When the Heating Is Off: A Winter Moisture Routine for Bosch Motors and Drives
An unheated warehouse in the UK is not a stable environment. In autumn and winter, internal temperatures can rise on a mild afternoon and fall sharply overnight. Warm, moist air entering a building or an enclosure can reach its dew point on cold motor frames, drive heatsinks and printed circuit boards. The result is condensation that is invisible at the end of the shift and harmful by the time production restarts.
Why idle plant is vulnerable
Running equipment dries itself. Idle equipment does not. A motor that is stopped for weeks has no internal airflow and no winding heat, so moisture can settle on windings and in terminal boxes. A drive that is isolated has no DC link activity and no internal heating; its control boards and capacitors sit at ambient temperature. If the enclosure breathes moist air, condensation can form on cool surfaces. The risk is highest when a cold spell is followed by a rapid thaw or when doors are opened for stock movement.
Consider the thermal mass. A steel motor frame or a cast enclosure may stay cold for hours after the air has warmed. Moisture in the air then condenses on the cold metal. This is why a dry-looking store can still produce wet windings.
Check anti-condensation heaters
Many industrial motors have space heaters or winding heaters. Confirm that they are rated for the motor, supplied independently of the drive and controlled by a thermostat or humidistat. The heater must be interlocked so that it cannot be energised while the motor runs. Measure the heater resistance and supply voltage, and check that the heater is actually warm when the motor is stopped. A heater that has failed open is easy to miss because the motor still runs.
Inside drive enclosures, use a panel heater or an anti-condensation heater with a thermostat. Do not rely on the drive's own heat when the drive is isolated. If the enclosure has a heater, check that it is not blocked by cable ducts and that its supply is protected and labelled.
Manage enclosure breathing
Check the IP rating and the condition of gaskets, door seals, cable glands and breather drains. A missing gland blanking plate or a hardened seal lets moist air in. Filter fans can bring in humid air and dust; if the enclosure does not need forced ventilation in winter, consider sealing it and using a heater. Where breather drains are fitted, confirm they are at the lowest point and not painted over.
Desiccant bags can help in a sealed enclosure, but they must be replaced or regenerated. A saturated bag is no longer a drying agent. Do not open enclosures on warm, humid days for inspection unless necessary; every opening exchanges air.
Test insulation resistance before power
Before energising a motor that has been idle, isolate it and disconnect the drive. Measure insulation resistance phase-to-earth and phase-to-phase with a 500 V DC tester, following the motor manufacturer's instructions. Record the values, the test voltage and the winding temperature. A single pass is less useful than a trend; a winding that has fallen from hundreds of megohms to a few megohms needs investigation before it is run.
If the reading is low, dry the motor before applying power. Space heaters, warm air or a low-voltage winding heating method can be used, but the method must be controlled and the temperature must stay within the insulation class limits. Do not use a high-voltage flash test as a routine drying check.
Store drives correctly
Bosch drives should be stored in a clean, dry place within the temperature and humidity limits in the product manual. Keep them in their original packaging until installation, and remove any transport protection only when the enclosure is ready. Long storage affects DC link capacitors; follow the manual's reforming procedure before applying full voltage. If the drive has been stored for an extended period, a controlled power-up with no load is safer than a direct start.
Record the storage period and any reforming date. If a drive has been moved from a cold store to a warm room, allow it to reach ambient temperature before opening the packaging; otherwise condensation can form on cold surfaces.
Restart checks before production ramps up
When production restarts, do not go straight to full speed. Check that heaters are off and isolated, that enclosure doors and glands are sound, and that fans and filters are clean. Verify motor terminal tightness and encoder couplings. Run the motor at low speed and monitor current, vibration, bearing temperature and any earth leakage indication. Listen for unusual noise and check that the drive displays no pre-charge or earth fault alarms.
Complete a short inspection after the first shift. Condensation that formed during the idle period may only show up as a low insulation reading or an intermittent earth fault when the motor warms up.
A simple winter routine
| When | Check |
|---|---|
| Before winter | Inspect heaters, seals and drains; record insulation resistance; confirm drive storage dates |
| Monthly while idle | Verify heater operation; check enclosure condition; look for water marks or corrosion |
| Before restart | Test insulation resistance; inspect fans and terminals; run at low speed and monitor |
| After first shift | Recheck for alarms, moisture and abnormal temperature |
A written routine is more reliable than a verbal reminder. Assign the checks to a competent person, keep records and review the results after each cold snap. In an unheated UK store, moisture control is not a seasonal luxury; it is part of the asset maintenance strategy.
