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Re: HV Arc Resistance

Hi Terry, Jim, John, all

Very interesting thoughts. My comments and
questions below.

Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>I have made a few gaps recently :-) so I'll give my two
>cents on this. I "think" if you double the distance, the
>resistance basically doubles. I think a good measure of
>this is when one draws an arc such as off a Jacob's
>ladder.  If you double the distance, the arc just seems
>to get longer without the intensity of the arc changing
>greatly.  So if it is twice as long, the arc is twice as big
>and the resistance would seem to be proportional.

Interesting analogy. I had thought of something similar, but
I wasnīt sure, because the arc of a Jacobīs Ladder "seems"
to get fiercer as the arc grows longer (the flame front gets
wider), so I wasnīt quite sure if the resistance is really
proportional to the distance.

I wrote:
>1.) Two electrodes with a distance X.
>2.) A 10 electrode gap with a TOTAL distance X and
>a gap to gap distance of x/10.

>I think many electrode gaps probably have the same resistance when they
>are conducting but have much higher losses at the zero current crossing
>where the arc goes out.  A single gap will stay hot and it is easy for
>the arc to restart.  However, a multi gap cools quickly and the arc has
> a much more difficult time restarting.  This appears effectively as
>higher losses but more effective quenching.  A gap with say a hundred
>small gaps would have super good quenching but the losses would be
>extreme.  A single gap has very low losses but practically no
quenching.

I think you really hit the nail on the head. This is most probably
the reason why Gary (high voltage, low primary amps) gets
good results and I donīt with a single (quench aided) gap. My
(low voltage, but high amperage) spark is much harder to
quench, than Garyīs system. Due to my high current, the arc
can "last" much longer. This may explain why I simply *must
have* a large number of gaps and I might even have to add
series gaps to my (still unfinished) rotary. We will see. As
Jim pointed out, it is the current, which "resists" quenching.

I wrote:
>3.) Same as 2, except 50% of the gaps are x/20 and
>the other half is x/5 (so total X is the same for all 3 cases).

>I seems that the more gaps sections one has, the resistance rises but
>the quenching gets better.  I think it is possible for a low resistance
>poor quenching gap and a high resistance good quenching gap to
>both do well but it appears to depend on the system they are used
>on. In other words, some systems seem to benefit from better
>quenching while others need lower primary resistance to make the
>best sparks.

True. In a high voltage, low current system, you will have lower voltage
losses (all in all), but you *need* every amp you can get. Whilst in my
low voltage, high current setup, I might lose a few volts, but the
current losses are %-wise a lot lower, even if I run a large number
of gaps. My uncooled 10 gap static gap gets hot enough to burn
your fingers within 30 secs of running. One would assume the
quenching goes down as it gets hot, (eaiser to ionize). Yet, my
performance only starts to go down after a 8-10min run. This gap is
still very crude (not finalized, of course) and is mounted on
particle board. After one of those 10 minute runs, I will have to
replace the particle board, because it is totally charred around
the Cu tubes.

>I have 10K in series with the transformer but the voltage is very high
>and the current is low.  So even this high resistance is not a
>significant factor in the "kicking effect".  If you add resistance to
>the primary circuit, your systems output sparks will fall
>dramatically!! It is very important to keep the primary system
>resistance low but there is a point where other factors start
>to take over.

A gold plated primary would look cool. No, seriously, I got to thinking
about the LTR system. In THIS case, a high voltage, low current
source might (I canīt get enough caps for a MMC (for long lifespan)
to prove the contrary ;o}) be better, because the higher the voltage
and the lower the supply current, the higher the psuīs inductance
will be. My planned 6kVA setup would consist of (6) modified NSTs
-at-7500V and 780mA. The inductance of this setup is quite a bit
lower than, say, a pig with 19200V and 300mA. On the other hand
my low voltage setup will be a lot stiffer, so I might be able (or have)
to go to a higher (but "in-sync") breakrate.

>The best gaps I have used to date are high power sync rotaries with
>two gaps with very close spacing.  Forgetting all the sync effects, I
>have been going for as few gaps as possible with very close spacing
>and not worrying much about quenching.  My rotary arcs about 1/4
>inch before the gaps align so the quenching should be very poor.
>Yet my systems does not seem to mind that as long as the primary
>resistance is lowered."

This is exactly one of the reasons why I asked the question of
resistance vs. time. In your rotary (oh boy, I hope I get this
straight in words), the initial primary (well really gap) resistance
is high at 1/4" gap distance, so the initial shot of current is limited.
Further on down the time scale, the voltage decreases (simply
because the cap has been partially discharged), but the gap
resistance is now lower (electrodes closer together). This kind
of setup would act as a "constant" current source. Okay, the
current isnīt really *constant*, but as the voltage decreases,
so does the resistance and the current stays with a *certain*
range. It seems to me, that if I could "jab" the energy, stored
in the cap, into the primary "at once" (not a little before, during
and after), it would result in a much stronger magnetic field
and a better spark output. So, it would seem that one needs
to build a gap which fires EXACTLY at point X to minimize
resistance losses. The resistance of a static gap is probably
pure chaos, but a rotary is "straight" resistance vs. time
device. Could this (1/4" before alignment spark start) also be
a reason why you need to "sync" your gap way after the
highest point on the sine (mains) wave?

>The best way I have found to study these effects is to use a primary
>system WITHOUT the secondary system.  A scope can be used to measure
>the ring down times of different gap systems.  The longer the ringdown
>the lower the resistance and losses.  Also, don't be surprised to see
>significant effects of different interconnect wiring.  The parasitic in
>the primary wiring do all sorts of fun things from causing RFI, heat,
>losses... I think the zero crossing effects also can play many games
>with all this...

Here is where I have a question. If you soley run the primary tank
system (i.e.: w/o the secondary), wonīt this REALLY stress the
cap and primary, as the energy has no where to go? I thought this
was the reason why you included the coupling factor in your first
MMC power handling equations. Also wouldnīt the presence of
a secondary have an effect on the *real* ringdown time, as it
absorbs some of the primary energy? I would think a secondary
-less setup would have a much worse quench than the same
setup with the secondary in place, because the gap has to handle
a much worse condition (more energy).

>I am not sure I said anything of real use here.  However, I hope
>you are able to REALLY understanding gap behavior better than
>I have been able to.

It DID get me thinking (as you see). I still donīt have anything
definately planned. These are just some arm chair analysis
going through my head ;o)) (which is one of the reasons I
havenīt completed my coil setup just yet). The gap just seems
to be *THE* most mysterious part in a coil. I think (for pure
performance) we have the rest of the coil mostly figured out.


Coiler greets from germany,
Reinhard