megger test
megger test
(OP)
Ran across something I don't understand today. Had to perform a megger test on an endcap per customer specs. What is being checked is the resistance between a bearing bore liner and the endcap. The idea is to limit current leakage through the motor bearings. spec is 1 megohm or greater is considered good.
Here's the odd part...the test polarity seems to be important. Parts which fail with one polarity pass with the other. Insulation resistance reads much higher when the positive probe is connected to the endcap and the negative is connected to the bearing liner.
How can this be?
Here's the odd part...the test polarity seems to be important. Parts which fail with one polarity pass with the other. Insulation resistance reads much higher when the positive probe is connected to the endcap and the negative is connected to the bearing liner.
How can this be?





RE: megger test
RE: megger test
RE: megger test
I'm sure it has nothing to do with your insulation which is probably mycarta, nomex or some other dry insulation, nothing like oil-filled transformer. Just wanted to mention it.
(If anybody remembers details of this test and the theory behind it, can you refresh my memory?)
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RE: megger test
Now it is bugging me that I can't remember the details of that test.
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RE: megger test
An added bit of information. The bearing liner is hardcoated aluminum and cemented into the endcap.
Hmmmm. Sounds a bit like a diode?
RE: megger test
RE: megger test
I looked through my copy and the closest test I found is called the Step Voltage Test.
From TMI text:
"The applied voltage should preferably be in the ratio of 1 to 5 or greater (500 and 2500 V, for example) both applied for one minute. Results to date show that a decrease in the insulation resistance of 25% or greater at the higher test voltage is usually due to the presence of excessive moisture. Why is this? Water in an insulating system is polar positive and will be attracted to areas of high negative electrical intensity. Therefore, when a Megger or similar equipment is used the negative lead is attached to the copper and the positive lead to the ground system."
Is this the test?
RE: megger test
The other details of my comments above may be a little off base. I think I was remembering RVM = Recovery Voltage Method. That method in itself does not involve reversing polarities. But somewhere along the line I read a paper comparing RVM test using different polarities.
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RE: megger test
RE: megger test
obviously, there is a capacitance between the insulated liner and the end cup but it does not come into the scene under meggering. so, what does your "comment" really say ?
and, why would one short circuit the liner and the end cap before meggering ? The idea is to check the IR value of the insulated liner.
The post mentioned about good IR value with one polarity and a not so good with reverse polarity.
jnims has given the right reason for this.
RE: megger test
RE: megger test
jb, but this post is not about cables or such charge retaining materials, is it ?
jnims, you get a star for your 'relevant' post.
RE: megger test
<nbucska@pcperipherals.com>
RE: megger test
The O2 on the negative pole oxidizes the Al and
insulates the surface so e.g. with a coke-can, an iron
electrode and some borax solution you can rectify
up to 40V and a few amp-s.
<nbucska@pcperipherals.com>
RE: megger test
jb, but this post is not about cables or such charge retaining materials, is it ?
///Yes, certainly. Ever wonder why there are grounding cables so much used; especially on medium and higher voltage levels.
Incidentally, there is no voltage level mentioned in the original posting. On the 22kV level, one might become charcoaled.\\\