[Prev][Next][Index][Thread]
Re: Self capacitance and Medhurst
Subject:
Re: Self capacitance and Medhurst
Date:
Sat, 22 Mar 1997 20:12:37 +0000
From:
"John H. Couture" <couturejh-at-worldnet.att-dot-net>
To:
Tesla List <tesla-at-pupman-dot-com>
At 07:24 AM 3/20/97 +0000, you wrote:
>Subject:
> Re: Self capacitance and Medhurst
> Date:
> Thu, 20 Mar 1997 15:36:08 +1200
> From:
> "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
>Organization:
> Wellington Polytechnic, NZ
> To:
> tesla-at-pupman-dot-com
>
>
>Steve wrote....
>
>> I have seen two methods of finding the self capacitance of a coil. One
>> is closed-form:
>>
>> V = L/R (in inches)
>> CD = 5.08*R*(.0563*V+.08+.38*SQR(1/V)) 'uufd.
>>
>> And the other is table-lookup, attributed to Medhurst:
>>
>> C = K x D (D in centimeters)
>>
>> H/D K
>> 5.0 0.81
>> 4.5 0.77
>> 4.0 0.72
>> 3.5 0.67
>> 3.0 0.61
>> 2.5 0.56
>> 2.0 0.50
>> 1.5 0.47
>> 1.0 0.46
>>
>> These two methods agree at the low end of aspect ratio, but begin to
>> diverge at the high end, with the closed-form yielding the smaller
>> values.
>>
>> Does anyone on the list have a feel for which approach is more
>> accurate? Does anyone know where the closed-form originated?
>>
>> Note: I have played around with gnuplot, and can easily come up with a
>> closed-form which matches the table points much more closely. But I'd
>> like to know if the closed-form above is better than the table, or is
>> merely an approximation of the table.
>>
>> Steve Falco
>> sfalco-at-worldnet.att-dot-net
>
>I have found Medhurst to be reliable for all the coils I've tested it
>on (quite a few) to within a few percent. It is conditional on one end
>being grounded though. It cannot be used to predict Cself for an
>unearthed halfwave coil. We need to develop or find another one for
>that.
>
>Malcolm
>
>---------------------------------------------------
Malcolm -
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.
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.
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