Several electric motor controllers operate on the same principle as dimmer switches, such as the trigger on a variable speed power drill. A common household dimmer switch used for motor speed control connects to a ceiling fan. There is a difference in lighting dimmers and those designed to handle the load for an electric motor. Pay attention to power ratings to connect the right switch to a motor.
Choose a dimmer that's made for running an electric motor. Select a switch manufactured for use as a motor controller that has an adjustable minimum speed setting. Match the power rating on the switch to the specifications for the motor. A ceiling fan will have a sticker label, or metal plate on the inside or outside of the hub cover that lists the amps and possibly the watts used at maximum power consumption. Find the number of watts used by multiplying 120 by the number of amps if the watts are not listed. Buy a switch that is rated to handle an equal amount or more watts used by the motor.
Shut off the power to a ceiling fan with the breaker switch. Shut off the main power breaker switch if the circuit for the fan cannot be identified. Leave the breaker box locked or labeled to stop anyone from turning the power back on.
Remove the on/off toggle switch that controls the motor. Loosen the holding screws on the cover plate of the wall switch for a ceiling fan. Remove the mounting screws at the top and bottom of the switch. Pull the switch slowly out of the wall box to examine the wire terminals.
Identify the wires attached to the switch and in the wall box. Two hot wires are connected to brass colored terminal screws on one side of the switch. The hot wires are either both black or one black and the other blue. There should be a green or bare copper ground wire secured to the green terminal screw on the lower side of the switch. Two white neutral wires are connected by a wire nut in back of the wall box.
Loosen the wire connections on the switch using a Phillips screwdriver to unscrew the terminals. Pull the wires away from the terminals. Loosen the wire nut connecting the neutral wires by twisting it counterclockwise. Use pliers to loosen the wire nut if it won't come off by hand. Throw away the used wire nut.
Secure the wires from the dimmer switch to the wiring in the wall box. Connect the two hot wires that were attached to the old switch to the two black wires from the wall switch. Twist the bare wire ends together and cover the connections with new wire nuts. Twist the green or copper ground wire from the wall box around the green wire from the new switch. Cover that connection with a new wire nut. Twist the two white neutral wires from the wall around the white wire on the dimmer. Cap that connection with a new wire nut as well.
Set the minimum speed control for the slowest speed you'll want the motor to rotate. Set the minimum speed for a ceiling fan to rotate slowly for use with heating systems.
Check every connection to be certain the color coded wires are securely attached to the right wires. Secure the mounting screws on the new switch, and put the cover plate on and tighten the holding screws.
Turn on the power with the breaker switch. Set the controller on the dimmer switch for any desirable speed during normal use.