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This paper describes the optimal magnet shaping techniques to increase the torque capability in brushless AC machines by utilising the 3 rd order harmonic in inverse cosine shape airgap and sinusoidal shape magnet. It is analytically found that for different types of magnet shaping methods, the optimal magnitude of the 3 rd order harmonic is always 1/6 of that of the fundamental, which is validated by the finite element analysis. By utilising the optimal 3 rd order harmonic, the average torque of two 10-pole/12-slot machine can be increased by ~11%, while their torque ripple is only slightly increased by less than 1.5%, compared to machines employing magnet shaping techniques without the 3 rd order harmonic.

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Φ Abstract -This paper investigates the torque characteristics of an 18-slot/10-pole fractional-slot surface-mounted permanent-magnet (SPM) machine and presents its comparison with conventional 9-slot/10-pole and 12-slot/10-pole SPM machines by analytical and two-dimensional (2-D) finite element analysis (FEA). The winding factors of the 18-slot/10pole machine are firstly analyzed, and then, its back-EMF, average torque, cogging torque, and torque ripple are investigated. In addition, the influence of magnetic saturation due to stator current excitation on the average torque and torque ripple is also studied by 2-D FEA. The results show that the torque ripple can be suppressed while the average torque can be improved in the 18-slot/10-pole SPM machine compared with the 12-slot/10-pole SPM machine, while the unbalanced magnetic force in the 9-slot/10-pole SPM machine can be eliminated. Finally, the experimental results are given for validation of theoretical analyses and FE results.