[TCML] Pigs killed in the line of [Tesla coil] duty (was: pole pig protection)

["J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>]

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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