Since NSTs are fragile I built my own transformer...

[AndreiRS]

... Which I managed to burn and had to rewind.

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.



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.



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



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.



Looks ugly, works good.



The metal case.







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





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



Added oil.



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.

[oneKone]

Man that thing is awesome! The finish on it is perfect! Some things are better to have a bit of character rather than 100% perfect.

Is the winding jig cnc controlled?

[AndreiRS]

Thank you.

The winding jig has a click counter and a 12v windshield motor running on a pwm. I basically use the pwm to start turning slowly then go to full speed, which is not much on that motor. I also made something like a lathe cross slide and a small pulley move it sideways. The gear ratio was calculated only for this wire diameter. I will take more pictures of it, but it is nothing fancy, it was made just to make this transformer. Then I was going to disassemble it. Since it was not taking too much space, I keep it assembled. I still have to keep holding the wire with one hand while winding, to keed some tension on it.

[Mads Barnkob]

That's a real nice looking oil enclosure you got made there. You went all out instead of the regular IKEA food container HV boxes most people use

What kind of insulation was added between the secondary windings? With so many failures of your HV transformer it sounds like you have too short creeping distances and inadequate insulation material between windings.

[AndreiRS]

I actually only had one fault, big fault, that destroyed the secondary. I just use that brown paper between layers. I don't know how they call it in english, let me google here. Oh Kraft paper. But even the big pole transformers use these, with epoxi. The problem was the lack of oil on my construction. Now it doesn't arc internally without load. Because of the oil... I still don't know how they make HV transformers without oil.

"IKEA food container HV boxes" I almost got one.

The idea behing kraft paper and oil is that the paper gives a solid space between layers and the oil gives the voltage resistance. The paper alone can't do much. When I turned it on without a load that day, it just created an arc inside the spool of wire. I still want to know how they make dry high voltage transformers. I bet they are not relly dry, but use some solid thing like tar or resin that solidifies. NST are just wire without any layer separation. I just don't know how they even survive being turned on without a load or a spark gap.

[Mads Barnkob]

Yes, dry HV transformers are potted with a resin/epoxy/something_else and put under vacuum to get all residue air bubbles out and get the insulating material in-between all windings and core.

If you want to make your own HV transformers without oil or potting, there is only one thing to do, big distances between all parts, lots of insulating bushings on everything and lots of insulating material between everything. This is why oil or potting is used, its much simpler and cheaper.

[profdc9]

I think you might be able to use paraffin wax (like for candle making) as an insulator.  It is a heavier hydrocarbon than mineral oil, but the dielectric strength should be similar.  The benefit is that you can melt it and remove it much more easily than something permanent like epoxy.   The only disadvantage is that if the transformer becomes very hot, that could melt the wax as well.

Dan