How to Tell If a Capacitor Is Bad on a Motor

How to Tell If a Capacitor Is Bad on a Motor

Capacitors are used in single-phase AC motors to provide the phase shift needed to start and run the motor. When a capacitor fails, the motor either will not start at all, starts slowly, hums, or trips its thermal overload. Testing a capacitor is a straightforward procedure with a multimeter that has a capacitance function, or with a dedicated capacitor tester.

Types of Motor Capacitors

• Start capacitor — assists starting torque only, disconnects after the motor reaches operating speed via a centrifugal switch
• Run capacitor — remains in the circuit during operation to improve efficiency and power factor
• Dual-run capacitor — used in HVAC equipment, provides both run capacitance for the compressor and the fan motor in a single unit

Signs of a Bad Capacitor

• Motor hums but does not start
• Motor starts slowly or requires a manual spin to get going
• Motor trips on thermal overload shortly after starting
• Capacitor casing is bulged, cracked, or shows oil leakage
• Motor runs but lacks full power

Safety First: Discharge the Capacitor Before Testing

Capacitors store electrical charge and can deliver a dangerous or fatal shock even after power is removed. Before touching or testing a capacitor, discharge it by placing a 20,000-ohm, 5-watt resistor across both terminals for several seconds. Never short capacitor terminals together with a screwdriver — the resulting arc can damage the capacitor and is hazardous.

How to Test a Capacitor With a Multimeter

1. Disconnect power to the motor and verify with a non-contact voltage tester
2. Discharge the capacitor using a resistor as described above
3. Disconnect the capacitor leads from the circuit
4. Set the multimeter to capacitance mode (marked as µF or the capacitor symbol)
5. Touch the probes to the capacitor terminals and read the measured capacitance
6. Compare the reading to the value printed on the capacitor label — a good capacitor should read within ±5% to ±10% of its rated value
7. A reading significantly below rated value, an OL reading, or a reading of zero indicates a failed capacitor

Replacing a Motor Capacitor

Replace the capacitor with one matching the original voltage rating and capacitance value. The replacement voltage rating can be equal to or higher than the original (never lower). Use the same type: a run capacitor cannot substitute for a start capacitor. For HVAC equipment, capacitor replacement is straightforward but involves refrigerant systems regulated under Ontario law — consult a licensed HVAC contractor for those systems.

EZSMART services and replaces motors in residential and commercial electrical equipment across Ontario. Contact us if you need help diagnosing a motor fault.

Amir Azimipour

Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp