Compressor Trips on Overload After a Control Board Replacement
You attend a walk-in cool room whose single-phase condensing unit keeps tripping its compressor overload after about four minutes of running. A previous contractor replaced the defrost/control board two weeks ago. The unit is R-134a, 230 V single phase, with a run capacitor, a start relay, a high-pressure switch, and a low-pressure switch wired in series with the contactor coil. You find: supply voltage 236 V at the isolator, 229 V at the compressor terminals while running, compressor current rising from 6.1 A to 9.8 A before the overload opens (nameplate RLA 6.4 A, LRA 38 A), condenser fan running, condenser coil partly blocked with lint, and the control board's defrost termination sensor reading 18 °C while the coil is frosted. The new board was configured for a different model and the defrost schedule is set to 4 defrosts per day with a 45-minute maximum.
1. Separate the electrical, refrigeration, and control causes and rank them by likelihood. Justify your ranking with the measured data.
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The current signature is the key: 6.1 A is close to the 6.4 A RLA, then it climbs to 9.8 A over four minutes. A failing run capacitor or a compressor with worn bearings/valves typically shows high current from start, not a gradual climb. A gradual rise over minutes with a partly blocked condenser points first to rising head pressure: as the condenser heat-rejection falls, discharge pressure and therefore compressor load climb until the overload opens. That is consistent with the lint-blocked coil and with a frosted evaporator (poor airflow across the coil from unresolved defrost). The control evidence supports the defrost theory: the termination sensor reads 18 °C while the coil is frosted, so the board believes defrost is complete and returns to refrigeration with an iced coil. A wrong model configuration with four defrosts per day and a 45-minute maximum may be inadequate for the load. Electrical causes rank lower but must be excluded: verify capacitor microfarads against rating, check start relay contacts, and confirm the high-pressure switch is not chattering. Supply voltage at 229 V under load is acceptable, so voltage is not the driver.
2. Describe the safe diagnostic sequence and the corrective actions, including what you would record and hand over.
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Isolate and lock out, then inspect and clean the condenser coil and confirm fan operation and airflow. Test the run capacitor with a meter rated for it (discharge first), check the start relay and overload, and inspect compressor terminal connections for heat damage. Refrigeration side: confirm the defrost termination sensor is correctly located, clamped to the coil in the right position, and reading plausibly against a known reference; a sensor reading 18 °C on a frosted coil is a fault, not a calibration quirk. Then correct the board configuration to the actual model, set the defrost schedule and termination temperature to the manufacturer's specification for this room, and force a defrost to verify termination. Re-check compressor current at stable conditions and compare against RLA. Record before/after readings, the board configuration change, the sensor finding, and the coil cleaning. On handover, explain to the customer that the previous board replacement left the unit misconfigured and that the blocked coil and faulty termination caused the overload trips; recommend a defrost schedule review and a follow-up current check. Do not simply reset the overload and leave.