Re: Racing Spark Prediction

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Dimitry wrote:
> [racing arcs] ...just measure it

Why not?  No progress has been made for a long time on
understanding the mechanism behind racing arcs.  Perhaps
because the regular fix for the problem is generally
successful there's little incentive.

I posted a list of some possible causes, a while ago now,

 http://www.pupman.com/listarchives/2002/July/msg00429.html

My hunch is that overtones have something to do with it,
perhaps involving primary overtones too.  That would explain
the apparent sensitivity of the phenomena to small changes
in tuning and coupling - or is that bit just a myth?

Plus, I think there is more than one type of racing sparks,
and I would be interested to hear coiler's descriptions.

Dimitry wrote:
> does this mean that i`m fooled every time when i see a nice,
> almost linear 45 deg voltage rise from bottom to top of my
> secondary, independently from what form of primary or value
> of coupling i use?

At any instant, the secondary voltage distribution is the sum
of the primary induction and the secondary self induction,
During much of the beat cycle, for modest k, the secondary
self induction dominates and, given a nice big topload, the
average secondary volts distribution will tend to give a
fairly linear rise over the whole length of the coil.

(Incidentally, the two dominant normal modes have pretty much
 the same distribution of I and V, diverging significantly only
 in the region in which primary induction is strong.)

However, at times during the beat, for example the first quarter-
cycle of the primary ringdown, the secondary must withstand the
onslaught of the induction from the full primary current, and
this has the distribution described by someone earlier - one
which mimics closely the primary induction profile.  The result
might be very high transient volts per turn in those parts of
the secondary receiving the strongest primary induction.  A more
uniform spread of primary coupling would mitigate this, but in a
TC we're forced to apply the coupling more or less at one end.

Looked at in the frequency domain, the greater concentration of
coupling onto a small region of the secondary manifests as a
larger overtone content - ie more of the bang energy goes
into resonances other than the two which form the desired beat
envelope.   Overtone content is usually only a few percent at
most, but each overtone has a peak-trough distance spanning
rather fewer turns compared with the fundamental, so raising the
volts/turn by the same factor.

The recipe given by D.C. for avoiding racing arc problems by
setting the primary plane some distance below the start of the
secondary winding, is a sure fire way to make the coupling less
concentrated, with of course an accompanying loss of k factor.

Another effect of the close proximity of a primary winding to
the bottom of the secondary is to apply greater shunt capacitance
to that part of the coil, which results in a greater fraction of
the overall secondary voltage rise in that region.

A note about coupling:  k is measured and defined in terms of
low frequency, uniform current, conditions.  But in operation,
the secondary current distribution is not uniform so the effective
coupling might be expected to be different from the low frequency
value.  This turns out to be the case when running distributed
models - the ratio of the mode frequencies is a little different to
sqrt(1+k)/sqrt(1-k), not that that matters in practice, it's just
one of the many little factors that make the real coil differ
slightly from the design model.

The software library GeoTC

 http://www.abelian.demon.co.uk/tssp/geotc/

is capable of calculating the two contributions to secondary
voltage - the self induced quarter wave profile, and the primary
induced voltage profile, although I think this has not been fully
implemented.

Of course, such calculations won't by themselves predict whether
a given design will suffer racing sparks.  We'd have to figure
out the mechanism(s) and the thresholds involved.

To do more than speculate, we'd have to study closely the
behaviour of a well-instrumented system which can be made to
produce racing sparks.
--
Paul Nicholson
Manchester, UK.
--