When the motor becomes wet due to being flooded or climatic conditions or when the motor rewinding is done. It shows very low value of insulation resistance.
The low value of insulation resistance is due to the fact that moisture comes in the insulation or part there of.
So it become necessary to improve the insulation resistance value of the motor otherwise there will be very bad effect on motor life.
In case of small and miniature motors, insulation of the motor can be dried up by use of halogen lamps.
But in case of large motors , except halogen lamps , drying up also can be done by passing short circuit currents through motor windings at low voltage by using a variac (auto transformer).