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May 08, 2005

7 Motor Testing and CBM

Using Dielectric Absorption and Polarization Index testing, including strengths and weaknesses.


VP, Electrical Reliability Group
T-Solutions, Inc.

Dielectric Absorption and Polarization Index

Dielectric Absorption (DA) and Polarization Index (PI) are methods of measuring the slope of the curve as the electrical insulation (dielectric) polarizes.  Insulation that polarize very quickly is normally damp or contaminated while insulation that polarizes very slowly may be overheated or brittle.  The difference between the two are such that the DA is the ratio of the 60 second to 30 second insulation resistance reading while PI is the ratio of the 10 minute to one minute insulation resistance reading.

In insulation systems manufactured after 1970, any one minute insulation resistance reading above 5,000 MegOhms (5 GigOhms) renders the DA or PI inaccurate.  This is because modern insulation systems have a tendency to polarize rapidly and, in some cases, a good insulation system may fail either the PI or DA.  As a result, the updated IEEE standard that governs insulation resistance testing, IEEE Std 43-2000, published in May of 2000, places reduced emphasis on either the DA or PI.

The applied voltage for both tests are the same as the MegOhm test (insulation resistance). 

With Dielectric Absorption, DA = 60Sec Megohm/30Sec Megohm.  A ratio of less than 1 should be considered dangerous, from 1 - 1.4 questionable, from 1.4 - 1.6 good, and, greater than 1.6 excellent.

With Polarization Index, PI = 10Min Megohm/1Min Megohm.  A ratio of less than 1 should be considered dangerous, from 1 – 2 questionable, from 2 – 4 good, and, greater than 4 excellent.

The high end of both DA and PI is not defined by the IEEE standard.  A number of vendors have made recommendations of high end ratios of about eight.  However, my personal experience has shown that every one of these high ratios has had an exception.

Strengths

Both DA and PI will identify compromised insulation directly between the conductors closest to the frame and the frame.  Trending will provide some degree of accuracy when the test results are temperature corrected.  Continuous results graphed in PI that show signs of discharge (sudden drops in results) can show additional problems with the capacitance between conductors and ground.

Weaknesses

Limited ability of detecting or troubleshooting common motor faults.  Trendable data is preferred.  Megohm testing, DA and PI all require discharging the conductors to ground for 4 times the charging time (ie: 4 minutes for DA and Megohm; 40 minutes for PI).  Failure to discharge the insulation system will introduce additional stress when power is introduced, or can cause some level of discharge should personnel come into contact with conductors.

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