What is IR and PI value? Why it is measured?

What is IR and PI value? Why it is measured?
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1 Answer

Answer :

IR It is the ratio of the DC voltage applied between the terminals and ground to the resultant current. When the DC voltage applied three components flow,

a) A charging component flows into the capacitance of the winding.

b) A polarization or absorption current involving in the insulation molecular mechanism.

c) A leakage component over the surface between exposed conductor and ground which is highly dependent on the state of dryness of the winding. 

The first two current components decay with time. The third component is determined by the presence of moisture or ground fault and relatively constant with time. Moisture may be with in the insulation or condensed on the end windings or connection surfaces, which are often dirty. If this leakage current is larger than the first two components then the total charging current will not change significantly with time. Thus to determine how dry and clean the winding, IR is measured after one minute and after 10 minutes. The ration of the 10 minutes reading over the oneminute reading is called the PI (Polarization Index). PI value detects relative condition of insulation with respect to moisture and other contaminants.
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