Comparative study of variable flux memory machines with parallel and series hybrid magnets

Variable flux memory (VFM) permanent magnet (PM) machines exhibit an additional degree-of-freedom for control, i.e. PM magnetization state, and thus excellent flux controllability. Moreover, the hybrid PM topologies having variable PM (VPM) with low coercive force and constant PM (CPM) with high coercive force at the same time, are employed to improve the torque density and the flux controllability. The parallel and series connections between the two different kinds of PMs are both feasible. Based on two-dimensional (2-D) finite element (FE) analysis, the VFM machines with these two connection types are investigated and compared in this paper. The results reveal that the VPM with series connected CPM is beneficial for more stable working point and better torque density. A pair of VFM prototypes with parallel and series hybrid PMs respectively are manufactured and tested to validate the analyses.