On single-phase compressor motors, the start capacitor provides the initial torque boost to get the motor spinning. When it fails, the motor can't develop enough starting torque and draws excessive current trying. The overload trips within seconds of starting.

What you'll see

The motor hums or buzzes loudly when you flip the switch, doesn't spin or spins very slowly, and the overload trips within 2-5 seconds. If you spin the flywheel by hand while flipping the switch, the motor might start -- that manual spin replaces the starting torque the capacitor should provide. The capacitor canister may look swollen, have leaked oil, or show scorch marks. This is only applicable to single-phase motors -- three-phase motors don't use start capacitors.
Before you assume this is the problem

If the motor doesn't hum at all (just silence), it's a power supply or switch issue, not a capacitor. If the motor is three-phase, there's no start capacitor to fail -- check for phase loss or imbalance instead. See: Low Voltage, Pressure Switch Failed.

See all causes of overload relay trips →

How to diagnose

  1. Check for visual damage to the capacitor

    Find the start capacitor -- it's a cylindrical canister mounted on or near the motor. Check for swelling (bulging top or sides), leaked oil or electrolyte, scorch marks, or a cracked housing. Any visible damage means the capacitor is definitely failed.

    Result: Visible damage = replace immediately. Looks OK = test electrically.

  2. Test with a multimeter

    Disconnect power, discharge the capacitor, then disconnect one wire. If your multimeter has a capacitance function, measure across the terminals -- the reading should be within 10% of the rated value printed on the capacitor label. If not available, use the resistance test: set to ohms, touch probes to terminals, and watch the reading climb from low to high as the capacitor charges from the meter battery. No climb = capacitor dead.

    Result: Within 10% of rated value = OK. Zero, open, or far out of spec = failed.

How to fix it

  1. Replace the start capacitor

    Match the new capacitor to the old: same microfarad (MFD) rating and same or higher voltage rating. The MFD must match -- too high damages the start winding, too low doesn't provide enough torque. The voltage can be higher but never lower than original. Disconnect old wires, mount new capacitor, reconnect wires (not polarized -- either terminal works).

  2. Check the centrifugal switch

    If capacitors keep failing, check the centrifugal switch inside the motor. This switch disconnects the start capacitor once the motor reaches running speed. If it's stuck closed, the capacitor stays in the circuit too long and burns out. If stuck open, the capacitor never engages. Clean the contacts and make sure the mechanism moves freely.

Common mistakes

Don't use a different microfarad value 'because that's what the store had.' MFD matters. Also: repeated capacitor failures point to a centrifugal switch problem -- don't keep replacing capacitors without investigating why they keep dying. One more thing: on dual-voltage motors (120/240V), the capacitor value may differ between voltage settings. Make sure the motor is wired for the correct voltage and the capacitor matches.

Parts & tools

Replacement start capacitor (match MFD, same or higher voltage). Insulated screwdriver for discharging. Multimeter with capacitance function (preferred) or resistance test. Wrenches for mounting.

Review safety precautions before starting →

Safety

Capacitors store electrical charge. Disconnect power and discharge the capacitor before touching it. Short across the terminals with an insulated screwdriver to discharge safely.

This issue can also cause

  • Compressor Won't Start Motor hums, trips the breaker, or nothing happens: usually unloader valve, check valve, or electrical problems...