How to Troubleshoot Variable Speed Motors

Disconnect the power supply before troubleshooting the motor. Remove the negative (black) cable from the battery.

Unscrew the brush assembly, or individual brushes. All DC motors operate on the same principles, with few minor design differences. As they are designed to drive other machinery, one end has a gear, or shaft, that provides drive, while the other end, called the commutator end, is closed. At the closed end of the cylindrical motor housing are screws holding either a complete carbon-brush assembly or individual brushes, normally two on each side of the motor. Remove these screws and take out the brushes, being careful not to lose any springs.

Inspect the carbon brushes. Carbon brushes maintain contact with the commutator as it rotates, causing the brushes to wear down over time. Inspect the brushes for extreme wear, noting how much brush is left or if it is notably concave. Replace the brushes if necessary.

Dismantle the variable-speed motor if it produces noise or squeals during operation. Operational noise is a sign of worn bushings or bearings. Remove the bolts on the motor housing's commutator end, and open the motor. Three sets of bearings and bushings are common to DC motors: one on either end and a middle assembly. Check for excessive wear on both bearings and bushings and replace them as necessary. Repack the bearings with bearing grease, and apply a light film of grease on the bushings.

Inspect all wires and switches. Check for loose connections at all points. Include the wires attached to all brushes, switches and the battery connection. To test for wire shorts, use the continuity tester. Simply attach the negative alligator clip on the motor housing and place the red probe at each connection. Depending on the continuity tester, a light shines or a beep sounds if the connection is good. if not, the short must be located and repaired.

Bolt the motor back together and screw down the brushes, if you haven't already done this. Reconnect the negative cable to the battery and operate the motor.