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

["Tesla list" <tesla@xxxxxxxxxx>]

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

There you are, Dave! :-)  Well, that is encouraging.
I have lifted a 15kVA unit from its can before, but
this 25kVA unit is just plain scary.  I'm not worried
about the lifting power of my hoist, but rather how to
attach my hoist to it.  It does have a metal bracket
strapped to the top of the core, but shockingly, it
appears to only be about 1/16" thick!  I forget the
exact figure, but I think the nameplate says the
core+coil assembly weighs 350lbs.  I look at that,
then at the wimpy metal bracket on top, and I think,
"no WAY!"  ...but I could be wrong.  Perhaps I should
take a picture and see what people think.

I too have used nylon load binders to lift pole
transformers.  I need to take another close look, but
perhaps I can feed the nylon straps between the core
and coils near the top of the transformer.  The core
is two giant donuts side by side with a single large
coil assembly the runs through the middle of each
donut.  Kind of the opposite of the configuration of
your transformer.  More like a big E-I core.

Regards,
Aaron, N7OE

--- Tesla list <tesla@xxxxxxxxxx> wrote:

> 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
>