Case Study: Cutting High-Speed PMSM Whine in EVs

Introduction: The High-Speed PMSM Whine Challenge#

Electric vehicle traction motors increasingly run at high speeds to deliver compact power and efficiency. A permanent magnet synchronous motor (PMSM) spinning at 15,000 to 20,000 rpm can produce a sharp, high-frequency whine that is easily perceived inside the cabin. This is an NVH problem rooted in electromagnetic force harmonics, not just mechanical imbalance.

This case study examines a rear-drive automotive electric axle with an 8-pole, 48-slot PMSM. At high speed, the motor generated a dominant 6.5 kHz tone and several sidebands that passengers described as an annoying whistle. The goal was to reduce the tone without sacrificing torque density or efficiency.

Root Causes of Electromagnetic Noise in High-Speed PMSMs#

The primary excitation is the radial Maxwell stress wave in the air gap. It is produced by the interaction of stator and rotor magnetic fields, slotting harmonics, and inverter current harmonics. At high speed, these force waves appear at frequencies that can match the natural modes of the stator core, housing, or rotor.

When a force harmonic couples with a structural resonance, even a small excitation can radiate significant airborne noise. In the case motor, a 48th-order radial force from stator slotting aligned with a breathing mode of the stator stack at about 6.5 kHz, creating the tonal whine.

Stator Skew Design for Tonal Noise Reduction#

Stator skew is a proven method to cancel certain slot harmonics. By skewing the stator laminations by one slot pitch, the radial force contributions along the axial length phase-shift and partially cancel. Continuous skew, step skew, and segmented skew each offer different trade-offs in manufacturability and harmonic attenuation.

For the case motor, a continuous skew of one stator slot pitch reduced the 48th-order force by over 70 percent. Measured sound pressure at 6.5 kHz dropped from 78 dB(A) to 71 dB(A). The trade-off was a slight increase in torque ripple and winding complexity, but overall NVH improved without extra acoustic insulation.

Rotor Magnetic Pole Shape Optimization#

Rotor pole shaping smooths the air-gap flux density and reduces higher-order force harmonics. Techniques include sinusoidal pole shaping, eccentric pole arcs, edge chamfers, and magnetic notches. These changes alter the spatial spectrum of the rotor magnetomotive force, weakening harmonics that cause high-frequency noise.

In a second build, the rotor poles were shaped with an asymmetric eccentric arc and small notches near the pole edges. This lowered the 24th-order radial force and reduced a 7.2 kHz whine by 5 dB(A). The optimization also cut torque ripple, showing that NVH and drive quality can improve together.

Inverter PWM Harmonic Suppression Strategies#

The inverter is another major source of high-frequency excitation. Fixed carrier PWM creates current harmonics at the switching frequency and its sidebands, which excite radial force waves. Random PWM, spread-spectrum modulation, discontinuous PWM, and selective harmonic elimination can spread or suppress these tones.

In the case vehicle, random carrier frequency modulation was enabled at high speed. The 8 kHz switching whine was reduced by 4 dB(A) and became less tonal. Because this was a software calibration change, it was a fast and cost-effective complement to the electromagnetic redesign.

Integrated Case Study and Measured Results#

The production intent combined all three measures: one-slot-pitch stator skew, shaped rotor poles, and randomized PWM above 12,000 rpm. On a chassis dynamometer, the overall high-frequency whine dropped by 10 dB(A) at the worst operating point. The 6.5 kHz tone fell below the background noise of the road and wind at highway speed.

Subjectively, drivers rated the high-speed sound quality as significantly improved. The solution was applied to a passenger electric vehicle program without increasing motor size or using heavy acoustic blankets. It demonstrates that electromagnetic NVH can be managed at the source.

Conclusion#

High-frequency PMSM whine is a system-level NVH challenge driven by electromagnetic force harmonics, structural resonance, and inverter modulation. Stator skew, rotor pole shaping, and PWM harmonic suppression each address different parts of the excitation chain. Combined, they can deliver substantial tonal noise reduction.

For automotive drive motor engineers, the practical lesson is to model, measure, and iterate across electromagnetic, structural, and control domains. A balanced design avoids costly acoustic treatments while meeting the refined sound quality expected in modern electric vehicles.