Eli5: what is the high pitched whine when electric motors work?
In: Engineering
Some electric motors have electronic speed control which causes whine which is more noticeable at low speeds. They use pulse width modulation that switches power on/off rapidly (over 100 times/second). Variable frequency drives can also cause it. You can hear it in variable speed drills which are spinning very slowly or stalled. Electric transit trains or buses also make the noise. It is due to the magnetostrictive effect which is a change in dimension of some materials when exposed to a varying magnetic field.
[Sample subway car noise and relevant article](https://en.wikipedia.org/wiki/Electromagnetically_excited_acoustic_noise_and_vibration#PMSM_motor_(traction_application))
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