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Motor Left Idle for Months? Pre-Start Checklist Before Restarting

Release Date ：2026-09-28

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Many plants restart motors after weeks or months of shutdown - and a surprising number burn out within minutes. The culprits are rarely mechanical: moisture in the windings, dropped insulation resistance, dried-out grease, and debris in the terminal box. This guide gives a practical pre-start checklist that prevents most restart failures.

SHANGDIAN motor pre-start inspection after long shutdown

1. Why Long-Dormant Motors Fail on Restart

During storage, windings absorb moisture from the air and insulation resistance drops; bearing grease slowly dries and separates; dust, insects or rodents can enter the terminal box; and corrosion can make the rotor stiff. Starting at full voltage without clearing these issues can trip the breaker or - worse - puncture the winding insulation and burn the motor out.

2. Pre-Start Checklist: 5 Steps Before You Power Up

Check	How	Acceptance	If not OK
1. Housing & terminal box	Open terminal box; check terminals, cables for looseness, corrosion, foreign objects	Terminals tight, no oxidation, clean	Clean, retighten, replace terminals if needed
2. Insulation resistance	Measure winding-to-ground with a 500 V megger (phase-to-phase if required)	≥1 MΩ (generally ≥0.5 MΩ acceptable)	Dry the motor first, then re-test
3. Bearing lubrication	Turn rotor by hand; listen for noise; inspect grease	Free rotation, no abnormal noise	Regrease or clean and repack - see our bearing grease guide
4. Ventilation path	Check fan cover, cooling fins, fan blades for blockage or damage	Air path clear, blades intact	Remove debris, replace damaged blades
5. No-load trial run	Start uncoupled (or disengaged); run 30-60 min	No noise, no overheating, balanced phase currents	Stop and investigate; contact the manufacturer

3. Insulation Resistance Too Low? Dry Before Starting

Never energize a motor whose insulation resistance is below 0.5 MΩ after storage. Common drying methods:

Natural drying: Place in a ventilated dry area with a dehumidifier or space heater for 24-72 hours, then re-test.
Oven drying: Dry the whole motor in an oven at 80-100°C (below the insulation class limit) for 12-24 hours.
Short-circuit current method (small motors): Lock the rotor and feed about 10% rated voltage to heat the windings, watching the temperature rise.

Only continue once the re-test passes.

4. Different Shutdown Durations, Different Effort

Up to 1 week: Visual check plus hand-rotation is enough.
About 1 month: Add insulation-resistance measurement and grease inspection.
Over 3 months: Go through the full 5-step checklist; clean and repack bearings if needed.
Explosion-proof motors: Also inspect flameproof joints, fasteners and sealing rings. Restarting a YBX3 flameproof motor in a chemical plant or mine should follow the Ex-standard check by qualified personnel.

5. During and After Start-Up

Even after the checks pass: jog 2-3 seconds to confirm rotation direction; run no-load for at least 30 minutes and watch bearing temperature (should not feel hot to the touch) and vibration; then run at reduced speed under load before full speed. Pay extra attention during the first 24 hours - the failure rate of motors returning to service is slightly higher than steady operation.

If your low-voltage induction motor has been idle for a long time and you are unsure how to handle it, contact the SHANGDIAN (Shanghai Electric Motor Factory) team for inspection and restart guidance.