Condenser Fan Motor Cycling on Internal Overload
A 3-ton split system uses a permanent split capacitor condenser fan motor rated 1/4 hp, 208/230 V, and 1.4 A. After 12 minutes the fan stops; the motor housing is hot to the touch, and the fan restarts only after cooling for 20 minutes. The compressor keeps running. The dual-run capacitor is marked 35/5 µF ±6%. A technician measures 34.2 µF on the hermetic section and 4.1 µF on the fan section. With power off and the capacitor discharged, winding resistance is C-to-R 2.4 Ω, C-to-S 5.1 Ω, and R-to-S 7.5 Ω. Supply voltage at the contactor is 242 V, and control voltage at the contactor coil is 24 V. The unit nameplate lists minimum circuit ampacity 18 A and maximum fuse 30 A. The breaker is a 30 A two-pole. The fan blade spins freely, and the condenser coil is clean.
1. Evaluate the capacitor and winding readings. What is the most likely cause of the overheating and cycling?
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Check the capacitor against its marked tolerance first:
- Hermetic section: 35 µF ±6% allows 32.9–37.1 µF. The measured 34.2 µF is acceptable.
- Fan section: 5 µF ±6% allows 4.7–5.3 µF. The measured 4.1 µF is about 18% low and is failed.
The winding resistances are consistent with a normal single-phase PSC motor: C-to-R plus C-to-S should equal R-to-S. Here 2.4 Ω + 5.1 Ω = 7.5 Ω, matching the measured R-to-S value. That does not prove the motor is perfect, but it rules out an open winding and makes a shorted turn less likely.
A weak run capacitor reduces the phase shift and starting/running torque, so the motor draws higher current and runs hotter for the same load. The internal overload then opens, and the motor restarts after cooling. Replace the capacitor with the same microfarad rating and a voltage rating equal to or higher than the original. After replacement, measure fan motor current against the 1.4 A nameplate and confirm the motor no longer overheats. If current remains high, replace the motor and inspect the blade and bearings.
2. The installer suggests using a 7.5 µF capacitor 'for more starting torque.' Explain why this is incorrect and what damage could result.
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The capacitor value is matched to the motor's auxiliary winding design. Increasing capacitance changes the phase angle and increases current in the start/auxiliary winding. It may appear to improve starting torque, but running current and winding temperature rise, the motor's torque curve shifts away from its design point, and the overload can trip more often. On a PSC motor, an oversized run capacitor can overheat and fail the auxiliary winding, increase supply current, and cause nuisance breaker trips. It can also cause the motor to run at the wrong speed or with reduced efficiency.
The correct replacement is the marked microfarad value within the capacitor's tolerance, with a voltage rating equal to or greater than the original (commonly 370 V or 440 V for this application). Never use capacitance to compensate for a bad motor, restricted blade, or incorrect voltage.
3. List the lockout/tagout, discharge, and verification steps you would perform before and after replacing the capacitor.
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Before replacement:
- Turn the thermostat off and open the unit disconnect.
- Lock out and tag the disconnect; verify with a voltmeter that no voltage is present at the contactor, capacitor terminals, and motor leads.
- Discharge the capacitor through a resistor rated for the voltage, then verify near-zero voltage.
- Photograph or record lead placement; confirm the replacement capacitor's µF and voltage ratings.
After replacement:
- Verify wiring against the unit diagram, secure the capacitor, and restore the panel.
- Energize and check supply voltage, control voltage, and fan motor current against nameplate.
- Confirm the fan starts promptly and runs without overheating through a full cycle.
- Check that the 30 A breaker is appropriate for the 18 A MCA and 30 A maximum fuse marking, and inspect connections for heat damage.
Document the failed capacitor reading, replacement rating, and final amp draw.