Stator/Rotor Pole Combinations of Variable Flux Reluctance Machines with 2nd Harmonic Current Injection Method

The 2nd harmonic current injection method is recently developed to enhance the torque density of the variable flux reluctance machines (VFRMs). This paper investigates the effectiveness of the 2nd harmonic current injection method in VFRMs with different stator/rotor pole combinations. By using magnetic gearing effect, it is found that the 2nd harmonic injection method can only be applied to the $6j/(6i\pm 2)j \ (i i=0,1,2\ldots)$ VFRMs for the torque density enhancement. For the other stator/rotor pole combinations, the 2nd harmonic current has no contribution in average torque production and only leads to excessive torque ripple. Further, by using global optimization method and finite element analysis (FEA), 6/8 is proved to be the optimal stator/rotor pole combination for 6-stator-slots VFRMs with the 2nd harmonic current injection method, being able to exceed the maximal torque density of conventional VFRMs by 12% at a fixed copper loss.