[Prev][Next][Index][Thread]
Re: Cylinder Cap / Oil Cap
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Friday, January 09, 1998 10:23 AM
To: Tesla List
Cc: JDM95003-at-UCONNVM.UCONN.EDU
Subject: Re: Cylinder Cap / Oil Cap
>
> From: Jim Monte[SMTP:JDM95003-at-UCONNVM.UCONN.EDU]
> Sent: Wednesday, January 07, 1998 7:32 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: Capacitor oil / Capacitor question / Coil length formula
>
Snipped out stuff on capacitor oil and coil length formula...
>
> While I'm posting, I would like some feedback regarding a cap I am
> considering making. The capacitor will essentially be a cylindrical
> capacitor with the outer cylinder being 3/4" copper tubing and the inner
> one being 1/2" copper tubing. This will be implemented as shown below:
>
>
> | + |
> | |
> -- ------ ------ ------ ------ ------ ------ --
> | | | | | | | | | | | | | |
> || || || || || || || || || || || || || ||
> || || || || || || || || || || || || || || stage 1
> || || || || || || || || || || || || || ||
> || || || || || || || || || || || || || ||
> | | | | | | | | | | | | | |
> -- ------ ------ ------ ------ ------ ------ --
> .
> . intermediate
> . stages
> -- ------ ------ ------ ------ ------ ------ --
> | | | | | | | | | | | | | |
> || || || || || || || || || || || || || ||
> || || || || || || || || || || || || || ||
> || || || || || || || || || || || || || ||
> || || || || || || || || || || || || || || stage n
> | | | | | | | | | | | | | |
> -- ------ ------ ------ ------ ------ ------ --
> | |
> | - |
>
> The tubing would be TIG welded to copper plate and the whole assembly
> would be submerged in oil. Each stage would look something like a
> bed of nails, with the inner stages having nails on both sides. The
> holes in the copper plate would be to allow the oil to pass. Insulating
> spacers would prevent each stage from shorting.
>
> This design seems to have an advantage over using a solid dielectric in
> that if you did manage to break down the oil, an oil change is all that
> is needed to bring the cap back to good-as-new condition. Being copper
> with fairly short conducting paths, it should be very well-suited for
> pulse applications, and since copper tubing is mass produced, it should
> be reasonably cheap. Disadvantages I see are mainly in the time it will
> take to weld everything together with sufficient precision. Any others?
>
> I have seen a lot regarding oil dielectric constants and breakdown
> voltages, but I do not recall seeing anything about losses of various
> oils. Can someone supply information on loss tangents for different
> oils?
>
> With oil having a breakdown voltage of approx 200 kV/inch, using 1/2"
> and 3/4" tubing would give a breakdown voltage of about 25 kV/stage,
> but I've also read that asking a single cap stage for more than 10 kV
> is asking for trouble. Any suggestions on what would be a reasonable
> rating for this cap per stage?
>
> Incidentally, a cylindrical cap has
> C= 2*pi*dielectric_permittivity*length / ln(outer_radius/inner_radius).
>
'nother snip
>
> Jim Monte
>
>
Jim,
You asked for feedback on your proposed design. I hope that the following
remarks will be seen as constructive feedback, and not negative
criticism. Thanks for your post! It is the free flow of ideas that
makes this Tesla list so very useful to us all.
The question is not whether your design will work (it will), but rather
does it have any real advantages over other designs?
What I see is a capacitor design in which there is a LOT of metal and not
that much capacitance per pound. I would hate to see you (or anyone else)
spend a lot of time building such a decice only to find that it involves
a lot of work and material and doesn't offer much bang for the buck.
In a flat plate and rolled cap design the surface area of the plates
is pretty well used. In the above proposed design I see that effectively
each electrode set comprises only a single useful surface area as far as
capacitance goes. So right off the bat there is a 50% loss in overall
plate capacitance. (In regular caps both sides of the plate enter in
and play a part.)
There appears to be much wasted space. By this I mean space that does
not contribute to actual capacitance or HV insulation. Thus the cap
will be quite bulky. It will require much more oil than a flat cap or
rolled cap design.
The only thing it has going for itself is the all-oil aspect.
But what would you be gaining by this design over a straight-forward
oil-only flat plate cap?
While all-oil designs are self-healing and easily fixed in case of an
accident, the bad news is that they become quite bulky. The major
advantage of poly caps is that the poly can withstand very high
voltages per mil. With only oil you need much greater distances
between plate surfaces, and this *reduces* the effective capacitance
you can attain. A LOT!
For the time being it appears that it is really hard to beat the
oil/poly capacitor.
Just a remark concerning using oil of a higher dielectric constant:
while it is true that the poly will get a higher *percentage* of the
voltage than the oil, please note that without the oil the poly
would be getting 100% of the voltage stress. So adding ANY kind
of oil could not possibly "stress the poly more" if by "more" you
mean "more than it would get all by itself." Indeed, if there
is anything to be concerned about it is whether or not the OIL
can take the stress. Although the oil will get a smaller *percentage*
of the voltage stress, it is also MUCH THINNER in thickness than
the intervening poly, and so it's small percentage of voltage has
to be held back by a very thin layer. THAT is where my own concern
was originally centered, but experiments have shown that the oil
with a dielectric constant of 4 DOES in fact give a higher total
capacitance than regular transformer or mineral oil, and it DOES
stand up to the rigors of the Tesla coil environment. And my own
personal philosophy on such things is to use whatever has been
demonstrated to work. See also Ralph Downs' recent post today on the
use of high dielectric constant oil. The facts speak for themselves.
Hope this helps.
Fr. Tom McGahee