Run Capacitor
What a run capacitor actually does for a motor, and how to tell when it has failed.
Safety
What it is
A run capacitor creates a phase shift between the start winding and the run winding of a single phase motor. That phase shift is what produces rotating torque. It stays in the circuit the entire time the motor runs.
How it works
Typical dual run capacitor terminals
A dual run capacitor serves the compressor and the condenser fan from one can. The common terminal ties to the contactor load side, the HERM terminal serves the compressor start winding, and the FAN terminal serves the condenser fan motor.
How to diagnose it
Measure capacitance with the capacitor discharged and disconnected
A reading below the manufacturer tolerance means the capacitor has lost capacity and should be replaced.
Look for a bulged top or leakage
Visible deformation is a definite failure, but a capacitor can be out of tolerance and look perfect.
Listen for a motor that hums and will not start
A classic symptom of a weak or open run capacitor.
Discharge before touching
Symptoms
Practice & Mastery
- Questions3
- Quiz1
- Run Capacitor Troubleshooting Scenarios2
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