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 Post subject: Re-Wind Newbie Seeks Advice
PostPosted: Mar Sat 03, 2018 8:09 am 
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Location: Lincoln City, OR
Greetings to the Forum:

I have never attempted any sort of transformer re-wind. However, I just recently smoked one of my favorite tools... a bench-top demagnetizer. Since this is a single coil and an open core "U" in cross section) I thought it ought to be a simple enough project to start on.

Here are the photos... first the core with the coil removed:

Attachment:
Demagnetizer Core R.JPG
Demagnetizer Core R.JPG [ 129.65 KiB | Viewed 732 times ]


Next, the coil. The "telescoped" inner windings were due to my efforts to pry the coil off the core.... it didn't maintain its structural integrity and the outer layers started to move while the inner layers stayed stuck to the core. A repositioning of the screwdrivers enabled me to get the whole thing off but the inner windings standing proud of most of the rest of the coil is the result:

Attachment:
Demagnetizer Coil CR.JPG
Demagnetizer Coil CR.JPG [ 126.86 KiB | Viewed 732 times ]


I measured the wire at around .025" which according to my wire table is #22. The coil weighs about 14 oz. so I ordered a pound of #22 magnet wire from the auction site. I wanted to go with the 200 degree C wire, but it measured almost .027" in diameter and I felt that that might make it impossible to get the coil wound with both the correct number of turns and the original dimensions, so I went with the 150 degree C wire which is closer to what I measured at about .026".

Now, the questions:

I measured zero ohms at any two points I cared to pick on the laminations. I presume this means that the insulating varnish is carbonized. So, I will have to de-stack the core. What sort of chemical stripper should I use to get the laminations clean? I obviously don't want to do this by hand.

Once I get the laminations cleaned up, what sort of varnish, shellac, whatever should I coat them with? How do I ensure that the coating is of the correct thickness such that the core will go back together with its correct dimensions? Do I thin the product and if so, how much?

I'll have lots more questions when I actually get to winding the coil, but I think I have bothered y'all enough for now.

Thanks,

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Jim T.
KB6GM


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Mon 05, 2018 4:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1772
Location: Lafayette, CO
I never bothered to deal with the conductivity between the core E's and l's. I have seen high voltage transformers whose cores had a spot weld across the core. I would just re-wrap the coil and go from there. Craig


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Mon 05, 2018 6:05 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8066
Location: Long Island
I think if you check any number of radio transformers with laminations you will find the same thing, practially no resistance from lamination to lamination. The reason why is because the eddy currents are mostly contained in the laminations; there are only millivolts or microvolts of difference. So preventing eddy currents requires only a small resistance between the laminations, not a high voltage insulator. Normally an oxide coating is applied to the steel to protect it and while it has very little ohmic resistance if you measure it with a meter, it is sufficient to make very efficient transformers. Special treatments (glass based coatings) are available for applications where that last few percent of efficiency is worth the extra coin.

You will find, however, that the laminations in most transformers are shellacked or varnished together very heavily--with vacuum impregnation on the better quality ones. The purpose of this is to fill the voids so the laminations don't buzz. This can be very destructive if a vibrating lamination wears a hole in the coil insulation. It also serves to keep moisture out. Metals expand when they rust, and rusty laminations can expand enough to break coils. The shellac or varnish also makes it wickedly hard to get the laminations apart again without bending them into pretzels.

I would not attempt to disassemble the laminations. They're fine the way they are. Just clean them up, and if you feel like doing something, paint or varnish them with something that will protect them from moisture.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Mon 05, 2018 10:12 pm 
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Location: Lincoln City, OR
Greetings to Chris, analog.tv and the Forum:

Thanks for the advice. This core got pretty warm, so if there was any varnish in there, it has carbonized. However, leaving well enough alone does two things for me... it saves me the trouble of de-stacking and re-stacking, and it ensures that the core will retain the same dimensions it had before.

So, I will take Chris's advice and put some varnish on the outside.... after cleaning up, of course. I will check with the hardware store and see if they have a recommendation as to what to use. They won't know transformers from Adam's off ox, of course, but I live on the coast and so they ought to have a pretty good idea of how to protect from moisture.

Next phase of the project: Carefully measure the old coil and then unwind it to determine number of turns. I just got a notice in my E-mail that my pound of #22 wire has shipped, so onward and upward as they say.

Thanks all,

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Jim T.
KB6GM


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Mon 05, 2018 10:37 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8066
Location: Long Island
Before completely unwinding the old coil, you want to carefully make a form to wind the new coil. This should be a wooden block which fits closely in the old coil, with a top and bottom on it to match the height of the old coil. The top and bottom could be thin plywood, sheet metal, even heavy cardboard. This will ensure the new coil fits where the old one did. One "trick of the trade" is to make the body of the form out of two pieces of wood sliced at an angle and held together with a couple of screws. You will wrap this with alternate layers of Kraft paper and wire, same as the original coil was made. Then when the new coil is finished, take the screws out and slide one of the wood pieces out. This is a lot easier than trying to get the finished coil off a solid one-piece form.

Another trick is to hacksaw the coil in half from top to bottom on one side, and pull the ends apart. You can count the turns from one of the exposed ends. This is often faster and easier than unwinding the whole thing, particularly if the old varnish puts up a fight.

When the coil is completed, good practice would be to dunk it in a can of varnish, let it soak up as much as it can, then hang it up to dry. This will seal it. Vacuum impregnating it would be better but that calls for additional equipment. Electrical varnish is best, but I've used ordinary polyurethane varnish from the hardware store to good effect.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Tue 06, 2018 2:02 am 
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Location: Tucson, Arizona U.S.A.
Chris108 wrote:
When the coil is completed, good practice would be to dunk it in a can of varnish, let it soak up as much as it can, then hang it up to dry. This will seal it. Vacuum impregnating it would be better but that calls for additional equipment. Electrical varnish is best, but I've used ordinary polyurethane varnish from the hardware store to good effect.

It is probably an even better idea to paint it on layer by layer as it is wound. That way you know it has gotten into all the layers. That's important because you don't want the wire to vibrate; it would wear through the insulation.

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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Tue 06, 2018 6:06 am 
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Location: Lincoln City, OR
Greetings to Chris, Jim and the Forum:

Chris writes:

Quote:
One "trick of the trade" is to make the body of the form out of two pieces of wood sliced at an angle and held together with a couple of screws.


What a clever idea! I was thinking about making the form out of small sticks, like 1/4" square and building up the form out of them. I could then (hopefully) slide the inner pieces out allowing the form to be disassembled in place. The double wedge idea seems to me vastly superior and I will adopt it. Thank You.

Quote:
Chris also writes: Another trick is to hacksaw the coil in half from top to bottom on one side, and pull the ends apart. You can count the turns from one of the exposed ends. This is often faster and easier than unwinding the whole thing, particularly if the old varnish puts up a fight.


The old coil is pretty badly burned up, and the inner few layers have already telescoped out of the rest of the coil while I was trying to get it off the core. I'm not sure the coil has sufficient structural integrity left to allow me to hacksaw it in half.... I think it might disintegrate in the process, with subsequent loss of information. If I were re-winding a transformer for the purpose of changing its voltages and therefore working with an intact coil, this would be a viable idea. However, I think my coil is too badly damaged to try it. Of course, if I go to all the trouble to unwind it and I discover it was firm enough to be sawed in half, I will be kicking myself for not taking your advice.... but in this case, I think I will try to unwind it. This should make a good test of the winding jig that I need to build.

Chris also writes:
Quote:
Vacuum impregnating it would be better but that calls for additional equipment.


I have a vacuum pump; it is a pretty good quality Robbins intended for evacuating automotive air conditioning systems. What I don't have is a bell jar. Any suggestions as to how to go about vacuum impregnating?

Chris also writes:
Quote:
You will wrap this with alternate layers of Kraft paper and wire, same as the original coil was made.


Please excuse my ignorance, but I don't know what Kraft paper is. There were some pieces of cardboard (not corrugated) about the thickness of the paper backing for note pads between the coil and the core. There is also paper in the core, but it is too badly charred to get much of an idea about what it was like before the fire. I thought I might have to buy some of what we used to call "fish paper" at least for next to the core. Information about Kraft paper and where to get it would be welcome. On the same subject, is the backing from notepads suitable for the insulating material and spacers next to the core?

Jim writes:
Quote:
It is probably an even better idea to paint it on layer by layer as it is wound.


I will do this if I have to, but I can see myself getting covered in varnish from head to toe.... not to mention everything else within 50 feet. :D

How does one do this without getting varnish all over ones hands, the machinery and the wire for the next layer you are trying to handle?

Thanks for all the responses. I am really on my learning curve here and your help and advice is greatly appreciated.

Thanks,

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Jim T.
KB6GM


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Tue 06, 2018 7:02 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 3099
Location: Lincoln City, OR
Greetings to the Forum:

Please disregard my question on Kraft paper above. I looked it up on Wikipedia. I have an order for a roll of the stuff from Amazon in the pipeline. It is described as "brown Kraft wrapping paper".... I hope it is paper made with the Kraft process as described in Wikipedia. It would seem that Kraft paper has superior strength and durability due to a lower lignin content when made with the Kraft process.

Also, I hope it is the right thickness.... I will just have to mic it when it gets here.

BTW, I had a thought: Has anyone ever cut strips out of an oven roasting bag and used that for inter-winding insulation? I don't know about its dielectric strength, but it is good for some pretty high temperatures.

Regards,

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Jim T.
KB6GM


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Wed 07, 2018 4:12 am 
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Location: Tucson, Arizona U.S.A.
Quote:
Jim writes:
Quote:
It is probably an even better idea to paint it on layer by layer as it is wound.


I will do this if I have to, but I can see myself getting covered in varnish from head to toe.... not to mention everything else within 50 feet. :D

How does one do this without getting varnish all over ones hands, the machinery and the wire for the next layer you are trying to handle?

You leave the coil in the winder and brush it on. If you don't have a winder (most likely here), make the center form long enough to clamp it in a vise. After winding one layer, clamp it in the vise with a weight or something to keep tension on the wire so it doesn't spring all over the place. Then, since your hands are now free, brush the varnish on in the usual manner. Be careful not to make drips. Put the inter-layer insulation on while the varnish is wet to hold it in place and to keep the varnish from building up high spots that would make the coil too fat.

Be careful of the thickness of the insulation you use; if it is too thick you won't get the core back into the coil. The closer the core originally wrapped around the coil, the more careful you have to be about the thickness of the materials you use.

If you can put ends on your coil form it will greatly aid in keeping the wire where you want it. If there isn't enough room in the core to leave the ends on, make them removable, like teflon or polyethylene that hardly anything sticks to.

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Jim Mueller


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 Post subject: Re: Re-Wind Newbie Seeks Advice
PostPosted: Mar Wed 07, 2018 4:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1772
Location: Lafayette, CO
For radio transformers, l have used clear packing tape. Craig


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