Lately, I've been having a problem with my grinder. It seems that that normal whining sound has been replaced by a low growling with a combination of of a crakling sound sound. I decided to open it and exposed the mechanism.
An old cut-off grinder salvage from flea market retrofitted with a different ac plug.
Other than the regular specs shown of 115v, 1.5 amps.
Inside mechanism exposed. Brushes are still in good shape.
This culprit with the dirty bearing as shown. Commutators seemed smooth and clean.
Most grinders have this so called straight shot wiring, single phase commutator motor. This small capacity series motor can also be operated on dc (direct current) or single phase alternating supply of same voltage with the same characteristics, called universal motors.