How do you tell which kind of motor you have? It is just brushes vs. no brushes? And, will the brushes always be obvious?Forget the light switch.
You need a speed controller. What kind you need depends on the motor, as Ryan stated. You can buy variable speed controllers for both universal (synchronous) motors and induction (asynchronous) motors. They are generally relatively inexpensive although slightly more than a dimmer switch for a light.
Figure out what kind of motor you have and go from there.
look in the literature if you have any on the unit. tell tale signs are 2 round caps that screw into the side of the motor, they are opposing each other. these caps allow easy access to replace brushes. some models don't have caps, and a cover has to be removed to access the brushes.How do you tell which kind of motor you have? It is just brushes vs. no brushes? And, will the brushes always be obvious?