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Re: De-potting and re-potting a pole pig: Insulation concerns.

Original poster: Dave Leddon <leddon1@xxxxxxxx>

At 08:54 AM 3/14/2006, you wrote:

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

I had long ago dreamed of creating my own version of
Dave Leddon's "Visible Pig" (don't know where those
pictures of it are...Dave?), but the 15kVA unit I
dragged from its can ended up being stripped down and
its core donated to ballast science :)  I now have an
old beater of a 15kVA unit (my workhorse) and a newer
25kVA unit.  The 25kVA unit has, I'm sad to say, been
unemployed since I acquired it a year ago, and only
just this last weekend did I finally pop the lid to
see what a modern 25kVA unit looked like inside.  It
was really quite handsome, and got me re-interested in
the "Visible Pig" idea.


Here's a picture http://nick_tesla.home.comcast.net/TheVisiblePig.jpg



However I've managed to get myself a bit paranoid now:
 If I lift the thing out of oil (a big *if*
considering I don't see any viable attach points on
the thing), am I seriously compromising its insulation
by letting the oil drain away from the windings?
I used a cheepo electric hoist purchased from Harbor Freight that has a 600-pound capacity and a nylon motorcycle tie-down strap to lift the transformer from the can. I just hooked the two metal hooks on the strap under the edge of the metal plate attached to the top of the transformer core and hoisted the thing out of the can. The core of a 10kva unit only weighs about 160 pounds so two people could probably accomplish the same thing with a metal bar slipped through the 
strap.


What
about the inevitable air bubbles what will become
trapped under the thing when I lower it into its new
home?  Will they be problematic?  My gut tells me that
a few tiny air bubbles are probably no big deal given
the dimensions of the windings, but I have no
experience to base this on.  I know that air bubbles
are "the end" for oil-filled capacitors...  What do
you all think?  What do the transformer rebuild places
do?  Are the cans momentarily evacuated to get the air
bubbles out, or what?  Hmmm...
On my first attempt to re-case the pig I pumped compressed air into the vent hole in the lid and then released the pressure. Each time I did this a storm of bubbles would stream out of the transformer. About the fourth time I did this one of the sides blew right out of the case, releasing about 2 gallons of oil on to the floor. On my second attempt I decided to forgo any use of positive or negative air pressure and just let the transformer set in the oil for several days before applying full power. I heated the oil during this soaking period to reduce its viscosity by shorting the high-voltage terminals and applying about 200 watts of power to 
the primary.
There's probably still some air in there but I don't think that this is really an issue since the oil-impregnated paper is the insulating medium. I'll bet that Dr. Resonance can offer 
some insight on this topic since he has re-cased dozens of transformers.

Dave
Pleasanton, Ca




Best Regards,
Aaron, N7OE