WiremanWireman · Capacitor keeps failing: troubleshooting for electricians · 2023 NEC ·
Capacitor keeps failing
The capacitor is usually the victim. Find the voltage, heat or start-circuit problem that keeps killing it.
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How it works
Run capacitors stay in the circuit the whole time the motor runs and are built for continuous duty. Start capacitors are only meant to be in the circuit for a second or so while the motor starts, and a centrifugal switch or start relay takes them out. When capacitors keep failing, the capacitor is usually the victim: too much voltage, too much heat, the wrong replacement, or a start capacitor that stays in too long.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Which capacitor keeps failing?
- Start capacitorTest: Time how long the start capacitor stays inExpect: A burst of current that drops to zero within about a second as the motor comes up.
- Current keeps flowingStart switch or relay not dropping out
- Drops out quicklyHow does the motor start?
- Starts many times an hour (short cycling)Too many starts
- Slow, hard startsHard or slow starts
- Normal startsWrong replacement capacitor
- Run capacitorDoes the replacement match the original µF, with a voltage rating at least as high?
- Wrong µF or lower voltage ratingWrong replacement capacitor
- It matchesTest: Measure supply voltage and capacitor voltageExpect: Supply within 10% of nameplate. Voltage across a run capacitor is normally higher than the supply, but must stay below the capacitor’s rating.
- Supply high, or capacitor near or above its ratingVoltage too high for the capacitor
- Both normalIs the capacitor in a hot spot, or is the motor drawing high current?
- Hot spot: near the compressor, in the sun, no airflowHeat
- Motor amps high or motor failingThe motor is stressing the capacitor
- NeitherLoose terminals or a poor part
- Start capacitorTest: Time how long the start capacitor stays inExpect: A burst of current that drops to zero within about a second as the motor comes up.