megger test 3

[jclough]

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?

 

[jbartos]

Comment: One way to explain this is: There may be a junction that passes a direct current in one direction and exhibits high resistance in the current opposite direction, e.g. recall diodes such as a piece of germanium and contact touching it thus forming a diode. Have you tried to increase the DC voltage? Has the megger been measuring correctly?

 

[OperaHouse]

Many oxide coatings exhibit diode properties. In WWII prisoners used blued razor blades with a piece of pencil graphite touching them as a diode detector to pick up radio signals. However, I think what you are seeing is noise pickup. Either radio stations or 60 hz from close power lines. Remember, your instrument is just an amplifier with an input and a ground.

 

[electricpete]

I remember hearing there is a test technique popular for transformers in Europe. The transformer is meggered with both polarities and compared. Difference is supposed to be indication of moisture. There was some explanation in ternms of the different way that the moisture behaved (within the oil-soaked insulation) in the presence of those two charges. There was a catchy three-letter acronmym for that test. Reverse something?

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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[electricpete]

I am pretty sure the transformer test I described is detailed in SD Myer's "Transformer Maintenance" book. Does anyone have it? (someone borrowed my copy).

Now it is bugging me that I can't remember the details of that test.

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[jclough]

Thanks for the input guys,

An added bit of information. The bearing liner is hardcoated aluminum and cemented into the endcap.


Hmmmm. Sounds a bit like a diode?

 

[jbartos]

Suggestion: The chemistry can get involved for electrical engineers, technicians, designers, etc. It is better to approach to chemical engineers for more info

 

[jnims]

electricpete

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?

 

[electricpete]

Thanks jnims. That is discussion of the principle I was remembering. The last sentence indicates that the polarity of the test leads in important to the result due to water in the insulation acting differently compared to negative and positive electrodes.

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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[jbartos]

Comment: A capacitive charge between the end cup and bearing liner may exist. Short-circuiting those two before the megger testing may also impact the measurements.

 

[edison123]

jb,

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.

 

[jbartos]

Comment on the previous posting: Cables, and some other electrical equipment especially operating on high voltage tend to retain some capacitive charge(s) and it is not recommended to touch before discharging the remnant capacitive charge. This charge may affect meggering to some extent.

 

[edison123]

jb, but this post is not about cables or such charge retaining materials, is it ?

jnims, you get a star for your 'relevant' post.

 

[nbucska]

If there is moisture between two different metals they behave as a battery.

<nbucska@pcperipherals.com>

 

[nbucska]

If one metal is aluminum there is an other effect:
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>

 

[jbartos]

Comment on edison123 (Electrical) Apr 6, 2004 marked ///\\\
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.\\\