Leakage current tests 1

[MagBen]

During the static full-flux test, the stator windings were put through several full-current thermal cycle cycles that were quite severe. After that, the stator windings were subjected to resistance testing and leakage current testing. It was found that several of the stators exhibited unacceptable leakage current from the windings to the stator lamination stack.
The question is, did the leakage have anything to do with stator material? (we used unannealed Hyperco Fe-Co lams)

 

[IRstuff]

I would say no. Leakage is an indication of insulation failure, particularly since not all of the stators are leaking.

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[MagBen]

I tend to believe so too. But I still have a little concern about "unannealed" for which we had to apply a much higher current to reach the point wanted, you know the leakage normally occurred at a higher current

 

[IRstuff]

Oh, that's slightly different, but if the unannealed material was that resistive, then more of the applied voltage would have been dropped across the material, rather than the insulation. But, then, how did you decide what voltage corresponded to normal leakage. You said "unacceptable," but it was at a higher voltage, so someone must have determined, or guessed, what the insulation voltage was?

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[MagBen]

IR, thanks for your reply. I got your point. However, the criteria for "unacceptable" is based on the level of flux. In order to reach a same level of specific flux, a higher voltage needs to apply for a higher current, when the core material is not annealed. The unannealed material is magnetically harder than annealed.
Resistivity is actually pretty much the same for both annealed and un-annealed materials.

 

[IRstuff]

Oh, OK, so not current leakage, but what you seem to be describing really isn't "leakage" of any sort, is it? It's seems to be more along the lines of, "We're need to apply a higher voltage to get the same flux, so we must be losing current?"

If that's the case, then. speaking as a neophyte, it sounds very plausible that the anneal would change that behavior. I can sort of imagine that in an unannealed material, the magnetic flux getting discombobulated and shorted out, while an annealed material would have the flux sailing nicely through it.

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[IRstuff]

so, doing a quick Google, I see:

http://www.metglas.com/products/magnetic_materials/

http://www.doiserbia.nb.rs/img/doi/1451-4869/2013/1451-48691300016K.pdf

http://www.vptechnotron.cz/download/katalog2_en.pdf

Which all seem to support the notion that flux will be less for unannealed material

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[waross]

The next question, was the core below spec before testing or did the testing alter the core?
Do you do a bench mark test before you start the the thermal cycling?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MagBen]

This was a new material trial, so no spec yet. Although not being checked, the properties of core seemed unlikely to be changed. Stress relief asks for 400C for a long time, order-disorder transformation (which would change both mech. and mag.properties for this alloy) asks for even a higher temperature. I didnot think of any reason altering core.

Unanneal bothers me a bit since this material is not supposed to use this way.

 

[electricpete]

What kind of a device are we talking about?
I’m guessing motor but I want to confirm.
Roughly what size motor?

During the static full-flux test , the stator windings were put through several full-current thermal cycle cycles that were quite severe

I'm familiar with motor testing but I’m not familiar with a full flux test for motors that would put the windings through full current thermal cycles. Do you energize stator windings directly or do you use a loop around the core?

On transformers, a change in exciting current pattern (usually an increase) is supposed to be associated with a shift in the windings under mechanical stresses.

Exciting current is not typically measured for motor stator cores on its own. If the exciting current magnitude did increase I guess there is a small possibility that a gap has opened up if you have segmented laminations (for example 8 different segments around the circumference at one axial elevation of the vertical core).

Otherwise, I guess we might think about a really really severe core interlaminar insulation damage such that the resistive component of current (due to circulating currents thru shorted lams) becomes significant relative to the inductive component. To check for this, do a standard stator core test which checks for “watts loss” (watts per pound) and possibly look for hotspots on the stator bore by thermography during the test.


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(2B)+(2B)' ?

 

[electricpete]

and possibly look for hotspots on the stator bore by thermography during the test.

... actually on the bore and backiron.

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(2B)+(2B)' ?

 

[MagBen]

Switched-DC motor, 10HP.