Re: LTR Pig Project - Long Post!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 2/16/01 10:20:28 AM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

>  let's put it in perspective.
>  > 
>  > The general consensus I've recieved on polepig powered coils is to
>  > run a high breakrate, small capacitor, and crank the power to it.
>  
>  > The problems I see in that...the high RMS current is murder on the
>  > caps.  The high power levels promote power-arcing on the RSG due to
>  > poor quenching. 

Shad, all

I don't know if something got snipped out above.  Are you saying
that a small cap with high bps has high RMS current?  I don't think
it's really any different overall.  In one case (high bps) the peak
current is lower, but there are more breaks, so the overall power
is the same.  If high bps is less efficient, you'll need more power
to get the same spark length.... so this would be inefficient, yes.

Again, the large bang size with low bps, is more likely to create 
gap quench problems I would think.  Although they can be dealt
with and are not a real problem.  

Regarding any types of currents or voltages being murder on the
caps.... I say just build the caps (or buy them) to withstand the
voltages and currents involved.

>  > Most common comment I hear, "Resonant value isn't
>  > important with a pig-based system."

It's not as important as with an NST, because you can adjust
the ballast to the cap when using a pig.

>  > 
>  >  Now, let's say everybody "normally" ran on 10kv20mA OBITs.  A
>  > 15/60 would be the equivelant of a pole pig!  Literally GOBS of
>  > power!!!  Would you run a tiny cap, high breakrate and simply watch
>  > your RSG fry?  

I don't see how a high breakrate would fry the RSG.  You'd simply
have many more bangs per second, but each one would be weaker,
so it's all the same in the end.  (not really the same though because
high bps tends to be less efficient.... so from that point of view, I 
agree.)  Since it's not efficient, I wouldn't use the small cap, high
bps approach... although many folks do.

>  > Nope.  Especially if it was proven you get longer,
>  > stronger sparks from a 120bps LTR setup on your OBIT.  

Yes, you'll get longer stronger sparks using 120 bps.  Part of the
reason is because 120 bps is more efficient, and part of the reason
is because 120 bps with a big cap will draw more current though
the OBIT.

>  > A pig is the
>  > same thing.  It's just a big rabid NST.  

The difference with a pig is you can adjust the ballast, and therefore
accomodate a range of cap sizes.  With an NST there is a smaller
range of acceptable cap sizes.

>  >  Yep, the cap size is
>  > massive, and expensive.  I wouldn't recommend this project to an
>  > amature, or the average joe coiler.  The pig's output is the same
>  > (lethal), but the cap bank is literally 2 to 3 times as large.(very
>  > lethal).

I would not use a cap more than 25% over resonant sized for a pig.
(or perhaps it's a resonant frequency that is 25% lower than resonant.
I can't see a need to make the cap 2 or 3 times resonant size,
because this will completely ruin the power factor, and you'll have
to use power factor correction caps.  Also, LTR will lower the peak
voltage.  Since a pig can handle high peak voltages, you want to
take advantage of that.  High tank voltages are more efficient and
give lower gap losses, and better quenching.
>  > 
>  > Something to keep in mind here...this tank circuit is capable of
>  > *easily* powering 10-12' arcs.  Think big.  Very big.
>  > 
>  > I test-fired my LTR pig setup this evening, on an already-damaged
>  > 8" coil.  Output was dismal, only ~3' tops, with massive flashovers
>  > and the windings being blown off of the coil.
>  
>  Flashovers.., seem a problem of higher power. I've not been able
>  to raise the coupling factor of B&W above 15% --> 18% without
>  introducing a polycarbonate sheet between sec and prim (distance
>  of the windings in air was 2"). Without just a critical value of
>  voltage and power, removing the breakout point on the toroid,
>  provoced power arcs between prim and sec, even 14" around the
>  polycarbonate shield, with ability to damage the secondary.

A 120 bps secondary has to be larger than a high bps secondary
to handle the big bang size.
>  
>  >  I knew it would
>  > fail, and I expected it to fail.  It was the tank circuit I wanted
>  > to test.  Though I wish it'd put up a better fight.  :\  Casualties
>  > included my primary tap (de-soldered itself from the lead, kept
>  > spot-welding itself to the primary.)  Also lost was an MMC string.
>  > I have yet to figure the RMS current the caps were seeing, but it
>  > should be nowhere near their 3A rating I use.  No, this was
>  > overvoltage.  While on a NST system, you can get away with an EMMC
>  > or even an EEMMC, because the current is low.  On my polepig
>  > system, my 12kv designed cap (16.7kv rating) is overvolted, but
>  > acceptably by NST standards.  But at 8kva, when one of the caps
>  > blasted through, the high current took it's toll.  No self-healing.
>  > Every single cap in the string literally exploded.  Ouch.  If using
>  > an MMC on a pig at high power levels, *DO NOT* skimp on it.  11
>  > caps popped like firecrackers.  Subsequent runs will have the pig
>  > throttled down to 12kv out.    

Depending on your ballast setting, you may have been running
at resonance (despite any theoretical LTR cap sizing), and gotten
a terrific resonant voltage rise.  The only way to know the cap
voltage is to measure it with a high voltage probe or some other way.

> > Other than that, the project is a
>  > resounding success!! The setup behaved just like a bit NST system.
>  > Definate potential!!!!
>  
>  My cap was about 40...50 J Bangsize. 
>  (109nF/300kV; 16kVrms 4//PT's -at- ~11kVA input power)
>  >  The gap.  It's done.  Tungsten-carbide stationary and flying
>  > electrodes.  The huge cap bank and low breakrate means the gap will
>  > see tons of current when it fires.  But being LTR, after the
>  > initial fire/quench, the tranny will be doing all it can to charge
>  > the cap for the next bang.  Powerarcing is eliminated, as is
>  > multiple-firings.  My 1/8" tungsten rods used in the gap have
>  > almost no wear on them from the useage they've seen so far.

Well that sounds good.
>  
>  My SRSG's 10mm-electrodes were just handwarm. No serial
>  static-gap this time. 200 BPS.
>  
>  >   The primary.  At least 1/4" tubing.  My primary tap is a 1/4"
>  > fuse holder.  It keeps desoldering itself from the lead, and
>  > spot-welding itself to the primary every time I fire the coil off.
>  > Problematic to say the least.  Plan on lots of turns.  Lots and
>  > lots of turns.  Primary inductance is the only thing keeping your
>  > gap current down, and high gap current can and will eat even
>  > tungsten-carbide like candy and destroy your caps from too much
>  > pounding.  Be nice to your caps, run high inductance primaries.

Yes.
>  
>  >   The secondary...keep in mind the huge bang size.  capable of
>  > making super-high voltage spikes.  Figure your target voltage,
>  > figure the toroid, then figure how many turns you need to keep your
>  > turn/turn voltage sane.  Use a good gauge wire to handle the ground
>  > currents.  (they will be heavy) Don't be afraid of very high
>  > secondary inductance, you'll need it to raise the primary
>  > inductance, hence keeping your gap current under control.

Yes.
>  
>  With B&W I didn't like the resonant frequency going too low, this
>  giving manageable sizes of the primary, relative to my available
>  caps. John's 1600 turns could not be maintained that way: I have
>  821 space wound turns only. I don't know, if this was a mistake.
>  However the toroid is quite big: 63"/16".

The primary wire can be made thinner when a lot of turns are used
because the current is lower.  This will give lower gap losses,
better quenching, etc, and will permit 1600 turns to be used on
the secondary.
>  
>  Current becomes more important in a big system. My 109nF caps are
>  300kV but only 25A rms. Stressed 50A rms! (however pulse current
>  of these Maxwells is up to 25kA).
>  > 
>  > I do not know if inductive kick is going on (probably is), but I
>  > want to limit the inductive kick into the pig.  It is *not* a good
>  > thing.  The caps already see a high enough voltage from the pig,
>  > the last thing you need is them seeing an extra 5-6kv spike because
>  > of the inductive kick.

If you really have the ballast adjusted for LTR operation, then I wonder
how the power factor is?
>  
>  As long as the caps are not stressed too much, I would see it as
>  a good thing: Voltage must be high for low spark-gap loss!

Yes,
>  
>  >  Hopefully rectifying the pig's output to DC
>  > will help with the charging current situation.  I'll be buying PFC
>  > caps for the pig as my budget allows.  They are beneficial to a
>  > NST, and the pig is behaving exactly like an NST.  

Yes, if you run true LTR, PFC caps are a benefit.  If you run just
slightly LTR as I do in my PT systems, you won't need PFC.

>  >  I invite comments, questions, etc. I'll
>  > be posting photos and scans of the setup as I get time.  The
>  > projected magnifier uses a 55gal HDPE drum for the driver, and the
>  > tertiary is as of yet undecided.  My target sparks are 12', and
>  > more is better.

Well those are just some comments that came to mind.  I'll be
looking forward to the results.  I may have repeated some things
I said in another post today.
John Freau
>  > 
>  >                                             Shad