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Old 26th May 2018, 9:04 pm   #1
Ed_Dinning
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Default Transformer lams

Hi Gents, see if you can explain this one.

Quad II mains transformer is potted in pitch and known to run hot and fail.

Why then, on stripping and de-potting I find many of the lams rusty on their mating faces, but said faces also contain pitch residues that have crept in during the potting and should have sealed out any moisture.

Ed
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Old 26th May 2018, 9:39 pm   #2
kalee20
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Default Re: Transformer lams

Maybe the pitch contained traces of moisture - or otherwise available oxygen?

Yes, pitch SHOULD seal out moisture, but even so, water vapour has the ability to diffuse through many liquids, even though they are hydrophobic.
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Old 26th May 2018, 10:14 pm   #3
GrimJosef
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Default Re: Transformer lams

You'll know better than I Ed that one face of each lam will have been coated (oxidised ?) to insulate it from its neighbour. Could the coating trap moisture ? The metals inside valves certainly trap gases of various sorts which are slowly released when things get hot. In an open transformer the gases might bleed out and disappear before doing any harm. But maybe the bitumen (I one had a strip torn off me for calling it pitch ) is acting in the opposite direction from what was intended and is trapping moisture inside rather than stopping it penetrating from outside ? Just a thought ...

Cheers,

GJ
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Old 26th May 2018, 11:13 pm   #4
Boater Sam
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Default Re: Transformer lams

As it heats up and cools down, air and moisture will be drawn in no matter what it is potted in, capacitors do the same and we all know that even plastic potted ones fail.
Nature abhors a vacuum.
I have a tar potted output transformer from a Lady Anne radio in bits at the moment. It was well coated but has still failed, the interleaving paper is damp in places as I strip it.
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Old 26th May 2018, 11:59 pm   #5
trobbins
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Default Re: Transformer lams

They could easily have not baked the transformer for sufficiently long (or at all) prior to potting during manufacture. Depending on humidity of kraft paper etc, and air within windings, there could well have been enough moisture to locally vaporise and permeate the lams.

Was the transformer separately potted and then put in can, or does it look like the potting was poured in to the can with the transformer in place ?
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Old 27th May 2018, 1:32 am   #6
Argus25
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Default Re: Transformer lams

I think most likely there was already a microscopic amount of rust on the lamination surfaces when it was stacked at the factory.

Just as in many old cars when the surface was not prepared properly for painting, rust would appear later bubbling up from under the paint, which in theory should have sealed water molecules out, but didn't because its very difficult with many coatings as in reality they are porous.

So once the rust crystals were there initially, they simply acted as a mechanism to absorb more water and the process advances, despite the pitch coating. Rust crystals are like a magnet for water molecules.

It is just another "rust never sleeps" example this is why in restorations, the rust must be completely removed, the surface treated (electroplated or painted with rust inhibiting paint) or simply, the rust will just keep destroying the metal as time goes by and bubble up under any applied top coat, unless other steps are taken.

Luckily it is easy to fix your laminations with two steps. Rub off the surface rust with 1000 grade paper. There will still be rust in the pits. Paint each lamination with Fertan organic rust converter. This deactivates the rust and leaves a thin blue-black surface film which is an insulator, which will help when its re-stacked to keep the lams insulated from each other and reduce eddy current losses. Any rust attempting to re-form is completely deactivated by the remains of the Fertan. I restore all of my vintage iron cored transformers this way when the lams have rusted. Then I usually vacuum varnish them too with a simple container and a large syringe to create a 70% vacuum is plenty to get the air out.
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Old 27th May 2018, 2:57 am   #7
joebog1
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Default Re: Transformer lams

Argus has it!!! and really its not necessary to coat the lams. Just bake them dry after removing the rust. Even a hot dip, and varnish as normal, rather than potting in bitumen, stinky burny horrible stuff.
Rust after all is an "insulator" of sorts. If you really want the true antique style potting, varnish as usual and then pot. The transformer is about 70 years old, you wont have any warranty to worry about, unless you have found the fountain of youth .
Coating with SOME rust converters will make the lams a bit thicker, and you will have trouble stacking them. They already run pretty high flux densities and thats not a good thing.

Joe
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