Compressor Overload Trip on a Low-Charge Packaged Unit
A 5-ton R-410A packaged rooftop unit with a three-phase Copeland scroll compressor serves a small office. The unit cools for about eight minutes, then the compressor internal overload opens; the indoor fan continues to run. Supply voltage is 208 V phase-to-phase on all three legs, and the 24 V control circuit is stable. The compressor nameplate shows RLA 28.0 A and LRA 150 A. During the run period, clamp readings are 14.2 A, 13.8 A, and 12.9 A. Suction pressure is 92 psig with a 78°F suction line at the compressor. Liquid pressure is 305 psig with a 96°F liquid line. Outdoor ambient is 85°F; return air is 78°F dry bulb and 65°F wet bulb. The condenser coil is dirty, but the condenser fan is running at full speed. The unit has a factory charge of 9 lb 8 oz and a liquid-line sight glass that shows slight bubbling. The building manager wants the compressor replaced because it 'keeps tripping.'
1. Evaluate the compressor amp readings. Do they prove the compressor is mechanically failing? Explain what the pressures and line temperatures add to the diagnosis.
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The amp readings are below the 28.0 A RLA and are reasonably balanced, at roughly 14.2, 13.8, and 12.9 A. A failed or mechanically seized compressor would more typically show locked-rotor current, a large unbalance, or a trip immediately on start. Low running amps here point toward reduced mass flow and low load, not a bad compressor winding or mechanical failure.
The refrigerant readings support that view. At 92 psig, R-410A saturation is about 27°F. With a 78°F suction line, superheat is about 51°F, which is very high. At 305 psig, saturation is about 93°F; a 96°F liquid line gives essentially no subcooling, and the bubbling sight glass is consistent with flash gas. High superheat plus no subcooling indicates the unit is low on charge or has a restriction. The compressor is likely overheating because low-density suction gas is not removing motor heat, so the internal overload opens. Replacing the compressor without correcting the charge or leak would repeat the failure.
2. What is the most likely fault, and what service sequence should be followed before replacing any parts?
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The most likely fault is a refrigerant leak causing undercharge. The service sequence should begin with recovering the remaining refrigerant according to refrigerant-handling regulations; the system should not be vented. Next, perform a leak search using an electronic detector and, if needed, nitrogen pressure testing. Repair the leak, replace any filter drier that was exposed or restricted, and evacuate the system to the manufacturer’s specified micron level. Charge by weight to the data-plate amount, adjusting for line-set length if applicable. The dirty condenser must also be cleaned because it will raise head pressure and amp draw once the charge is corrected. After charging, verify subcooling of roughly 8°F to 12°F, superheat appropriate to the metering device and load, compressor amps compared with RLA, and a stable suction line temperature. Only if the compressor still trips after charge, airflow, and condenser airflow are correct should mechanical replacement be considered.
3. After the leak repair and weighing in the factory charge, the compressor runs at 26 A and the overload still trips within 10 minutes. What should be checked next, and how would you distinguish an electrical problem from a refrigeration problem?
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At 26 A the compressor is close to its 28.0 A RLA, so the system is now drawing normal-to-high current rather than the low current seen before. The next checks should focus on condenser airflow and head pressure, since the condenser coil was described as dirty. A restricted condenser raises discharge pressure, increases compressor amp draw, and can trip an internal overload. Verify that the condenser fan blade, motor, and coil are clean and that recirculation is not occurring. Check voltage under load for imbalance and verify all three legs remain near 208 V; a voltage imbalance can cause high current and overheating. Compare suction and liquid pressures with expected values after the system stabilizes. An electrical problem such as a failing contactor or loose connection would usually show voltage drop, pitting, or unbalanced current; a refrigeration problem would show abnormal high-side pressure, low subcooling, or high superheat. Do not condemn the compressor until these checks are complete.