using megger to dry out motor ?!? 7

[electricpete]

Please don't laugh at this one. I don't believe it either.

Several electricians at our plant swear that they can make the insulation resistance reading on a random wound motor go up significantly by leaving 500vdc applied for hours.

I asked whether perhaps this is only a phenomenon of charging the capacitance and polarizing the insulation to "fool" the measurement. They assured me the motor was grounded after 500vdc applied but before megger... and further more lead polarity was swapped to eliminate effect from polarization.

Has anyone ever heard of this or seen this...a low megger reading cleared up by applying 500vdc for hours.

 

[jbartos]

Suggestion: Theoretically, there are two approaches to eliminate any moisture from the insulated conductors. First one applies heat to condutor with intent to remove any moisture from inside out and the second one is to apply heat from outside to have the moisture remove from outside toward inside the conductor over the insulation.
The first approach can lead to the water boiling point, i.e. 212°F, which could potentially damage conductor insulation. This may be compared to having the certain food warming up in a microwave oven. The heat generated from within causes small explosion and holes in the food.
The second approach would be somewhat slower; however, it will be safer, since the temperature will be highest at the surface and gradually decrease in the inward direction.