Technology: cogging torque
Self-tutoring about everyday technology: the tutor mentions why an electric motor, turned by hand, might advance in spurts.
The following is by my understanding.
I was working on a dishwasher lately, whose circulation pump was seized. However, I managed to work it loose again, so it would turn. Yet, it would turn then stop, turn then stop, only happy to rest at certain positions. I wondered if such was normal.
It turns out that such behavior might be expected from an electric motor (which the pump is driven by) that has permanent magnets. The situation is that the motor is not under power, but is being turned by hand. Then, the reason for its advance in spurts is that the turning shaft has teeth on it which are attracted to the permanent magnets positioned around the shaft. When a tooth gets to its closest point to a magnet, it naturally wants to stay in place. However, if you apply a little force in order to turn the impeller (or the shaft, or whatever the motor is meant to spin), it will give way, then soon jump forward and want to stop when the tooth reaches the next magnet, as so on.
Interesting, eh?
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.