Case Study: Reducing High-Frequency Whine in EV Drive Motors

Introduction#

In electric vehicles, high-speed permanent magnet synchronous motors are increasingly used as drive motors. At high rotational speeds and high inverter switching frequencies, electromagnetic forces can excite structural modes, producing tonal high-frequency whine that degrades automotive NVH.

This case study examines a rear-drive electric axle with a 12,000 rpm PMSM and a 10 kHz PWM inverter. Drivers reported a sharp whine between 8 kHz and 11 kHz during hard acceleration. We trace the source and present mitigation through stator skew, rotor pole shaping, and PWM harmonic control.

High-Frequency Electromagnetic Noise Mechanism#

The primary source is radial Maxwell force harmonics in the airgap. These forces arise from interaction between stator slotting, rotor field harmonics, and inverter current harmonics. At high speed, force orders multiply by rotational frequency, shifting energy into the audible high-frequency range.

When a force harmonic coincides with a stator or housing mode shape, resonance amplifies vibration and radiated sound. In our case, the 48-slot stator and 12,000 rpm speed created a 9.6 kHz slot-passing force. The 10 kHz PWM switching frequency and its sidebands added further tonal energy, exciting a stator breathing mode.

Stator Skew Design#

Stator skew reduces slot harmonics and cogging torque by averaging airgap permeance variations along the axial direction. Continuous skew or step skew can spread the force harmonic energy into sidebands, lowering peak tonal noise. In our case, a continuous skew of one slot pitch reduced the 48th-order radial force by 12 dB.

However, skew can reduce effective torque and increase leakage. We used a 6-step skew with 5 mm axial segments, optimizing skew angle to 7.5 mechanical degrees. This cut the 9.6 kHz whine by 7 dB(A) with less than 1.5% torque loss. We also adjusted stator tooth tip shape to smooth permeance transition.

Rotor Pole Shape Optimization#

Rotor pole shaping targets the airgap flux density waveform. By using sinusoidal pole shaping, eccentric airgap, or asymmetric pole arcs, engineers reduce higher-order flux harmonics that interact with stator slotting. A bread-loaf pole with a 0.8 mm eccentricity and optimized pole arc reduced the 48th force harmonic by 6 dB.

In our case, we applied a two-segment asymmetric rotor with magnetic bridges trimmed to balance flux leakage. Finite element analysis predicted a 4 dB reduction in 10 kHz sideband forces. Prototype testing confirmed a 3.5 dB(A) drop in overall whine. The trade-off was a slight increase in torque ripple at low speed, managed by current shaping.

Inverter PWM Harmonic Suppression#

PWM strategies directly shape current harmonics. Random PWM, spread-spectrum modulation, and variable switching frequency spread discrete sidebands into broadband noise, reducing tonal peaks. In our case, we replaced fixed 10 kHz synchronous PWM with randomized switching frequency between 9.5 kHz and 10.5 kHz, reducing the 10 kHz tone by 8 dB(A).

We also implemented selective harmonic elimination and deadtime compensation to suppress low-order current harmonics that can modulate high-frequency forces. A discontinuous PWM scheme reduced switching losses while maintaining acceptable NVH. Active damping through current injection further attenuated