All the electrical vehicles I've ever ride make that humming sound (either cars or trains).

What exactly causes that sound? Is it drag force in the gears? Is it a fan?

Or is it because of the coils (see coil noise)?

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    $\begingroup$ It can be any of those things. In most cases, it's a combination of multiple noise sources. $\endgroup$ Nov 30, 2015 at 19:58

1 Answer 1


Correct, coil noise is generated in the windings of the motor because of Pulse Width Modulation (PWM) driving current. Different frequencies of modulation cause corresponding sound frequencies (may create half or double frequency oscillations as well).

To change the speed of an electric motor, PWM is required to regulate voltage in DC motors and generate variable frequency wave forms in Variable Frequency Drives (VFD) for AC motors.

A brushless DC motor is essentially a 3 phase AC motor coupled with an application specific VFD. These are common in electric remote controlled aircraft and often play tones for a power up self check with nothing but the motor acting as a speaker. There are many examples online of people trying this DIY; for example, Brushless motor music (YouTube).

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    $\begingroup$ Thank you. I also did some research, and I found that the physical mechanism behind the sound is the volume variation of the wire by magnetostriction which therefore produces the mechanical vibrations that cause the Coil Noise (or Coil Whine). $\endgroup$
    – cinico
    Dec 2, 2015 at 14:06
  • $\begingroup$ @cinico could it really be the wire? Wikipedia says magnetostriction is a property of ferromagnetic materials. It could just be mechanical vibration of the wires, or something else. $\endgroup$
    – uhoh
    Oct 31, 2016 at 6:27
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    $\begingroup$ @uhoh I guess you're right. The resonance caused by the AC current can occur without magnetostriction. Thanks! $\endgroup$
    – cinico
    Oct 31, 2016 at 12:47

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