Reduction of Nonworking Armature Harmonics in Vernier Permanent Magnet Machines

Vernier permanent magnet machines (VPMMs) are becoming increasingly attractive for low-speed direct drive industrial applications. This article presents a novel design perspective of reducing nonworking even-order armature harmonics in VPMMs with concentrated winding to further improve the overall electromagnetic performance. The armature reaction magnetomotive force (MMF) harmonics can be adjusted by controlling the winding layout. In particular, a dual-stator VPMM (DS-VPMM) is presented, in which the inner stator is shifted by a half slot pitch relative to the outer stator, and the key is that dual concentrated windings are reversely connected in series with a 180° mechanical angle difference. Consequently, those nonworking even-order harmonics produced by the armature reaction can be eliminated. As a result, the synchronous inductance is notably reduced, which further improves the power factor without sacrificing torque density. A nondominated sorting genetic algorithm II (NSGA-II) is adopted in the optimization process to obtain the optimal parameters of the proposed design. The finite-element analysis (FEA) results show that, compared to conventional VPMMs, the proposed design with reduced even-order armature harmonics exhibits higher torque density and higher power factor while the unbalanced force is mitigated. Finally, a prototype is fabricated to verify this proposed approach.