Analysis of Back-EMF in Flux-Reversal Permanent Magnet Machines by Air Gap Field Modulation Theory

In this paper, the back-electromotive force (back-EMF) characteristics of the flux-reversal permanent magnet (FRPM) machines with different magnetization methods and winding layouts are analyzed and synthesized based on the magneto motive force (MMF)-permeance model. The air gap field modulation theory is utilized to investigate the permanent magnet-MMF harmonics and open-circuit air gap flux density harmonics that make a contribution to back-EMF as well as its harmonics. Based on this analysis, the specific harmonic of back-EMF can be reduced by suppressing the corresponding PM-MMF harmonic. Besides, the influence of rotor pole number on back-EMF is also studied. It is found that with the aid of this modulation theory, the reasonable magnetization method, winding layout, and rotor pole number can be obtained to improve the back-EMF characteristics of an FRPM machine with an arbitrary slot/pole combination. Furthermore, prototype machines are manufactured and tested to verify the analytical predictions.