Electric motor is among the main sources of noise and vibration for electrified propulsion systems. This paper focuses on the electric motor rotor NVH optimization, which is identified as a key enabler to reduce the motor whine, and balances other performance such as motor torque and efficiency. First, conventional rotor NVH design technologies such as rotor skew and asymmetric rotor pole-to-pole design are discussed, along with their associated tradeoff including reduced motor torque and additional sideband orders. Next, a special notch feature is proposed on the rotor surface with one notch per pole at every q-axis. A DOE study leads to the optimal notch design which significantly reduces the dominant motor torque ripple order by up to 20 dB, with minimum impact to motor torque or loss. Further design studies are then performed to explore additional d-axis notches which are symmetrically located within the top layer magnet opening angles. Up to 6 dB reduction is achieved at the most dominant motor winding order, with small trade-off for motor peak torque. Lastly, a prototype baseline rotor design without notch and a rotor with optimized NVH notch are built and tested in a motor fixture and an electric drive unit. Measured vibration and sound power data confirm that rotor notch achieves up to 20 dB reduction, and the optimal design is successfully implemented in the electric motors for General Motors Ultium electric drive units.
