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My first transformer

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AndreiRS

Tue Mar 06 2018, 04:56AM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

All started with this old NST I found at a junkyard. Soon I discovered it was full of solid tar. After a lot of time melting it, thinking I was going to find an amazing iron core, I just found a very small core.

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Square core, very common, nothing special. I don't even found what limits current in this thing. Like a magnetic shunt made of the same steel, across the middle of the core. Maybe this one didn't had one. They are so fragile, because they don't have any paper between layers, nor oil. And that tar just protects the case from hv, not the winding itself.

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So I bought 2 C cores, did some calculations using a sofware and tested the primary number of turns. It worked.

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I made the "official" primary by hand, just 134 turns. But the secondary was 15076 turns. Now imagine you winding 12 Tesla Coil secondaries... Then I made a improvised machine to turn the spool for me.

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Looks ugly, works good.

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The metal case.

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Then I burn it when one day I decided to turn it on without any spark gap at the output. Reconstruction pictures.

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Finishing is still rough, I'm not good with painting stuff.

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Added oil.

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And here a video of it before I destroyed it. Now it works just the same, but I don't have any new video. It is in portuguese, if you want to just see the sparks, go to 0:50.

Hazmatt_(The Underdog)

Wed Mar 07 2018, 03:35AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Damn fine job for a first!

Now if you are going to make a high voltage winding you have to consider more how it is going to break down dielectrically even under oil.

If you only need a single high voltage output, then one secondary is fine, and the start of that winding is grounded. That prevents the start of the winding burning up the inner windings of the bobbin.

The high voltage winding window closest to the core should be reinforced with fish paper, cambric paper, or some kind of dielectric paper or fiberglass tape to reduce corona.

The winding should also be enclosed in the same or similar paper, but more used where the winding is close to the core, or it is likely to eat itself.

2 high voltage outputs you will want to use 2 equal high voltage windings, wound opposite to each other, and their starts grounded. This divides the stresses in half, you can insulate more with paper, and you can use a heavier gauge winding (within reason).
It is also easier to source 1 lb of 32 AWG wire in some cases ( ~ 5000 feet) which can make a coil, unless you want to invest in a 10 b spool, which is fine too but it gets expensive.

Also, the big transformers have their bushings extend all the way into the oil, that prevents a fireball from the conductors making a high voltage to case short circuit. You could probably get bigger rubber tubing to make an extension of sorts into the oil. It would have to be stable in oil like Buna-N rubber, natural rubber will fall apart, and you could run the leads up that into the bushings.

RG8X coax cable is decently insulated for higher voltages, it has a thick dielectric jacket that you can use as a substitute for the nice silicone rubber high voltage wire, or just buy a short length of silicone rubber wire off of Ebay for the HV stuff, whatever you like.

But awesome job!
My first transformer was a terrible joke, it ate itself too.

klugesmith

Wed Mar 07 2018, 07:25AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Excellent jobs, Andrei. Coil winding, box fabricating, and illustrated story telling.

I think making a HV transformer is like making a telescope, including the mirror.

Welcome to the club!

AndreiRS

Wed Mar 07 2018, 04:08PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

Thanks Hazmatt and klugesmith, ah there is something I wan't to have someday too, a telescope.

I will try to answer to Hazmatt post a bit.

Now if you are going to make a high voltage winding you have to consider more how it is going to break down dielectrically even under oil.

Hmm I didn't really made any calculations about this. I just thought ~14kv was not much to go thru the fiber spool.

If you only need a single high voltage output, then one secondary is fine, and the start of that winding is grounded. That prevents the start of the winding burning up the inner windings of the bobbin.

I will have to make that grounding then. It is not grounded right now.

The high voltage winding window closest to the core should be reinforced with fish paper, cambric paper, or some kind of dielectric paper or fiberglass tape to reduce corona.

The winding should also be enclosed in the same or similar paper, but more used where the winding is close to the core, or it is likely to eat itself.

Nice, I did this. It is a spool of fiberglass, around 4mm thich and it has some paper turns inside too.

2 high voltage outputs you will want to use 2 equal high voltage windings, wound opposite to each other, and their starts grounded. This divides the stresses in half, you can insulate more with paper, and you can use a heavier gauge winding (within reason).
It is also easier to source 1 lb of 32 AWG wire in some cases ( ~ 5000 feet) which can make a coil, unless you want to invest in a 10 b spool, which is fine too but it gets expensive.

Originally I wanted to make 2 smaller spools, but then that was the biggest core I could buy at that time. And it is still small. I tried to fit 3 windings there with no sucess. Then I made only 2.

Also, the big transformers have their bushings extend all the way into the oil, that prevents a fireball from the conductors making a high voltage to case short circuit. You could probably get bigger rubber tubing to make an extension of sorts into the oil. It would have to be stable in oil like Buna-N rubber, natural rubber will fall apart, and you could run the leads up that into the bushings.

I think this is the biggest problem on my transformer right now. The way I fit the insulators too. I should have put them in the other direction. Closer to the handles.

I actually did a insulating job on the cover. Let me show you. The red thing is a stainless steel bolt. I welded the silicone rubber cables from the winding to it. Those cables go thru a transparent pvc hose. The light gray thing is the ceramic insulator, dark gray is the cover. The yellow tube is another piece of pvc hose. The two black washers, are rubber washers. And the light blue thing under the cover is a big piece of nylon cutting board.

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RG8X coax cable is decently insulated for higher voltages, it has a thick dielectric jacket that you can use as a substitute for the nice silicone rubber high voltage wire, or just buy a short length of silicone rubber wire off of Ebay for the HV stuff, whatever you like.

I got the rubber wire, it was very cheap here for some unknown reason.

But awesome job!
My first transformer was a terrible joke, it ate itself too.

Thanks again. Mine ate itself when I asked myself, what would happen if I turn it on without the safety gap? Why.........

Hazmatt_(The Underdog)

Thu Mar 08 2018, 02:36AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

So the only comment on the diagram is, you see how close your feedthrough bolt is to the case, that has a lot of potential between the two, so that needs to be heavily insulated, probably lots and lots of silicone on there.

AndreiRS

Thu Mar 08 2018, 03:49AM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

Yes, I don't like the way I did it. I just don't know what to do there. It would be nice to find a way to put the whole insulator thru the cover. But then I don't know how to hold it there. Just glue... Hmmm. Right now the bolts hold everything. Maybe I try some crazy idea, make a cover in fiberglass. Hmmm I think I will try. After painting it will look like the metal case anyway.

Hazmatt_(The Underdog)

Fri Mar 09 2018, 03:23AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Some transformers, like your neon transformer have the insulators coming out the side. You could have both coming out one of the sides at an angle if you like, and that would allow them to be covered in oil and still have a lid on top of the can. You would have to build a new can though.

Some of the Russian high voltage transformers do it this way I think, its kind of odd seeing the insulators sticking out one side at an angle, but that keeps the connection close to the winding and under oil.

The big challenge there would be keeping the oil from soaking through the can. You would need some buna-N rubber seals, and they would probably need replacing every 10 years or something.

If you can find insulators that have a solder flange then you can solder the insulators to the can and it would be hermetic, but insulators are typically pretty expensive. You have a really nice pair of insulators there already. Maybe you could make some kind of skirt out of copper flashing over one of the ribs and fill it with epoxy, I am not sure. Some kind of ring seal is what you're looking for and would do the trick.

Patrick

Mon Mar 12 2018, 11:03PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Beautiful work though.

TwirlyWhirly555

Sat Mar 31 2018, 02:32PM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

Very Nice work ,

One day I hope to wind a HV transformer of my own once I find a place for cores . I had trouble finding insulators for my X ray transformers and in the end used Teflon rod with a hole drilled though the centre as its resistant to mineral oil and has good insulation properties

I intend to have a Teflon rod turned down so I can flange mount it to cases when I get them made .

radiotech

Wed Apr 11 2018, 08:31AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Excellent !

A word about the bushings. The potential gradient, i.e. case to center terminal, is
of concern to transformer makers .

Your transformer appears to be similar a PT. A PT may also
have a BIL rating, because such circuits are often hit by
lightning.

See also Doble bushing test. That is how we predict failures.

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