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Presspaper's dielectric strength?

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Tonic

Thu Oct 11 2007, 08:01AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Well, I'm having difficulties with figuring out, what's presspaper's dielectric strength. I've sarched through gogle, but no big success. Only I know that properties are specified by DIN 7733 and IEC 641 norms.

As so far, I found this document, which tells that, according to IEC 641-2 norm, presspaper has 11kV/mm minimum dielectric strength unfoldet. What
'unfoldet' means?

Link to document:

I'm going to make a flyback transformer and I'm not sure, how thick insulation layer must be thick to prevent rupture between two closest layers with 3.6kV potentials differents.

Steve Conner

Thu Oct 11 2007, 09:37AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't know what "presspaper" is. Maybe Google doesn't either. You may have better luck searching under other names, perhaps kraft paper or cartridge paper.

Tonic

Thu Oct 11 2007, 11:11AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Maybe a pressboard is a correct name. Its common use is dielectric insulation in electrotechnic industry. It's also used for hydraulic seals. It has paper layers, which are pressed together. Properties are low moisture absorbing and high resistance for grinding and bending. There are photos of it, maybe you will figure out what it is :

Dr. Dark Current

Thu Oct 11 2007, 12:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Tonic wrote ...

I'm going to make a flyback transformer and I'm not sure, how thick insulation layer must be thick to prevent rupture between two closest layers with 3.6kV potentials differents.

That depends, if it is pure DC then two layers of the black electrical tape should hold it, if it's HF AC this will be much more difficult to insulate because of corona...

Tonic

Thu Oct 11 2007, 01:48PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

It will be HF, I'm going to use ZVS driver.

Oh well, I suppose I have to try myself, since it's hard to find someone, who used this pressboard and stated results.

Tesladownunder

Thu Oct 11 2007, 02:14PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I always understood that the use of paper or cardboard is as a physical spacer only with no additional properties then the air or oil it is soaked in.
Nevertheless the physical barrier when soaked in oil is very significant and presumably the physical properties of Kraft paper such as strength and holding its shape when soaked in oil are why it is used.
Dielectric strength doesn't matter since you are not making a capacitor out of it.

TDU

Steve Conner

Thu Oct 11 2007, 04:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I guess "unfolded" just means that the value given is for a single thickness, and is their way of reminding you that using two thicknesses doesn't double the breakdown voltage, because of air between the layers, etc. Also, folding it may stress it and make it partly un-press itself.

So for your application the 11kV/mm figure probably applies.

Why not soak the whole thing in oil and upgrade to the 40kV/mm that Google quotes for oil-soaked presspaper, though? Tesla himself wrote that oil was the best insulator for HF coils.

Tonic

Tue Oct 16 2007, 03:10PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Well, I don't have an transformer oil and it's a messy thing, but I will consider it if problems will occur.

The oil impregnated presspaper would be also a tricky thing to wind and glue together or use tape due of oil's slippery, right?

TDU - so, basically you're saying a raw presspaper is nothing in terms of dielectric insulation?

Oh, well.. it's too late. The only cheering thing is fact that presspaper is soaked a bit by epoxy resin. Actually, I've made one secondary, second one is in progress. There you see, how it does look:

And there's wideo of testing this secondary:

ZVS driver, 7+7 primary, secondary made by 6 layers, each consists something aroung 230 coils. So ~1400 coils divided by 7 makes 1:197 ratio. 35V at load multiplied by 197 makes ~7kV. With 5+5 primary I should be close to 10kV.

Now I'm thinking how to connect all after putting two secondaries. Connect in a way that prevents from arcing to core and primary. Is it possible?

uzzors2k

Wed Oct 17 2007, 05:25PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Looks good. The spacing between the layers due to the thickness of the paper might be enough to insulate it alone. I would put the secondaries on the long portions of the core, and the primary on one of the shorter portions.

Tonic

Wed Oct 17 2007, 07:03PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

There might be problem with arcing to primary since it's close to secondaries. It would kill transistors, right? So I'm thinking of buying the second core and make in that way:

More space between primary and secondaries and more space for primary, useful for bridge type power supply powered from main, where you need a lot of coils.

In case I won't find the same core, I will try to soak all into oil and fire. If it will success, I will be happy to have 20kV RMS and take a step to next project with 40kV RMS target. Now, there are details:

- I'll probably use two core configuration as I shown above because of a lot space for insulating and primary. It's possible I will buy bigger cores and higher quality.

- friend from native HV forum suggested to use polyester tape, since it has higer rupture voltage and use dielectric lacquer for improving insulation

- because of possibility to make insulation thinner, I would be able to make more layers. So, more layers, lower potential different between layers and lower risk of rupturing

- soak all into oil to prevent corona going into air and arcing to core and primary. The first layers of secondaries will be connected to core and ground, so first layers shouldn't arc core. But outer layers might and that's reason to use oil. There's another solutio - next layer, less coils, so space between core and coils is increasing, thus preventing from arcing.

- I will try to build a half-bridge configuration. The only problem is making driver without osciloscope. I remember about IR215X based driver, and I might try it again (the first tests were total failures, but maybe this time it will work..). And supply anything from main.

What do you think?

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