Compressor Lockout After a Breaker Trip
A 3-ton R-410A heat pump has a 240-V single-phase compressor. The breaker trips 8 seconds after a heat call. The outdoor fan runs normally. The compressor hums, and the contactor remains pulled in until the breaker trips. The technician locks out the unit, discharges the capacitor, and measures: common-to-run 1.1 ohms, common-to-start 2.9 ohms, run-to-start 4.0 ohms; resistance from each terminal to ground is greater than 100 megohms. The dual run capacitor is bulged on top; its label reads 35/5 µF, 440 VAC. The technician has a replacement capacitor of the same rating. The compressor nameplate shows LRA 68 A and RLA 15 A. The indoor unit is calling, and the outdoor fan is running.
1. Interpret the winding and capacitor readings. What is the most likely reason the breaker trips?
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The winding resistances are consistent with a normal single-phase compressor: common-to-run 1.1 ohms plus common-to-start 2.9 ohms equals 4.0 ohms run-to-start, and the insulation resistance is greater than 100 megohms to ground. That does not prove the compressor is mechanically sound, but it makes a shorted or grounded motor less likely. The bulged dual capacitor is the key finding. If the compressor section of the capacitor is open or weak, the compressor cannot develop starting torque and draws locked-rotor current. The breaker trips after a few seconds because the current exceeds the breaker rating, while the outdoor fan may continue running on the other capacitor section or a separate circuit. The capacitor should be tested for microfarads after discharge, not judged by appearance alone.
2. List the safety and verification steps for replacing the capacitor and restarting the unit.
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Lock out and tag out the disconnect, confirm zero voltage at the contactor line and load, and discharge the capacitor through a resistor. Verify 0 V with a meter before touching the terminals. Replace the capacitor with the same microfarad rating and voltage rating or higher, and reconnect the terminals exactly as labeled. Check the contactor points for welding or pitting and verify the compressor wiring. Restart and measure compressor current against RLA; a reading near LRA that does not drop indicates the compressor is not starting. Verify the new capacitor's microfarad value with a meter before and after installation. Do not bypass the breaker or increase the breaker size to mask a locked-rotor condition.
3. The new capacitor is correct, but the breaker still trips within 8 seconds. What should the technician check next?
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With the unit locked out, check for a mechanically seized compressor, incorrect wiring, a welded contactor, and low line voltage during startup. Measure the compressor current with a peak-reading meter; if it stays near the 68-A LRA and does not drop, the compressor is not developing rotation. Verify that the start components, if used, are correct for this compressor and that the run capacitor is actually in the circuit. Check the breaker and disconnect for loose connections or voltage drop. Do not replace the compressor solely on the breaker trip; confirm locked-rotor current and rule out electrical supply and contactor problems first. If the compressor is seized, the correct repair is replacement, not a larger breaker.