Compressor windings and the breaker
When the breaker trips on a cooling call, three readings tell you whether it's the capacitor, the compressor or the power.
The short version
- Run fan only first. If the fan runs clean and the breaker only trips with a cooling call, the fault is in the compressor circuit.
- Discharge and test the run capacitor. A weak HERM section makes a compressor hum, pull high amps and trip after a few seconds.
- Ohm the compressor: C to R, C to S, R to S. The two smaller readings should add up to the big one. Then every terminal to the case must read O.L.
- Ohms from a winding to the case is a short to ground. The breaker is right to trip, and no hard-start kit fixes it.
- Clamp the start: locked-rotor amps for under a second is normal. Pegged and staying there is a locked or shorted compressor.
On the job
- Fan only from the thermostat. Runs clean: the fault is in the compressor circuit.
- Power off, discharge the capacitor. Test HERM and FAN sections against the label.
- Ohm C-R, C-S, R-S. The two small readings add up to the large one. Each terminal to the case must read O.L.
- Clamp the start: high for under a second is normal. Pegged for seconds is locked or shorted.
- Meter voltage at the unit during the start. Under 108 V: fix the supply before you condemn the compressor.
Numbers
- Winding to case: O.L, always
- Capacitor: within about 6% of the label
- Locked-rotor amps: high for under a second, then down to running amps
Shop tip
Rooftop compressors aren't rebuilt in the field. A grounded winding means a new unit, and the note should say what you measured.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.