Electric vehicles (EV’s) present new challenges to achieving the required noise, vibration & harshness performance (NVH) compared with conventional vehicles. Specifically, high-frequency noise and unexpected noise phenomenon, previously masked by the internal combustion engine can cause annoyance in an EV. Electric motor (E-motor) whine noise caused by electromagnetic excitation during E-motor operation is caused by torque ripple and radial excitation. Under high speed and high load operating conditions, the overall sound level may be low, however high frequency whine noise can impair the vehicle level NVH performance. An example of a previously masked unexpected noise phenomenon is a droning noise that can be caused by manufacturing quality variation of the spline coupling between the rotor shaft of the E-motor and the input shaft of the reducer. It is dominated by multiple higher orders of the E-motor rotation frequency.
In this study, the high speed and high load condition whine noise problem was reproduced through electromagnetic and structural analysis. The countermeasures (E-motor geometry refinements to reduce the excitations and mechanical system transfer path modifications to reduce the vibration response) were defined and the effects investigated. Mechanical system modification to improve NVH performance without increasing the mass is challenging. However, E-motor air-gap geometry optimization, such as slot opening modulation and rotor notch modifications achieved significant noise reduction without critical trade-off of other performance.
This paper also describes the basic improvement plan proposed through the simulation of the droning noise behavior. The main two manufacturing errors (pitch error in the spline coupling and the alignment error between the rotor and the gearbox shafts) that are responsible for the unexpected droning noise were identified. Using simulation it was shown that the droning noise can be reduced through improved quality control and tolerance design optimization of factors such as axis self-alignment and spline gear tooth quality.
