Control the speed of an electric motor in a DC series circuit by placing a potentiometer or a rheostat between the motor and the power source. Adjusting the potentiometer or rheostat changes its resistance value and alters the voltage going to the motor. A device to vary voltage in an AC circuit is a dimmer switch. The speed of a motor is directly proportional to the voltage applied to it.
A variable resistor is called a rheostat, but a potentiometer may act as a rheostat as well. To use a potentiometer to vary resistance in a DC circuit, connect the wiper terminal and one other terminal, leaving the third terminal unused. Move the potentiometer wiper closer to the terminal in use to lower the resistance and increase the speed of the motor.
Using a potentiometer or rheostat to control motor speed is inefficient, since substantial amounts of power are dissipated by the rheostat in the form of heat. An alternative device to control power to the motor in an AC circuit is a household dimmer switch. This device pulses power to the motor through use of a high-speed switching device, such as a transistor or a transistor-like device called a triac. This device switches the electricity on and off very rapidly. Because these devices "chop up" electrical power, they are also called chopper switches.