The Resolution of 8th Order Whining Noise for a Battery Electric Vehicle 2020-01-1269

With significant improvement in overall vehicle NVH performance in recent years, sound quality has increasingly become an important customer concern. In search of excellence in customer satisfaction with BEVs (Battery Electric Vehicles), optimizing e-motor noise is one of the most effective ways to improve the vehicle sound quality. This paper aims to resolve a whining noise issue from a PMSM (Permanent Magnet Synchronous Motor) during the development of a BEV program. The most critical order of the whining noise from an e-motor on market today is most likely the 48th order, which could be mitigated by implementing sound package material on the motor casing. In this work, however, the 8th order instead is found to be the most critical issue, and it is perceived as the whining noise in the frequency range under 600Hz. With such a low frequency content, sound package treatment has little effect. With subjective evaluation and test data analysis, it is found that the worst case is associated with the peak of the 8th order. The 8th order electromagnetic force is induced by eccentricity, and as a consequence the inverter housing is excited and thus radiates noise, which makes a significant contribution to the overall motor whining noise level. Meanwhile, with inadequate design of the bearing system architecture and insufficient stiffness to support it, the combined rotor and gear system structure vibrates at specific frequencies. These vibrations further amplify the whining noise, which renders the emitted noise particularly annoying. Based on the principals of structural dynamics, the most effective way to alleviate the issue is to avoid or reduce the resonance of the rotor and gear system. However, this countermeasure is related to involved engineering details which are restrained by cost and timing as well as the supplier capabilities. In this work, it is proposed to resolve the issue by increasing the inverter housing stiffness and implementing a piece of damping material. After carrying out the proposed design improvements, it is shown that the whining noise level has been reduced significantly with a 5dB(A) reduction in overall motor noise level. Finally, the vehicle performance is rated acceptable by subjective evaluations.