Investigation of torque production and torque ripple reduction method for 6-stator/7-rotor-pole variable flux reluctance machines

This paper investigates the torque production and the torque ripple reduction of 6-stator/7-rotor-pole (6/7) variable flux reluctance machine (VFRM). Based on the harmonic analysis, the instantaneous torque equation is firstly derived by considering the spectra of the armature inductance and current. The average torque is found to be mainly produced by the mutual inductances between the field and armature windings. Since the armature coils in each phase have opposite polarity connection 6/7 VFRM, the even order harmonics of self-inductances and odd order harmonics in mutual inductances between the field and armature windings are eliminated. It results in the elimination of the third harmonic torque ripple, while a multiple of the sixth harmonic torque ripple exists. In order to mitigate the torque ripple, the harmonic current injection method is proposed and applied to the field winding, instead of armature windings. Two methods, i.e., finite element analysis (FEA) based and analytical equation based methods, are proposed to calculate the amplitude of the injected harmonic current. The performance of the proposed methods is verified by both FEA and experiment.