Insulation Resistance for Troubleshooting?

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Obviously, a lot of technicians and engineers out there do not read the MDMH Newsletter!  There is a consistent stream of questions received through SBD and MDMH about whether or not a winding is ‘safe’ or requires a rewind based upon a low insulation resistance (Megger) reading.  Often the reading is taken within a few seconds.

The primary purpose of the IEEE Std 43-2000 standard on insulation resistance testing is to evaluate whether or not higher voltage testing can be applied to evaluate the condition of the ground wall insulation.  The limits of 1 MegOhm plus 1 MegOhm per kV rating of the motor is for insulation systems prior to 1974, 5 MegOhms for random wound insulation systems below 1000 Volts, and 100 MegOhms for form-wound stators, using the appropriate voltages identified in the study, is specifically for whether or not a high potential test and/or surge test can be performed.  It does not verify that the winding is good and no insulation resistance test will detect shorted windings.  The only exception is a ‘dead ground,’ or a test result that is ‘0’ MegOhms to ground.

The test must be performed for a full minute and then adjusted for temperature.  If the reading is low but the winding is warm or hot from operating, then the result from applying the table will be much higher!

The polarization index and dielectric absorption tests are used to see if the winding is dirty or dry and brittle.  Most often the result of a low result, including a low insulation resistance, is strictly contamination requiring a cleaning.

In the newer version of the IEEE Std 43, still under development, the major changes that appear to be coming up, depending on the conclusions of the standards development group, will be the addition of a ‘polarization/depolarization’ test and ‘insulation resistance profile’ appendix, which will outline additional tests.  These will also help determine if winding results are from contamination, brittle windings, or other issues.

Any testing that involves applying an AC or DC potential between the winding conductors and ground is strictly for checking the condition of the ground wall insulation and will not identify turn shorts.  Usually, the result is an issue with winding contamination, which can be easily dealt with if detected early enough.

SBD Announces New Site!

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SUCCESS by DESIGN Publishing, established in 2001 by Howard W Penrose, Ph.D., CMRP, announces the launching of new sites during the week of November 25, 2013.  The purpose is to support several books slated to be published in 2014.

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