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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
When designing high voltage transformer (50Hz) to be operated on air, what is the minimum distance (kV/mm) I have to keep between HV winding and core/other windings?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I don't think tat there is a simple/general formula for high voltage transformers, The simple guides for air breakdown and surface tracking do not apply once you get to the multi-kilovolt range. Once above about 5 kV you will have to contend with corona...ozone eats wood/metal/plastic/rock What voltages are you considering?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I was bored so I drew my design in MS paint. The output is 12 (6-0-6) kV, 4000VA. Core is from welder. I can put some plastic insulation foil which will be able to hold the 6kV between sec. coils and core, but there is the main problem of the full 12kV between the coils. Ideally I would like the gap to hold 25kV peak (if I ever want to run the transformer with resonant caps) but its fine if it holds just the 12kV.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
You would be better off using a primary on each leg of your O core, then winding the secondary on top of the primary. I had a 3180V transformer wound this way, it gives you more winding area and more winding margins at the ends. Afterward you will be able to fill the gap between the winding cores with dielectric paper to reduce its tendency to arc. For really high voltage you would want to cast the secondaries in something to prevent corona.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
I used for my high voltage flyback transformers a Common Insulation Foil of 0,17mm thick, it can insulate up to 10kV (I tested it), its for transformers, I bought on a Transformer Shop. It has been US$25 the kilogram (R$40)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You would be better off using a primary on each leg of your O core, then winding the secondary on top of the primary.
It depends. Having the cold side of each secondary wound on each half-primary means that the insulation is not stressed there, but it also means that the HV windings are going up-and-down a long coil of few layers, which increases the stress between layers of the secondary. Using short and thick secondaries reduces that inter-layer stress. I suspect that for amateur winding, three discrete coils will be easier to produce than two compound coils. The electrical cost of seperating the coils as in the drawing is to add a little leakage inductance, which is not a problem in either TC or ballasted arc-drawing use, and not a huge problem as a straight power transformer. With either method, you might consider winding the secondaries trapeziodally, to increase the clearance between the hot outer layers and the adjacent core or other winding.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanx for the help, all. If I was to choose the winding position I would choose 1/2 primary and 1/2 secondary on each leg, as the 12kV where the windings come together can be easily insulated with a few mm of plastic, and is much better than 12kV between 2 adjacent coils where insulation is not so easy. For some reason, calculating the coils as said, it comes out I can wind more sec. turns this way (calculating 0.2mm interwinding insulation).
However the copper wire prices are not-so-nice and I think it is not worth for a "poor student" to wind such transformer... not considering that it may as well burst into flames as I turn it on.
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HV transformer design question
