"Assuming it's a standard AC induction motor (it will probably say so on it), it's speed is set by the line frequency, not voltage. A dimmer would not help you. "
That's correct. Not only will it not help you but it will burn out the dimmer since std ones don't have the current capacity nor are they built to deal with inductive loads. And if you find one that can supply enough current, it will burn out the motor.