# Whence comes the noise of a DC electric motor?

**URL:** <https://boards.straightdope.com/t/whence-comes-the-noise-of-a-dc-electric-motor/678459>\
**Category:** Factual Questions\
**Created:** [January 10, 2014, 7:31pm UTC](https://boards.straightdope.com/t/whence-comes-the-noise-of-a-dc-electric-motor/678459 "2014-01-10T19:31:59Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\
**Post date:** [January 10, 2014, 7:31pm UTC](https://boards.straightdope.com/t/whence-comes-the-noise-of-a-dc-electric-motor/678459/1 "2014-01-10T19:31:59Z")

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In my tool collection, I have a Dremel, and a shop vac. Each uses a conventionally commutated DC motor. Each is also as noisy as hell. Why?

Is it:

mechanical noise from the brushes riding on the commutator

arcing at the brush-commutator connection

magnetostriction as the current in each winding loop is suddenly switched on/off when the brushes make/break contact

high-frequency torque spikes as each winding suddenly starts/stops making torque

…something else?

In fiddling with brushless DC computer fans, I have noticed that there is mechanical noise that is apparently related to the torque spikes of each stator winding being suddenly turned on/off resulting in jerky rotation of the magnetic field that drives the rotor (as opposed to the smooth field rotation one achieves in a synchronous AC motor). If I put a brushless DC motor of suitable RPM and power rating in my shop vac, would those torque spikes manifest as the same sort of noise that I currently see? Or would it run much quieter?

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [January 10, 2014, 7:38pm UTC](https://boards.straightdope.com/t/whence-comes-the-noise-of-a-dc-electric-motor/678459/2 "2014-01-10T19:38:52Z")

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IMHO, most of the noise comes from the crappy bearings.

I’ve taken to lubricating the bearings in my Dremels when they get too loud, and it helps a lot. Also, there is a fan in the Dremel, and that probably makes quite a bit of noise, too.

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**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\
**Post date:** [January 11, 2014, 3:10am UTC](https://boards.straightdope.com/t/whence-comes-the-noise-of-a-dc-electric-motor/678459/3 "2014-01-11T03:10:18Z")

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I bet most of the noise is the rotation of the irregular core and windings, plus the little propeller, relative to the air. Like in a jet engine.