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Re: Cylinder Cap / Oil Cap
From: gweaver[SMTP:gweaver-at-earthlink-dot-net]
Sent: Saturday, January 10, 1998 12:32 AM
To: Tesla List
Subject: Re: Cylinder Cap / Oil Cap
At 05:12 PM 1/9/98 -0600, you wrote:
>
>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
Jim
Here is another idea. Make several flat sheet metal plates and put them all
in a stack. Clamp them all together with C clamps. Make sure everyother
plate is offset by 1/4". Drill several holes through all the plates at the
same time so all the holes will line up. Remove the C clamps and deburr the
drilled holes. Bolt all the plates together in a stack using plastic all
thread rods ordered from McMaster Carr. Use the plastic hex nuts as spacers
between the plates. 10-32 plastic hex nuts measure about 1/8" thick so that
will give you a space of 1/8" between each plate. On the plate ends that
are offset connect every other plate together by soldering a wire to the
edge of the metal plates. Place the stack of metal plates in a fish
aquarium and fill it with oil. The plates should be vertical so the air can
escape. This cap will be easy to build and self healing if it arcs.
Gary Weaver