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3-Phase Power Phase Loss/Voltage Imbalance — Compressor Overload Trip

Likely causes

Single-phase fuse blown or circuit breaker poor contact

Check: Measure each line-to-line voltage (R-S, S-T, R-T) — if one phase reads 0V, fuse blown or CB open. Measure fuse resistance: normal 0Ω, blown = OL

What to do: Replace blown fuse with same rating (A); if CB poor contact, replace or clean terminal and re-torque (M6: 4~6 N·m)

Line voltage imbalance >5% — causes compressor overcurrent

Check: Measure 3-phase line voltages (e.g. R:210V, S:220V, T:195V): imbalance(%) = max deviation ÷ average × 100. Above 5% = compressor current up to 25% higher

What to do: If imbalance originates at utility supply, report to power company. If internal wiring imbalance, redistribute loads or reinforce wiring

Main power terminal bolts loose/oxidized — increased contact resistance causes voltage drop

Check: Visually inspect terminal block bolts: oxidation (black or blue marks), burn marks, loose bolts. With thermal camera, imbalanced phase terminal runs 15°C or more above other phases

What to do: Sand oxidized terminals and retighten: M6 bolts 4~6 N·m, M8 bolts 8~10 N·m. If severe oxidation, replace terminal block

Magnetic contactor (MC) contact burned — incomplete conduction

Check: Measure MC 3-phase output terminal voltages — normal = same as input; if one phase output is low or 0V, that contact is burned. Direct inspection: look for burn marks or contact welding (stuck closed)

What to do: Replace MC contact kit or entire MC; after replacement, remeasure voltages to confirm 3-phase balance

Phase loss protection relay not installed or set incorrectly

Check: Check if phase protection relay exists in the control panel. If present, verify trip voltage setting — recommended trip at ≤85% of rated voltage

What to do: If not installed, add 3-phase phase loss/reverse/imbalance protection relay. Set trip at ≤85%. Install position: upstream of MC (line side)

Compressor winding ground fault — repeated overload trips due to poor insulation

Check: Measure insulation resistance (megger): compressor terminal-to-ground ≥1MΩ = normal; below 0.1MΩ = confirmed winding ground fault. Also measure 3-phase winding resistance — balanced = normal; unbalanced = open or short circuit

What to do: If insulation resistance <0.1MΩ, compressor replacement is unavoidable. Mandatory system flushing and filter drier replacement before installing new compressor

Field tip

When 3-phase voltage imbalance exceeds 5%, compressor current can increase by up to 25% — this is why repeated trips occur even with the same MCB rating. Never look at current alone; always measure all three line voltages simultaneously

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