Analytical cogging torque prediction for surface-mounted PM machines accounting for different slot sizes and uneven positions

This paper presents a general method for predicting the cogging torque in surface-mounted permanent magnet machines having different slot sizes and uneven positions by synthesizing the cogging torque associated with each slot, which is obtained by the subdomain model. The developed model is applied to analyse the influence of imperfect slot openings on the cogging torque. It shows that machines having more options of optimal slot opening width for cogging torque minimisation are more sensitive to manufacture tolerance of slot opening. The proposed method is also used to analyse and design the dummy slots, which can be the same/different sizes compared with the real slot and uniformly/unevenly distributed. Some candidates of dummy slot sizes and positions are presented, which depend on both the slot/pole combinations and original cogging torque harmonic contents. As confirmed by finite element analysis and experiment on a prototype machine having non-uniformly distributed slots, the proposed model has advantages of both high accuracy and analytical insight into cogging torque minimisation.