Magnetic pole shape optimization of permanent magnet motor for reduction of cogging torque

Cogging torque reduction methods are discussed and applied to the optimum design of a small brushless DC motor. Because the cogging torque has a close relation with the distribution of the magnetization, the magnetizing system for permanent magnets is analyzed numerically by using the time-stepping finite element method. Based on the remnant magnetic flux densities, the cogging torque is computed by using finite element analysis. Optimum design of the armature pole shape is then carried out using design sensitivity analysis and an evolution strategy whose design variables are based on the harmonic balance method, and the results are compared. With these design results, sample motors are constructed and cogging torques are measured. By comparing the computational and experimental results, the problems in the usage of the numerical design method are discussed.