How to Vary Power to Electrical Motors

Vary the power to your electrical motor by wiring a rheostat.

Vary power to an electrical motor by controlling the voltage it uses. For example, an electrical motor, designed to operate using a maximum of 110 volts, can be made to rotate slower by decreasing the voltage supplied to it by wiring a rheostat between the power source and the motor. A rheostat contains multiple wire windings, which creates an electrical resistance when current flows through the wire. A metal contact inside the rheostat moves along the winding when you move the external knob or lever. The farther the contact is along the winding, the farther the current has to flow, causing electrical resistance, which reduces the voltage.

Things You'll Need

• Rheostat
• Knife
• Screwdriver
• Wire strippers

Instructions

1. Disconnect the electrical power to the motor before wiring a rheostat. Cut the wire between the power source and the motor about 18 inches from the motor using a knife.

2. Remove the cover from the rheostat. Use a screwdriver to remove the screws, then lift off the cover. Inside there are two terminal blocks -- one labeled "In" or "Flow" and the other "Out." Loosen the screws on the terminal blocks using a screwdriver.

3. Remove about 1 inch of protective plastic off the ends of the wire you cut earlier using wire strippers. You either have two or three colored wires inside, depending on whether the motor has a ground terminal. Remove ¼ inch of colored plastic off the ends of all the internal wires so the metal core is exposed.

4. Connect the black wire from the power supply cable under the terminal screw labeled "Pos" on the "In" terminal block on the rheostat. Connect the white wire from the power supply cable under the terminal screw labeled "Neg" on the same terminal block. If there is a third green wire, connect under the terminal screw labeled "Gnd." Tighten the screws using a screwdriver.

5. Repeat the process by connecting the two or three wires on the end of the cable from the electrical motor under the screws on the "Out" terminal block on the rheostat. Connect the black wire to the "Pos" terminal screw, the white wire to the "Neg" terminal screw and, if there is a third green wire, connect it to the "Gnd" terminal screw. Tighten the screws.

6. Replace the cover on the rheostat. Insert the screws in the screw holes and tighten using a screwdriver.

7. Connect the cable to the power supply. Move the lever or knob on the rheostat, and you can vary the power to the electrical motor.

Tips & Warnings

• Purchase a rheostat from an electrical store.

• The "black" wire might be also be colored red or orange, and the "white" wire can also be gray.

References

• Encyclopedia.com: Rheostat
• Electric Motors: AC Motors
• All About Circuits: Wiring Color Codes