Reducing High-Frequency Whine in High-Speed PMSMs

Introduction#

As automotive traction motors push toward higher speeds and higher power density, high-frequency electromagnetic noise has become one of the most persistent NVH challenges in electric drive development. A high-speed permanent magnet synchronous motor (PMSM) operating at 15,000 to 20,000 rpm produces electromagnetic forces whose frequencies extend well into the 2 to 8 kHz range, precisely where human hearing is most sensitive. In a premium electric vehicle program, engineers observed a narrow-band whine near 4.5 kHz that was clearly audible inside the cabin during hard acceleration and became the top customer complaint in early ride evaluations.

The root cause was traced to the interaction between the inverter switching frequency and the spatial harmonics of the air-gap flux. Radial electromagnetic forces excite the stator core and housing, and when a force harmonic coincides with a structural mode of the motor assembly, the result is a tonal whine that conventional sound package treatment struggles to absorb. This case study describes how the team combined stator skew, rotor pole shaping, and PWM harmonic suppression to reduce the whine by more than 12 dB(A) without sacrificing torque density.

Understanding the Electromagnetic Noise Mechanism#

In a high-speed PMSM, the dominant noise source is the radial Maxwell force acting on the stator inner surface. This force is proportional to the square of the air-gap flux density and contains rich harmonic content from both the magnet field and the stator current. At high rotational speeds, the fundamental electrical frequency rises to 1 kHz and above, so force harmonics generated by slotting and by inverter switching land in the audible high-frequency band rather than being masked by low-frequency road and wind noise.

Two mechanisms dominate. First, slot permeance harmonics create force orders that rotate at high speed relative to the stator, producing tones that scale directly with rotor speed. Second, PWM sidebands around the switching frequency and its multiples create current ripple that modulates the torque and radial force, generating a characteristic high-pitched whistle. When either force order matches a breathing or ovalizing mode of the stator stack, the vibration is amplified and radiated efficiently as airborne noise.

Stator Skew Design for Force Harmonic Cancellation#

The first countermeasure was a stepped stator skew. By skewing the stator lamination stack by one slot pitch in three axial segments, the team averaged out the dominant slot-order force harmonic along the axial direction. This reduced the peak radial force amplitude at the problematic order by roughly 8 dB while keeping the winding factor and torque output essentially unchanged. The stepped construction was chosen over continuous skew because it is easier to manufacture with standard lamination tooling and maintains a more uniform air gap.

In a production example, a supplier of traction motors for a compact electric SUV applied a two-segment skew to a 160 kW motor and measured a 6 dB reduction in the 24th-order force harmonic. The trade-off is a slight increase in axial force ripple and manufacturing complexity, so the skew angle must be selected against the specific slot and pole combination. Simulation with electromagnetic finite element analysis combined with structural modal analysis proved essential to predict which orders would be attenuated before hardware was cut.

Rotor Pole Shape Optimization#

Rotor-side shaping addressed the source of the flux harmonics rather than their structural amplification. The original rotor used a simple bread-loaf magnet pole that produced a trapezoidal air-gap flux with sharp transitions at the pole edges. By introducing a sinusoidal pole contour with optimized eccentricity and a small pole-edge chamfer, the flux density distribution became smoother, cutting the amplitude of the 5th and 7th space harmonics by more than 40 percent.

The team also evaluated magnet segmentation and asymmetric pole shaping to reduce cogging torque and force ripple. In one automotive application, a segmented magnet design with a two-step axial offset lowered the high-frequency force content by 5 dB while improving flux-weakening capability at high speed. The key lesson is that rotor shaping must be co-optimized with the stator skew; treating them independently can shift noise to a different order rather than eliminating it.

Inverter PWM Harmonic Suppression#

The third pillar was the control strategy. The original fixed-frequency space vector PWM at 8 kHz produced sidebands that excited the 4.5 kHz structural mode through intermodulation. The team implemented a variable switching frequency scheme that spreads the ripple energy across a band, combined with randomized carrier modulation. This reduced the tonal peak by 7 dB and converted the pure whine into a broadband hiss that is far less annoying to the human ear.

Additional measures included synchronous PWM at high modulation indices, dead-time compensation to remove low-order current distortion, and an active harmonic filter in the current loop that injects a canceling component at the targeted force order. In a real vehicle, these strategies are constrained by switching losses and processor load, so the final calibration balanced acoustic benefit against efficiency. The production vehicle achieved a 3 percent improvement in drive-cycle efficiency while meeting the interior noise target.

Results and Practical Lessons#

Combining the three approaches produced a cumulative reduction of 12.5 dB(A) at the offending 4.5 kHz tone, bringing the cabin sound pressure level below the program target during wide-open-throttle acceleration. Subjective jury evaluations confirmed that the whine was no longer perceived as a distinct tonal event. The case demonstrates that high-frequency PMSM noise is best attacked at the source through electromagnetic design, with control strategies and structural treatments as complementary layers.

For automotive NVH engineers, the practical takeaways are clear: characterize force orders early with coupled electromagnetic and structural simulation, treat stator skew and rotor shaping as a single optimization problem, and design the PWM strategy with acoustic sidebands in mind from the first calibration. Addressing these factors during concept design is far cheaper than retrofitting sound absorption into a vehicle whose architecture is already frozen.