Re: [TCML] Racing arcs and primary resonances

[Paul Nicholson <tcml88@xxxxxxxxxxx>]

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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