I had similar case before with fan black pig. Most likely the commutator dirty. I use a tube to spray alcohal into the carbon brushes area a few times and let it dry, then use air spray unit to blow off any dirty stuff, and spray some oil bases contact cleaner. Then I can use the motor again. But be carefull la, when you use the motor after cleaning, it will have some smoke and burn smell for the first few minutes, haha...