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Re: Self capacitance and Medhurst
Subject:
Re: Self capacitance and Medhurst
Date:
Wed, 26 Mar 1997 08:57:42 +0000
From:
"John H. Couture" <couturejh-at-worldnet.att-dot-net>
To:
Tesla List <tesla-at-pupman-dot-com>
At 03:00 AM 3/25/97 +0000, you wrote:
>Subject:
> Re: Self capacitance and Medhurst
> Date:
> Tue, 25 Mar 1997 07:59:53 +1200
> From:
> "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
>Organization:
> Wellington Polytechnic, NZ
> To:
> tesla-at-pupman-dot-com
>
>
>John,
> I spent months testing Cself formulae while researching an
>article before I found Medhurst. You wrote....
>
>> Capacitance is mostly dependent on the dielectric and area. For a coil self
>> capacitance this would relate to the wire insulation and wire length. The
>> Medhurst equation uses only radius and coil lengh for the coil self
>> capacity. As Steve points out this can lead to some wild results.
>
>This is not correct IMHO. In fact it turns out that Cself is
>isotropic in nature and not inter-turn. It appears from what I've
>read in the CSN that Tesla didn't get this one either. If your
>scenario is correct, his various attempts to reduce Cself by using
>series capacitors etc. in the windings should have worked. They
>clearly didn't and he was forced to concede that Cself was here to
>stay. If you look up the windings, each turn is shielded from all but
>its immediate neighbour so you end up with a sum of tiny capacitances
>in series. I challenge you to present a coil (no terminal) whose
>Cself is other than that given by Medhurst's formula (to within 5%).
>
>> For a coil 1.5" radius and 15" long with .032 total dia wire (15/.032 = 468
>> turns). Medhurst would give about 5.8 pf for the coil capacity.
>>
>> For a coil 12" radius and 6" long with 0.1" total dia wire (6/.1 = 60
>> turns). Medhurst would give about 39 pf coil self capacity.
>>
>> Note that both of these coils use about 370 feet of wire. This would
>> indicate the coil self capacities would be about the same. The ratio of
>> 39/5.8 = 6.7 to one is to great for use with Tesla coils. This would
>> indicate Medhurst should be modified for use with Tesla coils or some other
>> equation should be used. If the look up table is used it would need more
>> fixing. I do not believe grounding is a factor.
>
>Indees it is and was stated as a condition of the formula's
>applicability. Once again, I would find a counter-example convincing
>(for a grounded winding). Things change dramatically when the
>winding is not grounded of course but that hardly applies to a
>resonator worked against ground. I'd suggest formulae for ungrounded
>coils more correctly applies to a suspended half-wave system.
>
>> There have been many coilers who have tried to come up with an equation that
>> works including N. Tesla (CSNotes), Corum's, and others. I researched this
>> problem several years ago when I was working on the JHCTES computer program.
>> I finally used an equation based on empirical data.
>>
>> This is another example of where radio equations must be used with caution
>> for Tesla coils. This is also why you will not find Tesla coil design info
>> in the standard radio books but have to go to the Tesla List or the few
>> books that have been written on Tesla coil design. These books, like mine,
>> are based on empirical data from real-world coils plus radio theory. These
>> books are still in the development stage. Much more building, testing, and
>> book writing by coilers is needed for Tesla coils.
>>
>> John Couture
>
>Well I spent 3 - 4 months hunting and testing before deciding to
>publish that one. There are minor variations on it that also come
>close but their range of coils is somewhat restricted. *None* of the
>wirelength formula worked in most cases and many didn't even work
>for one case. I have run this formula on over 50 coils so far. A
>counter-example is called for I think. Prove me wrong.
>
>Malcolm
>
>---------------------------------------------------
Malcolm -
Actually the Medhurst equation follows closely with my empirical
equation
based on wire length. That is for coil self capacities of a typical
Tesla
coil and H/D ratios over one. With ratios under one the two equations
diverge. I note that the lookup table doesn't go below H/D = one. The
differences are only with the ratios under one which are not typical
Tesla
coils.
Did your testing include coils with H/D ratios less than one to verify
the
Medhurst equation? Did you test a coil that would be about 12" radius
and
6" long with a 39 pf self capacity? Although this size coil would
probably
never be used, it would be interesting to build and test it. However,
Tesla's Colorado Springs coil was 50 feet diameter and 8 feet high with
a
.16 H/D ratio.
Somewhere I read that the coil self capacity is almost eliminated when
the
wire separation is 31 mils or more!! Have you ever noticed this coil
self
capacity characteristic with your tests? As the coil self capacity is
undesireable it would seem that wire spacing or thick insulation would
be an
advantage.
I note in Skip Greiner's post "Final test on new TC" that his secondary
wire insulation is about 33 mils thick. With this wire separation the
coil
self capacity would be almost eliminated. Could this be one of the
reasons
he is getting such extra long spark lengths for the wattage input he is
using?
I am always glad to compare notes with you Malcolm because you
apparently
have tested more coils than any other coiler and have accumulated the
most
data. Keep up the good work.
John Couture