Reduction of Cogging Torque in Permanent Magnet Motors

Abstract

EPS (Electric Power Steering) enables an increase in automotive fuel efficiency by 3-5% and contributes to the protection of the environment by eliminating the oil medium used in conventional hydraulic power steering. For this reason a sharp increase in demand for EPS is expected. Permanent magnet motors are generally used for EPS but a reduction of cogging torque in these motors is one of the challenges we face to ensure safe and comfortable vehicle handling. In this paper, magnetic field analysis was conducted on brushless motors with radially oriented ring magnets to find effective ways to reduce cogging. Through the investigation of the relationship between coercive force distribution and magnetizing waveform, it was confirmed that the following methods were effective in reducing the cogging torque.
(1) Optimization of the skew magnetization angle according to the coercive force distribution along the axial direction of the magnet.
(2) Control of the magnetizing intensity along of the circumferential direction of the magnet.