polarization Index for windings

[vinod]

Sir
What is the Polarization Index (PI)?
Why conducting this test for windings (Large transformers & Motors)?
What is the problem, If obtained higer value?

Kindly help me to find out answers...

 

[zog]

Pi is simply a 10 minute megger reading divided by a 1 minute megger reading. It is done on inductive equipment and simply put measures the efficiency of the insulation, meaning how much energy is used to polarize the insulation. You want the PI ratio to be high as possible, >4.0 is good.