Hello. My name is***** can help you with your question.
The most likely cause of the problem is either of the motor or the motor controller board. You can test both. To test the motor, apply 12 to 18 V of DC current to the terminals. At a constant voltage, the motor should run at a constant speed. To test the motor controller board, test the voltage coming out of it to the motor which should very as the speed is varied. It's very likely that this is where the problem is. If so, then the motor controller board will need to be replaced. To get the replacement part will require getting the part number from the board. If that turns out to be the case, if you can give me the part number, I can help you find.
Unfortunately, since we don't know which board it is it could be anywhere on the board. You may have to take it out to see it because it could be on the bottom. Usually it's along the edge though. If you would like to take a picture of the board and put it here I can probably pointed out for you – in fact a picture of the board may be all I need.