LTR Pig Project - Long Post!

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Original poster: "sundog by way of Terry Fritz <twftesla-at-uswest-dot-net>" <sundog-at-timeship-dot-net>

Hi All,

 Firstly, Yep, I'm serious.  The LTR pig project is sitting in my
garage.

Now, before you ask, "Why??!?", think about the "why not?", and
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.  Most common comment I hear, "Resonant value isn't
important with a pig-based system."

 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?  Nope.  Especially if it was proven you get longer,
stronger sparks from a 120bps LTR setup on your OBIT.  A pig is the
same thing.  It's just a big rabid NST.  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).

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.  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.    Other than that, the project is a
resounding success!! The setup behaved just like a bit NST system.
Definate potential!!!!


I'm in the process of designing a magnifier to run on this setup.
Here are some of the things I've taken into consideration and why.

 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.
  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.
  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.
  The caps...Given the high number of strings you'll be running in
relation to your input current, RMS current ratings should not be a
huge concern.  Pay very close attention to the voltage ratings.
There's an obscene amount of power in the tank cap bank, and any
small failure will instantly become really nasty.  Remember the
exploded MMC string?

  Ballasting and current draw...
   this could take up a whole thread on it's own.  Right now I'm
using pure inductive ballast.  The pig is throttled back to ~35A
(direct short on the HV side).  I'll be using resistive ballast
also to throttle the HV output to 12kv and help smooth the current
draw.  As it is, the ballast lags the demmand, and the pig is
getting current in huge chunks that the ballast then chokes down.
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 think you need is them seeing an extra 5-6kv spike because
of the inductive kick.  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.   A tappable
inductor is being considered for this thing, and my target for it
is 10 minutes of runtime at 30A.  10 minutes total runtime at that
power level is a lot of time. Of course, not all at once.
   And forget the variac.  By the time you get the voltage high
enough to fire the gap reliably, you'll about be at full power.
Start with the system decoupled a *whole* lot, and use a breakout.
Tune from the outside of the primary inwards for best spark, then
increase coupling.  It's far to easy to set the coupling too close
at half voltage, and then watch the secondary blow windings off
when you crank up the voltage.  At multi-kva levels, the
joules/bang is *very* significant, and because J=½C*V², cranking up
that variac will drive your bangsize through the roof.  Run it at
full power from the get-go, and use coupling to control how much
power the secondary is seeing.  Plan on how long a spark you want,
and when you get it, stop there!  If you designed the system to
handle that big of a spark, be happy! :) If you push it too far, it
will certainly self-destruct, and when a high-joule/bang coil lets
go, it's pretty spectacular! (not to mention expensive!)




  This project is *far* from over. :)  Put on the back burner till
I can properly assemble a coil capable of handling that much power,
yep.  ANd even that's in progress (tracking down materials,
planning, etc.)  I'll be working up more numbers for the LTR setup
to post as time goes on.  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.

                                            Shad