As long as we're keeping track of these incidents for posterity, I'll
add a couple more.
Case 1)
Friend and fellow Seattle coiler Matt Stiger toasted a 14.7kV 25kVA
unit a couple of years ago. Granted, this was done whilst feeding
240V to the 120V windings (which had been placed in parallel within
the can, as would be done for 120V-only service, or else 208Y
three-phase service) in an effort to increase the output voltage,
which appeared to work fine. Pig had operated without incident under
these conditions for some time, but it seemed to me a matter of
when--not if--the pig would be ruined.
Sure enough, the pig did give up the ghost one day while driving
Matt's monster coil. Coil suddenly stopped performing. Further
testing isolated the pig as the cause of the problem. Small arcs
could be drawn off the pig, but running it open-circuit caused it to
arc internally. The windings were later dissected and it was
discovered that multiple layers of paper on the HV side had burned
through near the end of the windings. It was hard to tell exactly
where the first fault had occurred and what the sequence of
destruction might have been.
Fortunately, Matt has other pigs and PTs, so this wasn't a terrible
loss, and it *was* an interesting experiment, if mainly serving to
illustrate what can happen if you overvolts transformers like this!
But ruining pole pigs is no good for many reasons, not the least of
which is that you must then figure out how to "throw it away". I've
dissected and removed the windings from several pole pigs in an
effort to re-purpose the cores, and it is very un-fun.
Case 2)
Nick Berndsen, a former regular on the web site All Things Tesla
(RIP), bought a 10kVA 14.4kV pole pig. Shortly thereafter, while
running his coil, the pig pooped out. IIRC, like Matt's, it would
arc internally when run into an open circuit.
Curiously for Nick, the pig seemed to rise from the grave a year
later; running into an open circuit no longer caused it to arc
internally. Indeed, it appeard to have healed itself! I offered
Nick my only theory, which was that the original fault had mainly
displaced/vaporized oil, making further arcing much easier. After
sitting unused for a year, however, oil seeped back into the void
created by the initial fault and reinsulated things. Any other
ideas?
I guess #2 doesn't really count, since the pig came back to life.
Self-ressurrection is definitely a unique property of a pole pig, if
indeed that is what happened here :-)
Cheers, Aaron, N7OE
--- On Tue, 9/9/08, David Rieben <drieben@xxxxxxxxxxx> wrote:
From: David Rieben <drieben@xxxxxxxxxxx> Subject: Re: [TCML] pole
pig protection To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Date: Tuesday, September 9, 2008, 7:34 PM Hi,
Yep, I concur here as well. As far as power transformers go, they
don't come much tougher and more robust than a pole distribution
transformer. Consider this- the high voltage primary (secondary for
coiling purposes) winding of a standard 14,400 volt pole
transformer is designed to take a 50 micro-second pulsed voltage
peak of 110 kV and survive! That's called the (B)asic (I)mpulse
(L)evel rating. Of course, they do get a little bit of extra help
for lightning impulse protection when they're sitting up on a pole
in the form of distribution arresters. Still, it would take some
real effort to kill a pig in Tesla coil duty and I've only heard of
one confirmed in- cident of someone killing a pig in a Tesla
circuit!
David
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