Question
After a period of standstill the insulation resistance of a high-voltage motor is clearly lower than the commissioning record. Can it still be started directly?
First Decide: Moisture or Damage
Measure insulation resistance at the specified voltage with a megohmmeter and record the absorption ratio. A value that rises during the test with a clear absorption ratio usually indicates moisture. A low, stable value with an absorption ratio close to 1, or an imbalance between phases, suggests local damage or contamination. Isolate and discharge the circuit and clean terminals and insulation surfaces before measuring.
Treating Moisture
- Open the terminal box and inspection covers and remove dust and oil contamination.
- Dry with hot air or by energising the windings, keeping temperature rise within the insulation class limit.
- Measure insulation resistance at intervals during drying and stop when the value is stable and acceptable.
- Re-measure the absorption ratio afterwards and confirm the space heaters work.
Treating Winding Damage
If the value cannot be recovered by drying, perform DC resistance and surge tests to identify turn-to-turn or earth faults. Once damage is confirmed, repair or rewind to the original parameters and repeat the withstand voltage test afterwards.
Checks Before Start-up
Confirm insulation resistance and absorption ratio, bearing lubrication, free cooling passages, foundation and coupling alignment, then verify the shaft turns freely and protection interlocks are healthy before starting to procedure. Motors on long standby should have space heaters energised to limit condensation.
Routine Management
Keep a log of insulation resistance together with ambient temperature and humidity, shorten the interval in the wet season, and avoid long operation at low load and low temperature which lets internal temperature fall below the dew point.
Measurement Method and Precautions
Isolate all supplies and external wiring, discharge the windings fully and clean terminals and surfaces before testing. Select the appropriate test voltage, read at the specified time and record ambient temperature and relative humidity so that results can be compared with history.
Measure the same machine under similar conditions for the figures to be comparable. If the value changes markedly with humidity, the insulation system is absorbing moisture and should be managed together with heaters and standby practice.
Moisture Control During Standstill
After shutdown the internal temperature falls and moist air condenses on winding surfaces. Energise space heaters on motors on long standby, or run the motor unloaded periodically so that internal temperature stays above the ambient dew point, and keep the machine room ventilated and free of standing water.
- Record insulation resistance after two weeks of standstill and compare with pre-shutdown data.
- Control heater power and energisation time per the manual to avoid local overheating.
- Check terminal box sealing and cable entry plugs before the wet season.
Practical Advice
Make insulation measurement a fixed item of the shutdown maintenance plan and record test conditions and instrument model. When a downward trend appears, arrange drying or repair early, which is more effective than waiting until the motor will not start.
References
- GB 18613-2020 Minimum allowable values of energy efficiency and energy efficiency grades for motors
- GB/T 755-2019 Rotating electrical machines — Rating and performance
- GB/T 1032-2012 Test procedures for three-phase induction motors
- GB/T 20161-2008 Guide for application of cage induction motors fed by converters
