Re: Air as di-electric

To: tesla-at-pupman-dot-com
Subject: Re: Air as di-electric
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
Date: Fri, 03 Jan 1997 12:37:59 -0800
Cc: Tesla-list-subscribers-at-poodle.pupman-dot-com
Subscriber: bert.hickman-at-aquila-dot-com Fri Jan 3 21:52:37 1997
Tesla List wrote:
> 
> Subscriber: kukkonen-at-cc.hut.fi Thu Jan  2 23:04:23 1997
> Date: Fri, 3 Jan 1997 01:38:57 +0200 (EET)
> From: Kristian Ukkonen <kukkonen-at-cc.hut.fi>
> To: tesla-at-pupman-dot-com
> Subject: Air as di-electric
> 
> I've lately been seriously thinking about spending some valuable
> lab space for a "definitely-not-space-efficient" capasitor. The
> good this would be the fact that it could be virtually impossible
> to destroy without melting steel BIG TIME..
> 
> So, it's a plate-cap using steel plates separated 10mm from each other.
> 
> It would mean that the capasitance, using 2000mm*330mm plates would be
> 0.5841nF *(n-1), where n is the amount of plates.  Using just 41 plates it
> would be about 23nF. using 1mm thick plates the total mass would be about
> 200kg. The cost would be 45FIM (9usd)  for a plate 1000mm*2000mm which
> would get converted to 3 plates.  This would mean 14 of those big plates
> and 630FIM (130usd) - propably less - usually price for steel is 2FIM/kg
> at "surplus"stores.
> 
> The total size of the thing would be 2m*.33m*.41m "only".
> Plus the fixture keeping the thing intact.
> 
> Quite reasonable. Does not require much FLOOR space. :)
> 
> Would be (trans)portable and moveble with some wheels under it.
> 
> So, the question is: dry air etc. break at something like 4kV/mm. That
> would mean 40kV rating for that monster. Thinking reasonably it could
> work with the 20kV pig I have.
> 
> In reality? Does someone out there know? for sure?
> 
> At least it would be INDESTRUCTABLE.
> 
> Adding a blower below the thing would make any arcs act like in a jacob's
> latter - and I could actually put kind of jacob's ladders to the top of
> the plates - between each pair of plates. It would quench any arcs caused
> by cap "breakdown".
> 
> So, before buying those plates of steel and starting to spend some disks
> with the disk grinder cutting plates to pieces and sending molten pieces
> of steel around:
> 
> Opinions? Am I forgetting something? Will there be HUGE corona between
> plates - causing losses and breakdown at much less than 4kV/mm ? Something
> else? I may need to put some plates of plexi between edges of plates..
> 
> It could be made tunable by inserting pieces of plexi (or PP) between
> plates..
> 
> I could build a tank around it too - and fill it with dry N2 or
> something fancier. It would make it a potential bomb though.
> 
> Thanks,
> 
>   Kristian.
> 
> ---------------------------------------------------------------------------
> | Kristian Ukkonen       |  Do as thou wilt shall be the whole of the law |
> | kukkonen-at-alpha.hut.fi  |_____  Chance favours the prepared mind  |-------
> | http://www.hut.fi/~kukkonen |  Fear is the mind-killer  |---------
> -----------------------------------------------------------              42

Kristian,

I'd suspect you'll run into breakdown problems between plates at 20 KV,
since you'll really be seeing higher peaks (30 KV?) at RF frequencies.
I've seen flashovers (in the external circuitry) across my primary cap
which easily bridged 1"+. The easiest way woul be to construct a pair
and run it in parallel with your existing cap. This should give you a
direct feel for the breakdown capability in situ...

You probably will get a fair amount of corona losses as well, since the
steel plates will have minor surface imperfections, dust, etc. Putting
plexiglas in between may reduce actual sparkovers, but will actually
increase corona problems, since it will increase the amount of
dielectric stress in the remaining air gap. Immersing it in other than
air may help somewhat - just not hydrogen (:^)) or argon.

Hope this helps...


-- Bert --