ELi5 Why do some electric motors make noise and others don’t

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For example, electric cars are quiet, electric drills/lawn mowers are noisy

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Electric motors make noise due to friction between rotating parts. This differs from combustion motors like gasoline powered engines that make a lot of joise due to the internal explosions that run them. All electric motors can be designed to be quiet, but a quiet motor is harder to build, design, and maintain and as such is more expensive. In applications where friction is already being avoided, such as in a car, the motor will be quieter by comparison.

Another thing to consider is that a car motor has a lot of stuff around it to block the sound while a hand drill motor doesn’t. You also hold the hand drill pretty close to your ears while you’re never going to stick your face right up to an electric car motor. This means that you’re not really making an “apples to apples” comparison for noise.

In general, the smaller the motor, the higher the ram. This is not always the case, but it’s not a bad rule of thumb.
Power tools used what is called a series motor, although that is changing now, and electric mowers used them too. Series motors are characterized by having extraordinarily high speeds but not very high torque. This is remedied by gearing the output down. That’s what makes those small motors so noisy, the combination of the high speed armature and the gears. There’s also air movement (vacuum cleaner), vibration (imbalance) and bearing noise.

The noise often comes from what the electric motor is driving. In the case of the lawn mower, most of the noise is coming from the cutting blade spinning in the air. In the case of the drill, the gears in the drill make much of the noise. The gears in drills are usually what are called “straight cut gears”. The gears in a car’s drivetrain are almost always “helical cut gears”. Helical cut gears are quieter than straight cut gears. As Fred2718 mentioned, cooling fans for the motor can also contribute to noise.

Electric motors can also have “torque ripple”. That is, the torque output of the motor might not be perfectly constant as the motor turns through a revolution. This will cause a vibration which will contribute to noise. Electric car designers use motor designs that minimize or eliminate torque ripple.

As a final note, in a piece of equipment that is in good shape and properly lubricated, friction is generally responsible for very little noise. But, if you ever had a motor (like on a fan) that suddenly developed a squeal, that is a friction problem in a bearing that is out of lubrication.