1) Check you Megger voltage. The test voltage should be 500Vdc for motors rated 600V or less.
2) Polarization: "The resistance of a good insulation system will increase when subjected to a test voltage for a period of time." The resistance 60 seconds into a particular test should be at least 1.25 times the resistance 30 seconds into the test.
Both of the above are taken from EASA Principles of Large AC Machines, a book that I have on my shelf for other reasons. I've never had to personally run these tests or deal with the results, so absorb the above information with care. Point 1 is IMHO solid, point 2 less so.
Winnie, I Beleive Your second Point "resistance of a good insulation system will increase when subjected to a test voltage for a period of time" Is meant For Hi-Pot testing. Megger testing is for short Durations. Just be aware that there are Two Types of tests.
It's Not The Fall That Kills You... It's That Sudden Stop At The End
Allclear timed test are applicable to Mega testing.
A one minute ten minute test is common. We just had a Mega salesman at the shop giving us some lessons.
The best way to quantify the results of a time resistance test is through a dielectric absorption ratio. The dielectric absorption ratio consists of two time resistance readings. A commonly used set of intervals is a 60-second reading divided by a 30-second reading. Another frequently used set is a 10-minute reading divided by a 1-minute reading. This resulting value is referred to as the polarization index.
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As I understand it, a 'Hi-Pot' test is used infrequently, and is a 'pass-fail' test. The voltages used are such that they are potentially destructive, and which probably cause insulation deterioration. A megger test, on the other hand, is done at lower voltage, generally in the range of what the insulation should be able to tolerate continuously.
Again looking at the EASA book, the High Potential tests are run with AC of 2xrated voltage + 1000, or with DC at 1.7 x the AC test voltage.
I presume that dielectric polarization occurs in both types of test, at least at DC.