Design techniques for reducing the cogging torque of a permanent-magnet synchronous motor

Many studies in minimization of the cogging torque in permanent-magnet synchronous motors (PMSM) have been presented [1, 2, 3]. This proves the high importance of minimization of the torque ripple not only in top quality PMSM motor drives but also in mass-produced PMSMs. In chapter 2 the paper quantifies the motor instantaneous torque M(alpha) into two components - constant component M-sr and periodic component M-izm. The latter gives rise to the undesired torque ripple (Fig. 1), caused by the following: harmonics in the back EMF, cogging torque, and reluctance torque. Chapters 3 and 4 present various design techniques for reducing the cogging torque. The influence of magnet span am is described using a simple model (Fig. 2). A parametric 2D finite element method (FEM) model of a 36-slot and 6-pole PMSM was developed for the study (Fig. 3). In order to reduce the cogging torque, the magnet span had to be almost an integer number of the slot pitch (Figs. 4 and 5). Shifting of the permanent magnet (PM) poles was analysed with an upgraded FEM model (Fig. 7). The achieved significant reduction of the cogging torque is presented in Fig. 8. Various shapes and PMs magnetisation patterns (Fig. 9), dictate air-gap flux distribution and considerably effect the cogging torque (Fig. 10). A summary of the rotor design techniques is presented in Table 1. It is shown that they are all are very efficient. As to the stator, additional notches in its teeth were implemented (Fig. 11). A minimal cogging torque reduction can be observed in Fig. 12. This design technique can be effective in conjunction with some other method, such as skewing. A magnetic anisotropy of the laminated core is considered on a 12-slot and 10-pole PMSM because of its strong influence on the cogging torque in motors where the number of poles is close to the number of stator slots (Fig. 13).