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Re: Arc Impedance Study



> 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.

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


>    Now I do all the same with my relatively tiny coil.  The scope photo at
> www.peakpeak-dot-com/~terryf/tesla/misc/c2ss2a.jpg shows the similar waveforms
[snip]
> The delay angle is 60 degrees with current leading voltage indicating a
> capacitive load.  With the output voltage peak of 225kV and a current of
> 400mA the magnitude of the impedance is 563k ohms.  This implies an
> impedance of 282k ohms resistive and 3pF capacitive.
> 
> 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.
 
 
> 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.

 
>   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!
-- 


-GL
www.lod-dot-org