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R.I.P. one 14kV pole pig!
---------- Forwarded message ----------
Date: Thu, 2 Oct 1997 20:35:24 -0400 (EDT)
From: Charles Brush <cfbrush-at-interport-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: R.I.P. one 14kV pole pig!
Hi everyone,
Well it appears that I have done the unthinkable and blown a pole pig with
my little coil. I posted my coil stats recently in response to Ed
Sonderman's message, but if you missed them, it's a pretty straightforward
6" coil, 28" winding length, #22 wire, 4 gap series rotary, .025uF tank
capacitance, resonant frequency somewhere below 180kHz with the new toroid
(haven't measured it yet). More current stats, pictures, etc. are on a
page I just uploaded to my site as companion to this message
Here's what happened, and it was a combination of things: Last weekend I
was experimenting with a larger toroid stack (6"x24" & 7"x36"). The nice
thing about the new stack is that I get a single steady streamer coming out
of the seam, so I can direct the discharge where I want it. I was also
getting very few strikes to the primary strike rail. Anyway, I was trying
to videotape the tests but was having trouble getting enough clearence in
front of the coil, so I aimed the toroid seam off to the left. The tank
circuit was in this direction. Shouldn't have been a problem, but it
proved to be mistake number one.
Mistake number two: I had been using a 19.9kV 15kVA pig with a safety gap
across the HV bushings. I decided to swop in a 14.4kV 5kVA one, but forgot
to change over the safety gap before testing (aauuughhh!!). This by itself
shouldn't have proved fatal. However when I fired up the coil, I got
several strikes to the HV busses in the tank circuit. I didn't see this
happening immediately. The next thing I knew, the output suddenly dropped
off until the coil stopped runnning. The coil was still drawing lots of
current as indicated my my ammeter and the hum from the welder, but no
output. I shut everything down, and began troubleshooting. I suspected
that the caps had failed, but they turned out to be ok. It didn't take
long before I discovered that the pig was now acting like a dead short. No
mistake. With nothing connected to the secondary bushings, it now draws
full current. Shorted secondary perhaps? I opened the pig up, and did
some tests with the tap switch set to various positions, but no luck. I am
actually relieved that it was the transformer and not the caps, since it
cost less than a quarter of what they did! I can tell you that from now on
I will be certain that the pig's safety gap is connected, and may add some
bypass caps as well. A grounded strike shield over the primary is also
called for if the power levels are going to keep increasing (and they are!
;-).
I've posted some shots of the interior of this pig at the following
address. There are also some recent pictures of the coil, new rotary gap,
etc.
http://www.foundrygroup-dot-com/cbrush/temp
So has this ever happened to anyone else here? Yes this pig should have
had a safety gap and should not have been subjected to such abuse, but
should it have failed? I am wondering if it could have had an internal
weakness, or if the same thing would have happened to any 14.4kV pig in
that situation. I was only getting 6-7 foot streamers, but then again that
kind of voltage is way beyond the transformer's BIL rating. Lastly, I was
using some fairly long RG213 to connect the pig to the tank (grounded to
the RF ground at the tank end). Is the jury still out as to whether this
can be detrimental or not? Any thoughts would be welcome.
Thanks for reading this long message, and for any replies!
Charles Brush