The compressor motor pulled more current than its limit, causing the overload protector to trip. This can happen due to high head pressure (compressor working too hard, e.g. if condenser is dirty or overcharged refrigerant) or a tight/internally damaged compressor struggling to turn.
If the compressor failed to start properly (locked rotor condition or a starting capacitor issue for single-phase), the sudden high amperage will trip the overload almost instantly.
Phase imbalance, low voltage, or power spikes can lead to overheating of the motor and an overload trip. A motor starved of voltage runs hot and can trip on overload even under normal mechanical load.
It's possible the overload relay itself is overly sensitive or malfunctioning. However, given its critical role, assume the trip was valid until proven otherwise.
Before resetting anything, do a quick visual check. Is the condenser clean and fans working (to rule out high pressure cause)? Also listen for any unusual sounds from the compressor area. If everything looks normal externally, proceed to reset.
Turn off power to the chiller for safety. Open the electrical panel covering Circuit 1's compressor controls (only if you are trained/authorized). Locate the compressor's overload relay (often part of or near the contactor labeled Q1 for compressor 1) and press the reset button on it. It usually clicks back into place.
While in the panel, if you are comfortable, visually inspect the wiring to the compressor contactor – loose connections can cause heating. Tighten any obviously loose lugs (with power off, using appropriate tools).
Close up the panel and restore power. Reset the AL175 alarm on the controller (hold reset button ~5 seconds) to clear it. The chiller will attempt to restart the compressor.
If you have a way to monitor the compressor amperage (some advanced chillers display current, or using a clamp meter if qualified), watch the compressor as it starts. It should ramp up and then settle at its normal running current. Listen for smooth operation.
If possible, reduce the cooling load or raise the temperature setpoint slightly for initial restart. This makes it easier on the compressor until you're sure it's running reliably. You can return to normal setpoint after a short while.
A professional HVAC technician can perform tests like a megohmmeter check on the motor or investigate mechanical failure. They might perform tests to check for high head pressure, incorrect refrigerant charge, or motor wear issues. Always involve professionals for major interventions like compressor replacement.
