In a humid environment, problems mostly arise during stoppages rather than during operation. A running motor is kept dry by its own heat; once it stops, the frame cools, the humid air inside condenses on cold surfaces and leaves water on the winding and bearings. In plants that stop overnight, close at weekends or run seasonally, this cycle repeats and the insulation resistance falls over time.

The catalogue recommends:

  • On humid sites, consider the anti-condensation heater option; it keeps the winding slightly above ambient temperature while the motor is stopped. This option is requested when ordering.
  • Measure insulation resistance before starting after a long idle period.
  • Make sure the terminal box cover, its gasket and the cable gland are tight.

IP55 protects against water jets from outside; it does not prevent condensation forming inside. For coastal sites and salty air, see cast iron motors in salty environments, and for long stoppages see motor storage and moisture.