Contactor Pulls In, Compressor Hums and Trips
You are called to a 2.5-ton residential split with a reciprocating compressor and a single-pole contactor. The thermostat is calling for cooling. The indoor blower runs continuously. The contactor pulls in, the compressor hums for about 5 seconds, and the internal overload opens; the cycle repeats a few minutes later. With the contactor pulled in, line voltage is 241 V at the line terminals and 241 V at the load terminals; coil voltage is 26 V. Winding resistance taken at the compressor terminals: C-R 0.9 Ω, C-S 2.1 Ω, R-S 3.0 Ω. Each terminal to the compressor shell reads greater than 100 MΩ at 500 V. The dual run capacitor is labeled 45/5 µF; your capacitor meter reads 0 µF across the hermetic (compressor) section.
1. State your diagnosis and show how each measurement supports it, including why the compressor and contactor are not the problem.
Read the worked answer
Diagnosis: the compressor run capacitor is open (failed).
Supporting logic:
- Contactor is fine. 241 V on both line and load side while energized, and 26 V at a 24 V coil, means the contactor is closed and passing full voltage. No voltage drop to chase.
- Windings are fine. C-S (2.1 Ω) ≈ C-R (0.9 Ω) + R-S (3.0 Ω) = 3.0 Ω, which is the expected series relationship; the start winding has roughly twice the resistance of the run winding, as it should. No short between windings.
- Insulation is fine. Greater than 100 MΩ to ground at 500 V means no ground fault.
- The capacitor is the outlier. An open run capacitor removes the phase shift that creates starting torque in a permanent-split-capacitor compressor. The compressor cannot accelerate, draws locked-rotor current, hums, and the internal overload opens - exactly the 5-second hum/trip pattern observed.
Note that the contactor dropping out is not a control fault: the overload opening de-energizes the compressor circuit, which is what you are seeing.
2. What safety steps apply specifically to testing and replacing this capacitor?
Read the worked answer
- De-energize and lock out the outdoor disconnect and verify at the contactor with a proven meter.
- Discharge the capacitor through a resistor across the terminals, then confirm 0 V with a meter before handling it. A 45 µF, 370/440 V capacitor holds a real charge even with power off.
- Replace with the correct ratings: same microfarad value (within the capacitor's tolerance band, typically ±6%), same or higher voltage rating, same terminal configuration. Never substitute a lower µF value or a lower voltage rating - a lower voltage rating will fail, and a large µF mismatch changes motor current.
- Do not re-use a bulged or leaking capacitor, and do not 'round up' capacitance to make the compressor start; if the compressor needs more starting torque, the fix is the correct start-assist components, not an oversized run capacitor.
- PPE: gloves and glasses, and keep the meter leads clear of the fan blade and the contactor line side while testing.
3. With the new capacitor installed, the compressor starts but draws 21 A against a 14.0 A RLA and trips on overload after about 90 seconds. What do you check next, and what might the earlier repeated locked-rotor starts have done?
Read the worked answer
Work from the outside in:
- Equalized pressures: confirm the compressor was not started against a high head. Check that the condenser fan is running, the coil is clean, and there are no non-condensables or an obvious overcharge. High head = high amps.
- Voltage under load: measure at the compressor terminals while running, not open-circuit. A long run of undersized wire can show 241 V open and sag badly under locked-rotor current.
- Start components: on a PSC compressor, verify the start relay/PTCR (if fitted) and confirm the new capacitor is actually in the circuit and not on the wrong terminals.
- Electrical integrity re-check: re-read winding resistances and megger the windings. Insulation that read greater than 100 MΩ cold can break down hot; a reading that collapses when the compressor warms is a failing hermetic terminal or winding.
- Mechanical condition: listen for slugging or a metallic knock and compare suction/head to expected values. A compressor that trips at 90 seconds with good voltage and equalized pressures may be mechanically seized or have a failing valve plate.
The earlier repeated locked-rotor cycles are not harmless - each one drove locked-rotor current through the windings and heated them, which is a common way a bad capacitor cascades into a burned-out compressor. Document that the original failure was the capacitor, and if the compressor is now failed, explain the sequence to the customer.