Investigation and Analysis of Cogging Torque for Axial Flux Permanent Magnet Machines

Axial flux permanent magnet (AFPM) machines have higher cogging torque compared with traditional PM machines due to high air-gap flux density. To reduce the cogging torque of AFPM machines, the phase shift characteristics and harmonic distribution of cogging torque for AFPM machines are investigated. Firstly, the expression of cogging torque is deduced based on energy method. Then, a quasi-3D analysis method is introduced to investigate the influence of cogging torque with the segmented permanent magnets (PMs) under different magnet skew angles. In addition, the effect of harmonics on cogging torque under on-load conditions is studied and revealed based on the frozen permeability (FP) method, which identifies the source of cogging torque generation. Finally, the results show that the cogging torque and torque ripple are optimized by using the off-center skew magnet.