What is switching noise?


Switching noise in electric drives comes from the electric machine excitation by electromagnetic forces induced by switching in power electronics. It is a particular case of magnetic noise and vibrations in electrical drives (e-NVH). Contrary to “slotting noise” responsible for electric machine whining noise under sinusoidal current supply, “switching noise” is not influenced by magnetic circuit design such as slot pole combination, it is only influenced by the converter topology and switching strategy.
Different switching strategies may be used depending on e-machine type and application, such as six-step commutation in Brush-Less Direct Current (BLDC) motors, or Pulse Width Modulation in AC machines such as Permanent Magnet Synchronous Machines or Induction Machines.

PWM excitations

In e-NVH, PWM harmonics stand for Maxwell force harmonics linked to Pulse Width Modulation only, or more generally by any stator armature current harmonic actively produced by control (not induced by strong electromagnetic coupling). PWM magnetic force harmonics are by definition independent of slotting force harmonics, saturation force harmonics, and stator winding force harmonics.

More precisely, as any force harmonic is due to the product of two flux density harmonics, first order PWM excitations come from the product of harmonic fluxes induced by current harmonics combined with the fundamentla flux.

As noise and vibration frequencies are the same as the exciting force frequencies, PWM harmonics can also refer to some specific harmonics of the acoustic noise spectrum or vibration velocity spectrum.

PWM harmonics should disappear when the AC machine is fed with sinusoidal currents, or when the PWM switching frequency is increased above 20 kHz.

Application to e-NVH

The largest excitation frequencies depend on the PWM strategy and modulation index. One can show that the main PWM force wavenumbers are 0 and 2p, occuring at linear combinations of the switching frequency and electrical frequency.

This is illustrated in the following simplified force / vibration / noise spectrum:

In spectrograms PWM harmonics appear as V-shaped lines starting from multiples of the switching frequency at zero speed.


Noise control actions

PWM effects can be reduced playing on the switching frequency to reduce PWM current harmonics and avoid structural resonance, or by implementing spread spectrum strategies like randomization of the switching frequency. When doing so the sound quality should be checked with adapted psychoacoustic indices and panel testing.

Application to Manatee software

Manatee e-NVH software can include PWM effects in Quick Campbell feature, as well as in voltage-driven simulation workflows. Simulated noise can be synthesized as audio file for listening. The resulting sound file can also be analyzed in the Sound Quality module of Manatee.