step-skewing is derived based on energy method and Fourier series analysis. Based on the derived analytical expression, rotor step-skewing can be optimized to minimize the cogging torque. Furthermore, the affection of rotor step-skewing on PM torque and reluctance torque are respectively analyzed, which can be used to get the output torque when the cogging torque is minimized with optimal rotor step-skewing. Finally, the correctness of the analytical methods is validated by the finite element analysis (FEA).

I. Introduction

With the development of permanent magnet (PM) materials, permanent magnet motors are more and more widely studied and used. Compared with the motors excited by current, permanent magnet motors offer some advantages such as, low size, high torque-to-current ratio, high efficiency and high power factor. However, the cogging torque, which is the interaction between permanent magnets and stator slots leads to torque ripple and affects the control accuracy of the system, as well as causes noise and other problems. Several methods have been proposed for cogging torque minimization, such as pole arc to pole pitch ratio[1], skewing of rotor magnets[2], skewing of stator slots[3], dummy slots[4], fractional slot PM motors [5], different slot width pairing[6], and etc.. Rotor step-skewing is the most commonly used cogging torque minimization method for PM motors.