what is the minimal insulation resistance should a motor run? is there any formula for this one? depending on the voltage source i think? let say for 480V motor what would be the minimal insulation resistance it should run safely?
Insulation is not rated for "resistance" except in some exotic applications and laboratory procedures.
Insulation is rated for breakdown voltage. Obviously an insulation over 480V is required for a 480V line. With higher temp operations (as you would see in a motor), or high frequency, or otherwise tough environment, you will need to get a significantly higher voltage rating.
With a 1000V Insulation Resistance Tester (IRT/Megger) an acceptable reading is above 1 megohm. However in practice, it is normally much higher than that - IMHO if you have 1 megohm insulation resistance, there is a pending problem.
thank you phasor for your words. it seemed i have a problem with my 250KW sea water pump motor. it has a 1 MOHM insulation. what i did is put lamps around the motor to heat it up, maybe mists causes the downfall of the reading as the motor stands for a while due to delayed PMS activities.