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Open-Drive External Motor Winding Overtemperature — Thermal Protection Trip

Likely causes

Supply voltage unbalance exceeds 3% — localized overheating in windings

Check: Measure all three phase voltages → unbalance = (max deviation/average voltage) × 100%. Report to utility if > 3%. Use clamp thermometer on motor housing — investigate cause if > 80°C (Class F max 155°C)

What to do: Resolve voltage unbalance (report to utility) → measure insulation resistance with 500V megger (rewinding if < 1MΩ) → clean cooling fan grilles + ensure 300mm clearance around motor → for VFD, maintain minimum frequency ≥ 25Hz

Excessive start frequency — more than 6 starts per day, interval less than 10 minutes

Check: Check control log for daily start count. Repeated inrush current at startup (5~7x rated) is the main cause of cumulative winding thermal damage

What to do: Limit starts to max 6 per day, minimum 10-minute interval. Use soft starter or star-delta starting to reduce inrush current

Field tip

Motor housing at 80°C = too hot to hold for 2 seconds. Quick field judgment. Insulation resistance below 1MΩ can often be solved by rewinding rather than replacing, saving cost.

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