Comparative Study of Loss Characteristic Between Permanent Magnet Synchronous Machine and Vernier Machine for In-Wheel Direct Drive

In this article, a comparative study of core loss and permanent magnet (PM) loss in permanent magnet synchronous machines (PMSMs) and permanent magnet vernier machines (PMVMs) for in-wheel direct drive is carried out. First, the air-gap magnetic field is analyzed, and the armature reaction field and PM field harmonics are derived theoretically, where the conditions of steady torque for split-pole and open-slot stators are extracted and unified. Besides, the magnetic field harmonic group is defined for the further analysis of core loss and PM loss, which has also been validated by finite-element analysis (FEA). Moreover, the effect of magnetic field harmonics generated by armature reaction and PMs on core loss and PM loss is investigated. The contribution of each harmonic order on core loss and PM loss is calculated, where the fundamental component and slot harmonics, particularly the low-order one, are highlighted. Second, the influence of slot opening, flux barrier dimension, and PWM harmonics on losses are investigated. The losses of PMVM and benchmark PMSM under various torque and speed ranges are compared, and the distribution features of each loss in PMSM and PMVM are illustrated, in which, the PM loss highlights in PMVM, while stator core loss dominates in PMSM. Finally, two prototypes, a PMVM and a benchmark PMSM, were manufactured and tested, where the experiments have a good agreement with the FEA and theoretical analysis.