Impact of Axial Shapes of Circumferential Modes on Electromagnetic Noise in Outer-Rotor Permanent-Magnet In-Wheel Motors

Circumferential modes are recognized as the primary contributors to electromagnetic (EM) noise of permanent-magnet (PM) in-wheel motors (IWMs), whereas their axial shapes have not been thoroughly investigated. Given the significance of axial shapes in outer-rotor (OR) structures, this paper aims to investigate the impact of axial shapes on the EM noise of a 72-slot/80-pole OR-IWM. Finite element analysis (FEA) is used to predict the main circumferential modes of the rotor. The uneven distribution of axial shapes is observed and also verified by the modal test. A coupled-field FEA model is subsequently established, demonstrating that the variation of the modal amplitude of each segment can result in significant differences in velocity responses. To reveal the impact, the circumferential mode shapes of each segment are axially reduced and expressed by the Fourier series. The findings indicate that, in addition to amplitude variation, rotor skewing also generates phase variations in mode harmonics, which should be carefully considered in the design of OR-IWMs.