A new approach to reduction of the cogging torque in a brushless motor by skewing optimization of permanent magnets

This paper presents a new approach to the skewing optimization of permanent magnet poles in a two-phase brushless d.c. motor in order to effectively reduce its cogging torque. The effect of skewing is obtained by a proper angular dislocation of permanent magnet slices. Skewing optimization is carried out with respect to the number of pole slices and their mutual angular dislocation. A natural criterion of optimization is assumed by the minimum standard deviation of the cogging torque. Since employing a full-size three-dimensional (3D) finite element method model of the motor in the skewing optimization problem would lead to prohibitive computational burdens, a new simplified 3D model is developed for the multisliced pole structure. The simplified method for torque calculations makes the optimization task feasible at reasonable computational cost. Solutions obtained clearly exhibit that the torque pulsation can be effectively reduced by appropriate skew-like shaping of permanent magnets. Simulation results on cogging torque minimization are in very good agreement with experimental data obtained from a prototype motor.