Investigation of power factor characteristic in permanent magnet motors for in-wheel direct drive application

Abstract

This paper proposes the intrinsic coefficient of power factor in permanent magnet vernier motors (PMVMs) influenced by slot-pole combinations for in-wheel direct drive application. To start with, the theoretical expression of power factor under typical conditions is derived, and the dominant influencing coefficients are extracted. Secondly, each inductance component is calculated theoretically, eg. air-gap main inductance, slot leakage inductance, etc. Based on previous derivation, the intrinsic coefficient of power factor is extracted, which represents the high-power-factor ability and be utilized in the initial machine design. It is influenced by the armature poles, armature magnetomotive force (MMF) harmonic spectra, winding factor, and air-gap permeance. Moreover, the slot-pole combinations with various high-power-factor ability are illustrated, in which, the ones with coil-pitch of two-slot pitches overwhelm. In order to validate the theoretical analysis, several PMVM model candidates with various slot-pole combinations are selected and simulated by finite-element-analysis (FEA). Finally, the prototype of PMVM with 28p18s and benchmark PMSM with 28p24s were manufactured and tested for experimental validation. Overall, this paper aims to provide a guideline to select the slot-pole combinations with high-power-factor ability, particularly in PMVM, for in-wheel direct drive.