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Re: Arc Impedance Study

Hi Greg and All,

At 11:29 PM 10/8/98 -0700, you wrote:

>> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
>
>> First, I considered the sphere20us.jpg scope photo from Greg Leyh's
>> Electrum coil (www.lod-dot-org/electrum/sphere20us.jpg).
>[snip]
>> So we have a magnitude of the impedance and the phase angle.  Geometry will
>> show that the 300k ohm impedance is composed of 226k ohms of real
>> resistance and 197k ohms of capacitive reactance.  By knowing the frequency
>> we can find that the capacitance is 21.3pF.  So the arcs Greg's coil was
>> driving are similar to a 197K ohm resistor in series with a 21pF capacitor.
>

Oops!  I should have said is similar to a 226k resistor in seriess with a
21pF cap.

>A curious thing: the 1/(2*pi*RC) frequency for 197K and 21.3pF is 38kHz.

In fact, it is exact!  F=1/(2*pi*R*C) is equivelant to R=1/(2*pi*F*C) which
is also equal to the capacitive reactance Xc=1/(2*pi*F*C) which is the
basic definition I used to calculate the 197k ohms.  ;-))

>> What I found very surprising, is that the real resistance of my small arcs
>> were very close to the real resistance of the Electrum's giant arcs at
>> around 200k to 280k ohms.  
>
>That is very interesting... A possible explanation: if the 
>current density in the base of an arc does remain constant 
>with increasing current, then a 5A arc will have 12.5 times 
>the x-section of a 400mA arc, providing better conductance.
>However, if the 5A arc ends up 12.5 times as long for some
>reason, it's effective resistance will end up about the same.
>

Very interesting indeed!!
 
> 
>> If one looks at Greg and I's sphere to arc currents, it appears that most
>> of the current waveform represents the current needed to drive the arc's
>> capacitance.  Both Greg's and my waveforms show roughness at the peaks of
>> the current waveforms which may represent the instability of the arc
>> "frying nitrogen".
>
>BTW, Did your arcs show the 'roughness' on just one polarity
>of the peaks?  Seems like dart leader action to me.

It seems to favor the positive peaks somewhat.  I will have to look at this
specifically this weekend to really see if that is true.

>
> 
>>   Terry Fritz  (who has done more than just read mail lately :-))
>
>Indeed!!  I would be keen to see this msmt performed on some
>other coils as well, to see where their arc impedances lie.
>Perhaps next time you're in SF with your FO setup we can 
>measure the 26kVA coil, or I'll have to borrow a shark suit 
>and climb up into the toroid with the scopemeter!
>-- 

The sphere to arc probe I used here is built into the secondary terminal.
However, a portable version could be whipped up easily that would have the
appropriate breakout point that could be simply be placed on any toroid.
Perhaps the coils of our local Tesla group could then also be tested in
this way.  I'll work on this.

	Terry

>
>
>-GL
>www.lod-dot-org
>
>
>
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