High-Order Mode Harmonic Investigation in Electromagnetic Noise Analysis of Outer-Rotor In-Wheel Motors Using Multiple Basic Slot/Pole Combinations

Electromagnetic (EM) noise radiated by the rotor is a key point in the analysis of outer-rotor (OR) in-wheel motors (IWMs). Previous research only focused on the EM forces while lacking enough insight into the harmonic characteristics of mode shapes. This paper highlights the impact of the high-order mode harmonics of OR-IWMs caused by the use of multiple basic slot/pole combinations. Focusing on a 72-slot/80-pole OR-IWM, the coupled-field finite element analysis (FEA), consisting of a 2-dimensional EM model and a 3-dimensional structural model of the rotor assembly, is built to predict the main EM noises. It is found that identifying the primary EM forces causing the noises is challenging because the existing force modulation theory oversimplifies the spatial characteristics of modes. An analytical model is proposed to analyze the mode shapes of OR-IWMs in terms of mode harmonics. The investigation reveals that the unsymmetric OR structure can generate high-order mode harmonics within modes that are traditionally considered to be low-order. Furthermore, the equivalent radiated power levels generated by the high-order mode harmonics are assessed, and the primary contributing force harmonics are determined. The prototype of the studied IWM is manufactured, and sound pressure is measured for validation.