The Resolution of 8th Order Whining Noise Issue for a Battery Electric Vehicle 2020-01-1269
With significant improvement in overall vehicle NVH performance in recent years, sound quality has become an important customer concern. In search of excellence in customer satisfaction with Battery electric vehicles (BEV’s), optimizing the e-motor noise is one of the most effective ways to improve vehicle sound quality. This paper aims to resolve the whining noise issue from a permanent magnet synchronous motor (PMSM) during the development of a BEV program. The order of the most critical whining noise from an e-motor on market today is normally the 48th order, which can be mitigated by implementing sound package material on the motor casing. In this work, however, the 8th order is found to be the most critical issue which is perceived as the whining noise with the frequency range under 600Hz. For such a low frequency content, sound package treatment has little effect.
With subjective evaluation and analysis of testing results, it is found that the worst case is associated with the peak of the 8th order. The 8th order electromagnetic force is induced by the eccentricity, and as a consequence the inverter housing is excited and the whining noise is radiated, 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 some engineering details which are restrained by cost and timing as well as the capabilities of the supplier. In this work, it is proposed to resolve the issue by increasing the inverter housing stiffness and implementing a dynamic damper. 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.