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Re: Racing Arcs Explained???

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

That is a fair question. The simple answer is most probably comes from the interruption. Only when the ac voltage is constant amplitude and frequency will there be no other spurious frequency. The ring down will cause other frequencies, the rep rate of the bang will cause other frequencies, the coupling between primary and secondary will cause multiple frequencies, and any distortion in the fo waveform will cause harmonic frequencies. I use the term harmonic maybe too loosely and I'm not really trying to define what the multiple frequencies are but Im trying to allude to the presence of multiple frequencies. The primary waveform is a periodic waveform with a period of 1/120 sec (for 120BPS). A fourier analysis on this waveform can yield insight on what those frequencies are (best done with a simulator program that has FFT capability.

some examples:

1.  AM modulated carrier will create sidebands.
2. FM modulated carrier will create bessel functions. The coupled frequency interaction between primary and secondary may fall into this category
3. Pulse modulation frequencies may also apply, etc

Its been 40 years since I studied these things and haven't really applied it since. But I hope you get what Im driving at :o)))

Gerry R.

Original poster: "Steve Ward" <steve.ward@xxxxxxxxx>

Given the waveform on the primary (sparkgap initiated resonance that
rings down), I think there will be harmonics.  A fourier analysis can
determine the spectral content.

With no secondary in place, i cant see where any other harmonics would
come from.  Could you be more specific when you say "i think there
will be harmonics"?  From what i can figure out, pretty much all the
methods we use for driving our secondaries, are all from sources that
dont have much harmonic content (the current is a sinusoid).  Im not
sure about some SSTCs where the voltage across the primary is a square
wave, but those too have sinusoidal current in the primary (so where
are the harmonics?).

 A secondary will have overtone
resonances (1/4, 3/4... wave).

So we are getting away from a lumped analysis now, right?  Thats fine,
since a lumped analysis doesnt explain all of the secondary
characteristics anyway.

 If the velocity constant was
"constant", the corresponding frequencies would be fo, 3fo, etc.  As
Paul has pointed out, the velocity constant is not fixed but varies
with frequency.  The size of the top load will also change the
frequency ratio of f(3/4) / f(1/4).  I think the challance is to pick
a topload where this ratio is not 3, then the harmonics will not
excite the 3/4 wave mode.

Well, wouldnt any reasonable top capacitance pretty much destroy that
perfect 3:1 ratio?  How far off should the ratio be to avoid
harmonics?  I would guess that depends on the Q somewhat, as to
whether or not you would excite that other mode.  I must admit im
having difficulty conceptualizing how the 1/4 and 3/4 resonances work
when you have a big toroid on the end of the coil.  Maybe i should
read Paul's papers ;-).

I still dont really believe that exciting higher harmonics in the
secondary is what causes the racing sparks.  It seems simple enough to
me, put too much stress on the coil and its going to arc over,
particularly near the bottom where the coupling is greatest.  So if
you relax the coupling some, the volt/turn should be lower, and in all
of my cases of racing sparks, this has fixed the problem.

Again, maybe i should read Paul's papers to get a better idea of what
goes on within the secondary coil.


Gerry R.

>Original poster: "Steve Ward" <steve.ward@xxxxxxxxx>
>>So, the question is, has anyone ever experienced racing sparks on a >>tuned
>>solid state or VTCC? If they occur on these type of coils and if
these coils
>>don't emit harmonics, it would disprove my guesses.
>My DRSSTCs experience true racing sparks, but usually it isnt so
>severe as what ive experienced with spark gap coils.  The problem has
>worstened now that i try to make energy transfers as quick as
>possible.  I actually only get racing sparks if i try to run the coil
>without a breakout point, then i see the sparks across my secondary
>now and then.  A DRSSTC shouldnt excite any higher harmonics, so it
>seems the problem of racing sparks really may be just transformer
>action, and having simply too much of a voltage gradient.  I know
>Jimmy's original DRSSTC experienced a lot of problems with racing
>sparks.  He went so far as to add disks to his secondary, which i
>guess helped.
>Ive also experimented with using inductors for marx generators, and
>again there i have seen the same racing spark phenomena.  I was
>running about 20kV/inch of winding length (though it could have
>possibly peaked higher), and that was enough to start problems.  I
>didnt have any insulation on the magnet wire, though.
>The primary of a TC is pretty much a lumped LC, its not going to
>respond so much to harmonics.  I think the reason spark gap coils are
>worse with racing sparks than SS or VT coils is because the primary
>energy peaks to its maximum within the first half-cycle.  SS and VT
>coils slowly build up primary energy, which allows the secondary to
>slowly ring up in voltage (even when the coupling is high).