Analysis of power factor in variable flux reluctance machines with MMF-permeance model

This study investigates the underlying mechanism of low-power factor issue of variable flux reluctance machines (VFRMs) from the perspective of magneto-motive force (MMF)-permeance model. On the basis of a simplified analytical model, the relationship between the design parameters and the power factor is identified and systematically summarised into three predictable ratios: the rotor permeance ratio, stator/rotor-pole ratio and DC/AC winding ampere turns ratio. Specifically, the smaller the rotor-pole arc, the air-gap length, the rotor-pole number and the AC/DC winding ampere turns ratio are, the higher the power factor will be. In addition, the weak coupling between the field and armature windings caused by the modulation effect of the salient rotor is responsible for the low-power factor issue of VFRMs, regardless of the control scheme, winding configuration or saturation effect. A 6-stator-pole/4-rotor-pole VFRM is prototyped and tested for verification.