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?
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
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?