How to Troubleshoot a Split-Phase or Capacitor Motor

Single-phase motors are common in many industries and can be found in blowers, pumps, fans, lathes, and numerous other types of equipment. Single-phase motor failures or malfunctions can often be traced to issues with the centrifugal switch, thermal switch, or capacitor(s). These problems can typically be fixed by servicing, repairing, or replacing these components. On a motor older than 10 yr or less than 1 hp, however, the motor is usually replaced. Since the cost of repair often exceeds the cost of a new one, small motors less than ¹/₈ hp are also typically replaced.

Split-Phase Motor Troubleshooting

A split-phase motor is a common type of single-phase motor that includes both a starting and running winding. A centrifugal switch automatically disconnects the starting winding when the motor reaches operating speed. A thermal switch is added to some split-phase motors, which automatically trips and turns the motor off if it overheats. Thermal switches might include automatic or manual resets. Since automatic resets could restart the motor at any time, extra safety precautions should be taken when troubleshooting a motor with an automatic reset.

Steps for troubleshooting a split-phase motor include:

Turn off power to the motor and visually inspect it. Replacement is recommended if you see signs of damage, such as the motor appearing burned or if the shaft is jammed.

Determine if the motor includes a thermal switch. Reset a manual thermal switch, then turn the motor ON.

If the motor fails to start, check the voltage at the motor terminals using the voltmeter setting on your multimeter. (Be sure to use a meter with the proper safety rating for the task). The voltage reading should be within 10% of the voltage listed on the motor nameplate. If the voltage is incorrect, then troubleshoot the circuit to the motor (Photo 1). If the voltage is correct, turn OFF power to the motor so you can test the motor.

Photo 1. A multimeter is used to troubleshoot a single-phase motor. — **Photo 1.** A multimeter is used to troubleshoot a single-phase motor.

Turn OFF the safety switch handle or the combination starter. Lock out and tag the starting mechanism according to company policy.

With the power OFF, connect an industrial multimeter to the same motor terminals the incoming power leads were disconnected from. Use the ohm or resistance setting. The ohmmeter will read the resistance of the starting and running windings. Since the windings are in parallel, their combined resistance is less than the resistance of either winding alone. If the meter reads zero, then a short is present. If the meter reads infinity, then an open circuit is present. In either case, the motor should be replaced. Replacement is required because these motors are too small for a repair to be cost-efficient.

Carefully inspect the centrifugal switch for signs of burning or broken springs. Service or replace the switch if you see any obvious sign of problems. If not, check the switch using an ohmmeter.

Manually trip the centrifugal switch. (The end bell on the switch side might have to be removed.) On a good motor, resistance on the ohmmeter will decrease. If the resistance does not change, then a problem exists. Continue checking to determine the problem.

Troubleshooting Capacitor Motors

A capacitor motor is a type of split-phase motor that includes one or two capacitors to provide the motor more starting and/or running torque. Troubleshooting capacitor motors is the same as troubleshooting split-phase motors (see above), but also takes into consideration the capacitor.

Capacitors deteriorate over time and thus have a limited life span. Capacitors often are the cause of problems with a capacitor motor. Capacitors can have a short circuit, an open circuit, or can deteriorate to the point where they need to be replaced. Deterioration can also change the value of a capacitor, which can cause more problems. When a capacitor short-circuits, the winding in the motor may burn out. When a capacitor deteriorates or opens, the motor often does not have the proper torque to easily start. If the motor is prevented from starting, then overloads are usually tripped.

Take the following steps when troubleshooting capacitor motors:

Turn the handle of the safety switch or combination starter to its OFF position. Lock out and tag the starting mechanism per company policy.

Using a multimeter, measure the voltage at the motor terminals to make sure the power is OFF (Photo 2).

Photo 2. Use a multimeter to measure voltage at the motor terminal to verify the power is off. — **Photo 2.** Use a multimeter to measure voltage at the motor terminal to verify the power is off.

Capacitors are found on the outside frame of the motor. Remove the cover of the capacitor, but beware: A good capacitor holds a charge, even when power is disconnected.

Visually check the capacitor for leaks, cracks, or bulges. Replace the capacitor if damage is found.

Remove the capacitor from the circuit and discharge it. To safely discharge a capacitor, place a 20,000-ohm, 2W resistor across the terminals for 5 sec.

After you discharge the capacitor, connect leads from your multimeter to the capacitor terminals. Use the Capacitance Measurement mode of your multimeter. (See your meter’s manual for instructions.) The capacitance value reading should be within ±20% of the value on the capacitor label to be considered good. Replace the capacitor(s) if necessary.

Sidebar: Capacitors 101

All capacitors consist of two conducting surfaces, which are separated by dielectric material. Dielectric material is a medium in which an electric field is maintained with little or no outside energy supply. It is the type of material used to insulate conducting surfaces of a capacitor. Two of the more common capacitor types found in motors are oil and electrolytic. Oil capacitors are filled with oil and sealed in a metal container. The oil serves as the dielectric material.

Electrolytic capacitors are more common in motors than oil capacitors. These electrolytic capacitors are formed by winding two thin sheets of aluminum separated by pieces of paper infused with an electrolyte. An electrolyte is a conducting medium in which the current flow occurs by ion migration. The electrolyte is used as the dielectric material. The aluminum sheets and electrolyte are encased in a cardboard or aluminum cover. A vent hole prevents a possible explosion in the event the capacitor is shorted or overheated.

AC capacitors are used with capacitor motors. Capacitors that are designed to be connected to AC may have no polarity.

Silva has been with Fluke for more than 20 years, working in various roles from field service and metrology to product management. He holds a B.S. in Business and an M.B.A. in Technology Management. He can be reached at [email protected].