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Old 18th Jan 2019, 10:12 am   #1
ekcopyephilips
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Default Transformer laminations

Hi All

I have a small transformer from a bush DAC90 that im having rewound.

Long story short, i discovered that the laminations are rusty so i have seperated them all and cleaned all the rust off.

I now need something to insulate each lam as they are all lovely clean bare metal, i do have some Electrolube CPL, but that might be overkill and im worried about it also being too thick a coating.

Would just a coat of ordinary grey spray primer on both sides be sufficient?

Cheers

Mike
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Old 18th Jan 2019, 10:44 am   #2
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Default Re: Transformer laminations

Just a thin misting of spray lacquer would be my approach.

Grey primer is intentionally a high-build paint designed for rubbing-down easily. Not very robust.

David
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Old 18th Jan 2019, 12:41 pm   #3
G8HQP Dave
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Default Re: Transformer laminations

Rust would probably make an adequate insulator.
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Old 18th Jan 2019, 1:28 pm   #4
M0FYA Andy
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Default Re: Transformer laminations

I recall reading somewhere that cheaper transformers deliberately used rusty laminations to provide the insulation!
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Old 18th Jan 2019, 9:16 pm   #5
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Default Re: Transformer laminations

Hi Mike, the thinnest of lacquer coatings is all that is required as the voltage across it is negligible.
If very badly rusted a light rub on emery will suffice, leaving most of the rust on.

Ed
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Old 18th Jan 2019, 10:06 pm   #6
David G4EBT
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Default Re: Transformer laminations

Off topic really, but it's always puzzled me that on microwave oven EHT transformers, there are welds running across all the laminations, effectively creating a solid iron core. It's only in recent years that I became aware that the laminations on conventional transformers aren't meant to have conductivity with their 'neighbours'.
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Old 19th Jan 2019, 12:46 am   #7
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Default Re: Transformer laminations

Hi,

By far and away the better replacement coating is Fertan organic rust converter. It forms a very thin dark blue-black coat on the surface of the lamination, deactivates the rust by converting it into a harmless organic compound. It is also an electrical insulator. It is painted on with a small brush, has the look of brown water initially, it evaporates leaving the thin layer of reactants changing color as it dries.

This way you solve the rust and the lamination insulation issue all in one go. The occasional lam will still electrically connect if the coating is scraped when stacked, but most will be insulated.

It is folly to coat the lams with lacquer (before stacking), unless it is an extremely think coat. Because the thickness adds up with each lam and very quickly you will find that you cannot fit all the lams back in the bobbin. Also, it is not a good idea to lacquer over untreated rust anyway.

I have done this for all of my transformer restorations with rusty lams that were un-stacked. For ones that don't need un-stacking & rewind and have exterior rust, the Fertan is also very helpful. You can also lacquer or paint over the fertan coat when the transformer is stacked with excellent adhesion to the surface.

Last edited by Argus25; 19th Jan 2019 at 12:55 am.
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Old 19th Jan 2019, 8:08 am   #8
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Default Re: Transformer laminations

Most if not all rust treatment's are simply acid, problem is if you don't paint or cover the treated area it quickly rusts again, only worse. Yon Fertan is maybe some sort of gun blue which would also make a reasonable insulator. I'd be tempted to squirt/wipe with a bit of light oil or even the old WD if you were concerned, though the odd tfmr I've wound with rusty lams have been fine.

The worst tfmr to take apart/rewound are varnish impregnated ones, you all but destroy it getting it apart, so wouldn't use varnish or laquer, most laquer's take ages to cure properly so any treated lams are likely to stick like the proverbial. Just thinking about the next rewind.

Andy.
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Old 19th Jan 2019, 2:28 pm   #9
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Default Re: Transformer laminations

Quote:
Originally Posted by Diabolical Artificer View Post
problem is if you don't paint or cover the treated area it quickly rusts again, only worse.
Fertan is not like this at all. One great thing about Fertan, it gives a coat which is extremely re-rust resistant. A good example is some iron work I coated with it exposed to salt air, which un-coated rusts in less than 24Hrs and just about in front of your eyes, but is fine for many months to years with just Fertan. If a coat of lacquer is put over that too, the rust is history. I used just it to coat the the hubs of the brake rotors on my car, which had no coat from the factory, were rusting and staining the inside of the alloy road wheels. It still looks black over a year later, no rust spots and that is outside in the weather, rain and salty environment here.
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Old 19th Jan 2019, 3:34 pm   #10
M0FYA Andy
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Default Re: Transformer laminations

It sounds as though Fertan, like Jenolite, is based on phosphoric acid which converts rust into a black passive layer.
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Old 20th Jan 2019, 6:33 pm   #11
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Default Re: Transformer laminations

David i wonder if that is to reduce the effect of magnetostriction, which will cause noise to be generated, what do you think Ed?
Cheers
John
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Old 20th Jan 2019, 6:52 pm   #12
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Default Re: Transformer laminations

Joining this thread late....
I have just rewound a 1920s intervalve transformer
All the lams were rusty, I tried very fine wet and dry but eventually sand blasted them all and then used a light coat of spray pcb lacquer on each side.
All the lams went back in and it works well.

Peter
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Old 20th Jan 2019, 7:52 pm   #13
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Default Re: Transformer laminations

Hi John, noise from magnetostriction is often caused by the lams not being tightly together where the "E" & "I" butt up. This will also cause an increase of magnetising current.
Lams are often tightly clamped as if the bobbin does not do that then there will again be a buzz.

Generally there is little noise from audio transformers due to magnetostriction as they operate at lower flux densities.

Ed
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Old 21st Jan 2019, 3:03 am   #14
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Default Re: Transformer laminations

In all my projects now when I build lamination transformers from new, I treat the lamination edges with Fertan, prior to varnishing the transformer (I use "partial vacuum varnishing" as its easy to create a 500mmHg vacuum in a glass jar with a large syringe). This is because the residual Fertan coat is very good at rust inhibition and the varnish sticks well to it.

Scroll to page 24 of this article which shows some small transformers prepared this way, they were placed in home made brass brackets as they had to be mounted in an aluminium tube:

http://worldphaco.com/uploads/ZC1_MK...R_SUPPLIES.pdf

Although just varnishing the laminations and their edges alone does help prevent rust, it is not 100% and it can still rust under the varnish, but the Fertan prevents that. Plus it deactivates any existing rust spots already there. Sometimes even new laminations have rust on the cut edges depending on how they have been handled & stored.
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Old 21st Jan 2019, 7:56 am   #15
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Default Re: Transformer laminations

I understood Fertan to be tannic acid, so just dip them in your tea!
Its not phosphoric like most rust converters.

So to take up Davids question, why are microwave transformers welded together? I use them as a source of mains primary winding and cores for winding odd transformers after I have ground out the welds.
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Old 21st Jan 2019, 9:39 am   #16
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Default Re: Transformer laminations

I was intrigued by this so I did some digging and came up with - The weld doesn't affect the eddy currents much as they don't circulate across the laminations rather along them and it stops them buzzing - Thoughts?

Robin
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Old 21st Jan 2019, 8:41 pm   #17
Ed_Dinning
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Default Re: Transformer laminations

Hi Gents, welded microwave transformers are a production engineering technique.
The laminations are not interleaved as that saves time as the stack is being inserted, Just "E" one side "I" the other and they are then clamped tightly in a jig and TIG welded.
For high production volumes this saves time (and money).
I have also seen it done on yellow pot isolating and auto transformers as well as small ones used in wall warts.

Ed
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Old 21st Jan 2019, 11:37 pm   #18
Wendymott
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Default Re: Transformer laminations

Tonight I was wanting to add a few turns to a commercial 20VA transformer.. I could see that there was room inside the casing.. I needed to add 2V worth of winding.. I removed the clamp... to my horror the Laminations had been welded at the "I" / "E" junction.. right across the lams stack.. This was both sides.... obviously this rewind was not going to happen..... I found another transformer that I could use...... As it happens I was going to open a thread about this...
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Old 22nd Jan 2019, 4:11 am   #19
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Default Re: Transformer laminations

Easy to cut the weld out with an angle grinder.
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Old 22nd Jan 2019, 9:34 am   #20
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Default Re: Transformer laminations

Drifting away from the original post,

I have ground these out and re built interleaved but each lamination has to be individually de burred and I never seem to all of them back in.

Pete
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