Three-Phase Rooftop Compressor Nuisance Trips
A 460 V, three-phase rooftop unit serving a sales floor has a 5-ton scroll compressor with an RLA of 18.5 A. The unit trips its compressor overload after 20 to 30 minutes, then resets and runs again. At 95°F outdoor ambient, you measure line-to-line voltages of 474 V, 469 V, and 459 V. Compressor currents are 18.0 A, 17.6 A, and 12.3 A. Winding resistance is equal within tolerance, and insulation resistance is 100 MΩ to ground. The contactor shows pitting on the L3 contacts, and the terminal screws are tight. The supply breaker is a 40 A three-pole with no nuisance trips on the condenser fans. The customer reports the problem began after a utility transformer was replaced. The compressor is not hot enough to trip on internal thermostat alone at startup.
1. Calculate the voltage unbalance and current unbalance, and interpret the results.
Read the worked answer
Average voltage = (474 + 469 + 459) / 3 = 467.3 V. The greatest deviation from average is 467.3 − 459 = 8.3 V. Voltage unbalance = 8.3 / 467.3 × 100 = 1.8%. Average current = (18.0 + 17.6 + 12.3) / 3 = 16.0 A. The greatest current deviation is 16.0 − 12.3 = 3.7 A. Current unbalance = 3.7 / 16.0 × 100 = 23%. A voltage unbalance of 1.8% is already significant; NEMA-style guidance expects current unbalance to be roughly 6 to 10 times the voltage unbalance, or about 11% to 18%. The measured 23% is worse than that, so the supply voltage is not the only problem. The pitted L3 contacts are a likely high-resistance connection causing the L3 current to drop and the other two legs to rise. The compressor is running with a severe current imbalance even though two legs are below RLA.
2. What is the most likely cause, and what tests would you perform before replacing the compressor?
Read the worked answer
The most likely cause is a high-resistance connection in the L3 circuit, probably the pitted contactor, with a contributing supply voltage imbalance from the transformer change. Do not replace the compressor yet. Perform a voltage-drop test across the L3 contactor under load and compare it with L1 and L2; a difference of more than a few volts indicates bad contacts. Check line and load terminals, fuse clips, breaker connections, and the disconnect for heat or arcing. Measure voltage at the compressor terminals while running, not just at the disconnect. Record current on all three legs after repairing the contactor. Ask the utility to confirm the transformer connection and voltage balance. Also check for single-phasing or a lost leg under load. If current balance improves and the overload still trips, then evaluate the compressor overload and refrigerant charge, but the electrical imbalance must be corrected first.
3. Explain why the overload can trip even though the highest current is below the 18.5 A RLA.
Read the worked answer
The overload protector responds mainly to internal motor temperature, not simply to the highest line current. An unbalanced three-phase supply produces a negative-sequence current that creates a reverse-rotating magnetic field. That field causes rotor heating and stray losses that are not reflected in a simple average of the three line currents. With 23% current unbalance, the motor can overheat while one leg reads only 18.0 A, which is below RLA. The pitted contactor also adds localized heating at the terminals and can worsen the imbalance as it heats. This is why a compressor can trip after 20 to 30 minutes rather than immediately: the thermal protector is integrating heat over time. Replacing the compressor without correcting the contactor and voltage imbalance would likely repeat the failure.