[Prev][Next][Index][Thread]

Re: 3MV/meter



>>From MALCOLM-at-directorate.wnp.ac.nzFri Sep 27 21:57:14 1996
>Date: Fri, 27 Sep 1996 17:46:46 +1200
>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>To: tesla-at-pupman-dot-com
>Subject: Re: 3MV/meter

Malcolm wrote:

>Hi all,
>        I have been using single shot methodology for measuring coil
>parameters and performance for several years now. In fact I used it 
>to get the primary Q measurements which should be posted soon. With
>a cap voltage of 12kV on the coil in question I can get a single 
>solid channel spark 17+" long to a discharge rod (Ecp about 7.2J).
>I have serious doubts about 3MV/metre though. The Ctot of the 
>secondary of this coil is 26pF. Ignoring gap losses, the best it can 
>theoretically do at this energy level is 744kV. The spark length 
>according to my rule of thumb of around 35kV/inch is about 600kV at
>this energy. That seems reasonable considering the secondary only gets
>one ringup under these conditions.

>Richard wrote....

>> There was an old article By DC cox in a TCBA NEWS of yesteryear.  In this 
>> article he suggested single pulsing a Tesla coil to determine the actual 
>> voltage output.
><snip>
>>...  At the point where a true streamer arcs (not just a brush 
>> connect), this is the output of you coil based on the DC, 3 megavolt/ 
>> meter, air breakdown rule.  It is important to closely mime the real 
>> conditions of your coil system to make this really hold.

<snip>

Malcolm, All,

In John Kraus' book Electromagnetics, 4th edition, table 4-3, on page 
149, lists a table containing common dielectric materials and their 
dielectric strengths.  Air (atmospheric pressure) is listed at 
precisely 3 MV/meter.  

I guess that means any air filled meter that considers itself a  _real_ meter
should be capable of measuring 3 megavolts. :-) Ha, ha.

Back to seriousity though, a very thorough treatise was done on using 
sphere gaps to accurately measure HF AC discharge voltages, as well 
as DC voltages in air by Clark and Ryan. Their work is used commonly 
in the industry as the authoratative standard for air breakdown 
values.  In order to get an accurate measurement it is imperative that both
terminals have a smooth (polished clean) surface with a radius of curvature
somewhat larger than that of the length of spark being measured.  
Large objects in the vicinity of the spheres can also cause 
perturbations in the extended field surrounding the spheres, and 
therefore ruin the accuracy of this test method.  I cannot find
the reference where I have seen the exact figure for sphere size just now but a 
radius of 2X spark length seems to ring a bell.  The Clark and Ryan paper
shows a chart tabulating the results of measuring sparking voltages between 
two 7 inch copper spheres with electrical current at 25 CPS and 
123,000 CPS.  At 123 kHz, 1 inch of air was broken down at 48 
kilovolts.  At 25 Hz,  1 inch of air breaks at 52 kilovolts.  The 
breakdown values versus distance were perfectly linear up to the 50 
kilovolt range.

Malcolm, even your 35 kV/inch rule-of-thumb is slightly low, but may 
actually be more realistic when you factor in the common use of smaller 
radius or pointy terminals.

regards,

rwstephens