Torque Improvement of Vernier Permanent Magnet Machine With Larger Rotor Pole Pairs Than Stator Teeth Number

Vernier permanent magnet machines (VPMM) have attracted extensive interest owing to high torque density. However, VPMM with larger rotor pole pairPrthan stator teeth numberZshas weak torque capability due to the opposite phase of fundamental back electromotive forceE1induced by the fundamental and modulated flux density, i.e.,BPrandBPa. Thus, the pole ratio (PR, defined as the ratio of rotor pole pairPrto winding pole pairPa) of VPMM withPr>Zsis labeled as “bad,”, and has long been abandoned. This article puts forward two novel design tools, i.e., unequal element coil (UEC) and stator teeth unit (STU), via which the torque-improvement method of VPMM with “bad” PR is proposed. First, constructing the winding via UEC, in which bothBPrandBPainduceE1of the same phase, meanwhile the totalE1is maximized. Second, designing the stator teeth structure via STU so that multiple flux density harmonics are exploited andE1can be further enhanced. A VPMM withPr= 10 and PR = 10 is designed by the proposed method. The FEA and experimental results prove that at current density 7.5 A/mm2, the torque density of the proposed VPMM is improved to 18.3 N·m/L while the regular counterpart is only 13 N·m/L.