Cogging torque suppression in flux-reversal permanent magnet machines

In this paper, an analytical expression of cogging torque considering flux-leakage effect for FRPM machines with different magnetization methods of magnets is derived based on co-energy and magneto motive force-permeance model. Then, the influences of machine parameters (including PM width and rotor tooth width) on cogging torque as well as its harmonic spectra are analysed according to this derived model. Consequently, an effective method to diminish cogging torque is obtained by optimally choosing rotor tooth/PM width. Besides, the skewing and chamfering methods are also discussed. Then, with the aid of 2D finite element analysis (FEA), the above theoretical analysis and the proposed cogging torque suppression methods are verified by two exampled 6-stator-slot/8-rotor-pole FRPM machines with different magnetized magnets. Finally, these two exampled machines are fabricated and tested to further validate the derived model and FEA analysis. It turns out that the cogging torque of both magnetization methods are completely the same and it can be minimized by optimally choosing machine design parameters during the design stage. Then, combined with the skewing, cogging torque can be reduced further. However, chamfering is not recommended in this type of machine since it may aggravate asymmetric back-EMFs.