How to Test Start Winding in a Capacitor Start Motor

Capacitor start motors have only four moving parts -- rotor, front bearing, rear bearing and centrifugal switch.

The electric motor on your lawnmower hums but doesn't start because of a problem with the start winding circuit.The electric motor on your lawnmower hums but doesn't start because of a problem with the start winding circuit.
They are relatively trouble-free devices. When a problem does arise, it is usually a faulty bearing, start capacitor or centrifugal switch. The start winding itself can develop an open or shorted winding. The start winding provides the extra torque needed to start under load. Once the motor has reached two-thirds of its operating speed, the centrifugal switch disconnects the start winding from the circuit.

Unplug the motor, or turn off the branch-circuit circuit breaker.

Remove the screws holding the cover on the electrical terminal box. There is a schematic diagram showing the wires connecting the run and start windings to the terminals on the cover plate. Using the long-nose pliers, pull the start wing leads free of the terminal plate connectors.

Plug the test leads into the digital multimeter. Plug the red test lead into the meter's red "Volt-Ohms" jack, and plug the black test lead into the meter's black "Common" jack. Turn on the digital multimeter's power switch. Set the meter's function switch to the meter's "Ohms" function.

Touch one of the test probes to each of the wires coming from the start winding and note the reading on the liquid crystal display. A good start winding will have a resistance of approximately 1.5 to 7.0 Ohms. If the meter displays "O.L.," the winding is defective, and you need to replace the motor. If you get a proper resistance reading across the start winding, proceed to the next step.

Touch one of the test lead probes to the two terminals associated with the centrifugal start switch. The switch should be in the closed position, and the meter should display 0.00 Ohms. If the meter displays "O.L.," the switch is stuck in the open position. If the switch is stuck in the open position, you will have to disassemble the motor and repair or replace the switch. If the centrifugal switch checks out OK, the problem is a faulty electrolytic start capacitor.

Replace the capacitor with an exact replacement. The replacement capacitor must have the same voltage rating and the same capacity rating. Reconnect all the wires and replace the terminal cover.

Things You Will Need

  • Screwdriver
  • Long-nose pliers
  • Digital multimeter

Warning

  • Never work on a motor that is still connected to a voltage source. Remember that electricity can kill. Unplug the motor or turn off the circuit breaker protecting the motor's circuit.

About the Author

Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.