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 D.C. 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. After that, optimum design of the armature pole shape is carried out using design sensitivity analysis and evolution strategy whose design variables are based on the harmonic balance method, and then the results are compared. With the 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.