Analysis and Design of a Hybrid Rare-Earth-Free Permanent Magnet Reluctance Machine by Frozen Permeability Method

Hybrid-type switched reluctance machine (SRM) by adding permanent magnets (PMs) in the stator of the SRM is gaining significant attention in order to solve the rare-earth PM problems, such as price instability and environmental problems. However, the addition of PMs in an SRM leads to positive and negative effects on torque production at the same time, because PMs in the stator assist the flux linkage of phase windings but aggravate total reluctance due to a slot for each PM. This paper is to propose a hybrid PM reluctance machine (PMRM) (hereinafter referred to as a hybrid PMRM) having a rare-earth-free PM in the stator, and the machine is analyzed and designed in terms of PM thickness by torque separation during freezing permeability in steel. The level of excitation current is not important in the variation of reluctance torque and mutual torque, but the thickness of PM has a major effect on both reluctance torque and mutual torque. Although bigger thickness in PM causes negative effect on reluctance torque production, total torque in the hybrid PMRM has been improved by a distinct margin, since mutual torque enhancement is much larger than reluctance torque drop. As a result, the proposed PMRM has an increase of 10.1% in efficiency compared with the same design without PM.