How to Control the Speed of a Single-Phase Motor

By James Stevens
Control the speed of your single-phase motor using a rheostat.

Single-phase motors generally operate at one speed and don’t contain a device to select variable speeds. The speed the motor turns is determined by the voltage it gets from the power source; reduce the voltage and the speed of your single-phase motor decreases. Increase the voltage and the speed of your motor increases. Never increase the voltage above the input voltage labeled on the motor; that will burn out the motor windings. Install a variable rheostat and you can control your motor's speed.

Disconnect the single-phase motor from the power source. Use a knife and cut the wire that connects the motor to the power source. Make the cut about 12 to 18 inches away from the motor.

Cut off about 2 inches of outer plastic from the two cut ends of wire using wire strippers. This reveals the inner wires. Remove ¼ inch of insulation on the two or three inner wires using wire strippers so you can see the copper wire.

Loosen and then remove the screws on the rheostat cover using a screwdriver. Lift off the cover from the rheostat base. Locate the two plastic terminal blocks attached to the base of the rheostat. One is usually labeled “In” and the other “Out.”

Loosen the screws on the terminal blocks using a screwdriver; don’t remove the screws. Using the wire that connects to the power source, insert the white or gray wire under the terminal screw labeled “-“ or “Neu” on the “In” terminal block. Insert the black or red wire under the terminal screw labeled “+” or “Pos” on the same terminal block. If there is a third green wire, insert under the terminal screw labeled “Gnd” or “T.” Tighten all the terminal screws.

Repeat the procedure by connecting the two or three wires on the end of the wire that connects to the single-phase motor to the terminal block on the rheostat labeled “Out.” The white or gray wire connects to “-“ or “Neu,” the black or red wire connects to “Pos” or “+” and, if there is a third green wire, it connects to “Gnd” or “T.” Tighten the screws using a screwdriver.

Replace the cover on the rheostat. Put the screws in the holes and tighten the screws using a screwdriver.

Connect the wire to the power supply. Turn on your single-phase motor. Control the speed by turning the dial or moving the lever on the rheostat.

About the Author

James Stevens has been writing articles for market research companies in the U.K. since 1990. He has written various country profiles for inclusion in comprehensive market reports including Vision One Research and Investzoom Market Research. Stevens holds a General Certificate of Education from Chelmsford College of Further Education.