Re: [TCML] Racing arcs and primary resonances

[Futuret <futuret@xxxxxxx>]

Paul,

I like your Poulsen resonance hypothesis.  

Here's some speculations.  

I suspect that when my gaps heated up, the firing voltage decreased
not so much because the gaps became physically closer together
(due to expansion of the metal due to heat ---  although that is a factor
too), but because the hot gaps tended to ionize the air at a lower
voltage.  Because of this, I think the firing voltage was greatly
reduced, and the break-rate may have risen quite high.  During
parts of the 60Hz AC cycle, the gaps may have failed to de-ionize
and may have been in a power-arc or semi-power-arc state (which
would lower the break rate at those time during the 60Hz AC cycle).  
Or even if no power-arc existed, the break rate may have risen so
high during portions of the 60Hz AC cycle, that the primary
capacitor may have had very little time to charge.  This same
condition may have occurred in Brandon's coil when he turned
up the variac to 140V.  These conditions of intermittent power-
arcing or very fast break rates giving the capacitor very little
time to charge may in some way contribute to the formation 
of Poulsen resonances and racing sparks as a result.  If the
gaps re-ignite much more rapidly than normal, the
reduced capacitor charge voltage may favor the production
of the Poulsen resonances, and/or permit them to persist.

If Brandon scopes his coil, it will be interesting to see if there
is evidence of power arcing, very high break rates, and low
levels of capacitor charging during parts of the 60Hz AC
cycle which would normally see higher voltage levels.   

Cheers,
John

 

 


 

 

-----Original Message-----
From: Paul Nicholson <tcml88@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sun, Apr 3, 2011 10:53 am
Subject: Re: [TCML] Racing arcs and primary resonances


John wrote:

 > There were no racing sparks at full power until the gap

 > heated up for 10 seconds or so, then the racing sparks became

 > progressively worse within a few seconds until the secondary

 > was covered with a mass of racing sparks.  If I then let the

 > gap cool down, and went to full power again, the process

 > repeated.



Gary Lau wrote:

 > I would think that if a static gap overheats, its breakdown

 > voltage would be reduced,



These comments are compatible with each other, and with

Brandon's racing arc condition.



In both cases described, the supply transformer secondary

voltage is trying to rise above the level at which the gap

fires, either because Brandon has turned up the variac from 80V

(firing point) to 140V (racing arcs start), or because John's

spark gap has heated and closed up a little.



The description of secondary racing arcs in both examples is

the same and indicates HF activity in the secondary.



I wrote:

 > When the variac is turned up beyond the level required for

 > normal firing, without opening out the spark gap, is this

 > likely to excite extra HF oscillation, perhaps through

 > negative resistance arc behaviour interacting with the

 > spectrum of primary resonances.



I propose from the comments so far the hypothesis that the

oversupply to the gap is pushing the arc into some sort of

Poulsen resonance with one or more of the primary overtones.



Some notes on Duddell 'singing arcs' and Poulsen arc

generator can be found here,



  http://www.qsl.net/vk5br/Before_Valve_Amp.pdf



and the whole document is quite a nice read.



The thing to note is that the effect arises with a series-tuned

circuit across the gap, which would be provided in this case

by the half-wave (and multiples) modes of the bare primary

coil.  The primary cap is in series with this series resonator

and has little or no effect.



Some ways to test/refute this:-



- The effect should not be seen with solid state gaps;

- The HF should be visible in the scope and frequency should

   agree with calculated or measured half-wave primary mode;

- It should be possible to reproduce the effect on any static

   gap coil where the supply is enough to overdrive the gap;

- The HF should be tunable by bringing up an earthed plate

   near to the primary coil.



Examination of primary current waveforms is required, looking

for HF components which increase significantly as the variac

is turned up beyond the normal firing point.



Brandon has a 'scope available, and can presumably rustle up a

CT for the primary current.    I can torture some modelling

software to make it calculate roughly the primary overtones.

-

Paul Nicholson

--
 
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