Re: [TCML] Racing arcs and primary resonances

[DC Cox <resonance@xxxxxxxxxxxx>]

You may be able to replicate this performance by purposefully building a
poor design gap (small electrodes, etc) and then overpowering it at 8-15
kVA.

Dr. Resonance




On Sun, Apr 3, 2011 at 12:43 AM, Paul Nicholson <tcml88@xxxxxxxxxxx> wrote:

> 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
> --
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla