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Compressor motor overheat — internal overload (Klixon) tripping and root-cause diagnosis

Likely causes

How a Klixon trips — direct vs indirect causes

Check: Principle: A Klixon is a bimetallic disc near the motor winding. When winding temperature reaches ~110–135°C the disc snaps and opens the circuit → compressor OFF. It auto-resets at 60–90°C. Direct cause: ① winding inter-turn short (insulation failure). Indirect causes (far more common): ② superheated discharge gas (low charge, overload, fouled condenser) heating the suction-cooled motor, ③ failed start with sustained LRA (locked rotor amperage), ④ low/unbalanced supply voltage, ⑤ very high ambient limiting heat rejection.

What to do: Initial diagnostic order: ① Megger (500V) winding-to-ground insulation. <1 MΩ → likely winding damage. >10 MΩ → winding ok, suspect indirect cause. ② Winding-to-winding resistance (R-S, S-C, R-C; or U-V, V-W, W-U). Within ±10% of datasheet. One leg different = possible short. ③ Discharge line temperature — normal 60–80°C; >90°C suggests overheat. ④ Operating current — within ±10% of RLA (rated load amperage). Higher = overload. ⑤ Supply voltage — within ±10% of nameplate; for three-phase, phase-to-phase imbalance <2%.

Confirmed winding damage — not repairable; replace compressor and clean system

Check: Confirmation: ① Insulation <1 MΩ + one winding leg shows abnormal resistance → short. ② Oil — very dark brown/black with strong acidic burnt smell. ③ Acid test exceeds 0.05 mgKOH/g or shows immediate color change. ④ Discoloration/soot near the compressor terminal area. ⑤ External breaker / overload relay repeatedly tripping. This is a 'burnout' — acidic byproducts in the system can corrode other parts (filter-drier, condenser, TXV), so a clean-up procedure is mandatory.

What to do: Burnout clean-up: ① Recover the refrigerant entirely (use a dedicated tank — acidic refrigerant can contaminate hoses). ② Replace compressor + filter-drier + add a suction-line filter-drier in addition to or replacing the existing one. ③ Blow nitrogen through liquid and suction lines in both directions to expel residual acid and debris. ④ Install new compressor → evacuate ≤500 micron with 30-min decay → recharge with fresh refrigerant. ⑤ One week after running, retest oil acid — if residual acid remains, replace the filter-drier again. ⑥ If acid corrosion has progressed into other parts (e.g., condenser tube interior), consider partial replacement or complete system re-build.

Field tip

Common field mistake: protector trips → operator just resets and restarts → underlying cause persists → eventual winding damage. A Klixon trip is the result, not the cause. Each unaddressed reset compounds winding damage and pushes toward burnout. On the first trip, run the four-axis diagnosis to pin the cause first.

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