Why do large electric motors make that “woooooooo” sound?

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Why do large electric motors make that “woooooooo” sound?

In: Technology

That is the sound of the engine turning. Large internal combustion engines also make that sound, but because they are making a lot of other sounds you can’t really hear it. The only reason you can hear it with an electric engine is because otherwise an electric engine doesn’t really make any sound at all.

There are two possibilities. First, the spinning part of the motor isn’t smooth, so as it turns it beats on the air, making a thrum. Second, because the motors are driven by AC running through wire cores wrapped around metal frames, there are usually enough loose bits to make a 60 (or 50) cycle hum. If the noise is constant regardless of motor speed it’s AC hum, if it increases in volume and frequency as the motor spins up its the thrum.