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The Influence of Dummy Slots on Stator Surface-Mounted Permanent Magnet Machines

In this paper, the concept of harmonic permanent-magnet (PM) torque is defined and deduced first. Then, a conventional stator surface-mounted PM (SSPM) machine, namely, the 6/8 SSPM machine, is investigated. The finite element (FE) predicted and measured results reveal that the back electro-motive force (back EMF) of the 6/8 SSPM machine is asymmetrical, leading to large harmonic PM torques, cogging torque, and torque ripple. Hence, a 6/7 SSPM machine exhibiting coil complementarity is proposed afterward, which exhibits improved electromagnetic performances, such as lower cogging, more sinusoidal back EMF waveforms, and smaller harmonic PM torques, resulting in less torque ripple. After that, the influence of the dummy slots located in the center of two magnets on the same stator pole for the SSPM machines is investigated. The FE-predicted results indicate that the electromagnetic performances including cogging torque and torque ripple can be improved for the machines exhibiting coil complementarity, whereas for the machines having no coil complementarity, the performances will degrade further.