Compressor humming and tripping overload: HVAC-R troubleshooting guide
Compressor hums without starting and trips overload? Diagnose voltage, capacitor, relay, overload, windings, pressures and locked rotor safely.
A compressor that hums and trips its overload is often receiving a call but failing to accelerate. Possible causes include low voltage, excessive voltage drop, an out-of-tolerance capacitor, failed relay, resistive connections, unequalized pressures, a weak protector or a locked rotor. Repeated energizing can overheat the windings.
Copeland's official compressor will-not-start procedure calls for supply, ground, resistance, current and start-component checks before condemning the compressor.
1. Stop repeated attempts
Disconnect power if the compressor hums without starting, draws heavy current or quickly opens its protection. Allow the specified cooling period before testing again. An open internal overload can look like an open winding.
2. Confirm voltage at the compressor
Measure supply voltage at rest and during the start attempt at the specified points. Check phase imbalance, connection torque, contactor, disconnect, fuses and voltage drop. Correct voltage at the panel does not guarantee correct terminal voltage under load.
3. Measure starting current
Compare measured current with nameplate data, including LRA and RLA where applicable. Current near locked-rotor value without acceleration confirms the symptom, not the cause. Disconnect promptly to avoid repeated thermal cycles.
4. Test the capacitor and starting device
Lock out equipment, safely discharge the capacitor and measure capacitance out of circuit. Compare it with the marked tolerance. Inspect the potential relay, PTC, electronic module and wiring for the specific design. Never improvise a replacement value.
5. Check windings and ground
With conductors removed and identified, measure terminal-to-terminal resistance and each terminal to the shell using the proper instrument. Inconsistent values, a winding that remains open after cooling or insulation failure require comparison with manufacturer data. See the detailed C-R-S and ground test guide.
6. Consider pressure and mechanical load
An immediate restart against a large pressure difference may exceed available torque. Verify anti-short-cycle delay, equalization, valves, charge and temperature. A mechanically locked compressor can have apparently normal winding resistance while drawing LRA.
7. Use a start kit only when approved
A start device must not hide low voltage, a failed capacitor or a condemned compressor. Use only a solution approved for the equipment and record measurements before and after. Systems with restrictive metering devices may have specific requirements.
8. Find the cause after replacement
If replacement is required, identify the root cause: voltage, condenser airflow, floodback, superheat, contamination, oil return, short cycling or operation outside the envelope. Otherwise the replacement compressor may suffer the same failure.
Diagnostic checklist
- Voltage at rest and during the start attempt.
- Starting current and LRA/RLA data.
- Measured capacitor value and tolerance.
- Contactor, connections and start-relay condition.
- Winding resistance and insulation to ground.
- Pressures, equalization time and compressor temperature.
FrigoTechPro's electrical solver, nameplate scan and HVAC-R AI assistant keep those readings with the job. Continue with the HVAC capacitor guide and the breaker-trip guide.