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Grooved Bobbins For Segmented Windings

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Patrick

Thu Jul 21 2011, 02:52AM

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

I propose a test, a small amout of epoxy in a cup under vacuum, and the remainder left atmospheric, then lets see.

Ash Small

Wed Aug 24 2011, 03:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Following on from my comments in another thread, if you vacuum impreg the coil in this position:

but leave excess tupe on the outside, and put a bung in the hole in the middle, you can overfill with epoxy, and it can bubble off without making a mess. After the resin has cured you can cut it to finished length, etc.

However, while I agree that epoxy encapsulation can have advantages in some applications, I'm sure you'll agree that the 'segmented, tall thin multiple secondaries' do seem to be ideally suited to oil cooling, which has to be the main advantage of oil encapsulation.

Just to clarify what I said elsewhere, the main advantage of oil encapulation is the benefit of cooling. Any advantages that epoxy/silicon encapsulation may have will come at the cost of less efficient cooling. In light of this I'm tempted to concentrate my own efforts in this direction towards oil immersion and cooling.

IntraWinding

Wed Aug 24 2011, 06:07PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Some high performance industrial epoxy resins are boastfully described as 'Solvent Free', which suggests cheaper stuff is 'watered down' with some solvent or other. I've tried degassing slow setting Araldite with a two stage rotary vacuum pump and it's very ineffective because it bubbles up so much. In fact it forms a stable foam. If you cut off the vacuum pump you can leave it to set that way. I've always assumed that was the mystery solvent boiling off. It didn't cause a sticky outer surface after setting, by the way. I've reverted to degassing Araldite with a temperature controlled hot air gun, which works very well if you heat it enough but not so much as to prematurely set it. No use for potting, of course.

jpsmith123

Tue Sept 20 2011, 03:41AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I tried to epoxy encapsulate one more coil (which got ruined, incidentally) but I came to the conclusion that the vigorous bubbling I saw was air, and not any volatile ingredient of the epoxy.

I first degassed the epoxy in a cup by itself, and after about 5 minutes, it essentially stopped bubbling. Then I opened the chamber to the atmosphere and poured the epoxy into the coil housing, then I pumped it down again - with more vigorous bubbling. And this time it took about 20 minutes before it stopped bubbling.

Anyway, the coil got ruined because the epoxy never cured. I think what happened is that I simply did not agitate it well enough before I used it - not just this time, but the other two times as well. I think that the stuff left in the bottom of the can was mostly Al2O3 filler, and not enough resin.

I'm really irritated too since the ruined coil was my best one yet. I redesigned the feed through insulators and placed the coil in a better spot. Mechanically and electrically, the coil assembly was a masterpiece. It sucks spending several days working on something and then having to throw it in the garbage.

I don't like working with thermally conductive epoxy, mainly because the unmixed resin is extremely viscous. I shook it a little bit before using it, but it apparently wasn't enough, and in retrospect I think it needs to be vigorously mechanically mixed somehow.

It seems to me that all non thermally conductive epoxy-resin systems have a relatively high dissipation factor, with the best ones being in the range of 0.01 to 0.02 or something like that. Apparently the only way you can get better performance (maybe in the range of 0.003 to 0.005) is by replacing some of the lossy resin with Al203.

In light of this, I guess I have to conclude that one more advantage of a "pie" wound secondary over a layer wound secondary is that you can use cheap, lossy epoxy, without suffering very much power loss - maybe on the order of a few tens of mw. Whereas with with a layer wound coil, depending on the operating frequency and the voltage between layers, I can easily see a few hundred mw to a few watts of dielectric loss.

Patrick

Tue Sept 20 2011, 07:49PM

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

yep that solid stuff scares the hell out of me, ill be winding my own secondary soon, it will be a solid secondary of epoxy immersed in oil.

jpsmith123

Tue Sept 20 2011, 08:51PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well I've made some improvements to the design of my segmented coil former. I also now turn them on a mandrel between centers, and that works better when using the 0.125" grooving tool. The last one I made really came out nice. It should easily hold at least 600 turns of 28 gauge wire and I think it should be good for at least 15 kv without any potting.

I'm going to wind one later this week and then I'm going to pot it with this stuff:

(BTW maybe I could send you a coil former to wind your coil on?)

Patrick

Wed Sept 21 2011, 02:30AM

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

jpsmith123 wrote ...

Well I've made some improvements to the design of my segmented coil former. I also now turn them on a mandrel between centers, and that works better when using the 0.125" grooving tool. The last one I made really came out nice. It should easily hold at least 600 turns of 28 gauge wire and I think it should be good for at least 15 kv without any potting.

I'm going to wind one later this week and then I'm going to pot it with this stuff:

(BTW maybe I could send you a coil former to wind your coil on?)

Id rethink that carbon black epoxy.

jpsmith123

Wed Sept 21 2011, 03:06AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Why what's wrong with it?

Patrick

Wed Sept 21 2011, 03:11AM

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

I generally try to avoid carbon black in my HV devices, carbon soot just doesnt sit right with me.

Ash Small

Wed Sept 21 2011, 11:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

I generally try to avoid carbon black in my HV devices, carbon soot just doesnt sit right with me.

Yep. Avoid carbon black at all cost.

Carbon is a conductor, not an insulator.

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