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The Pig Uncovered




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From:  Eric Davidson [SMTP:edavidson-at-icva.gov]
Sent:  Monday, February 16, 1998 9:37 AM
To:  Tesla List
Subject:  Re: The Pig Uncovered

Greg:

I would say that 200-300 lbs is right for a 5 kVA (remember, we pay for
watts but the power company must supply volt-amperes!) transformer. Most
distribution transformers are pretty conservatively rated, (which is one
reason they are so heavy) a 5 kVA unit could easily put out 20 kVA.  Not
only do you have the weight of the core, but also the oil, the steel
tank, the bushings etc. You can't compare the weight of an auto
transformer to a transformer. As you may or may not know, an auto
transformer has one winding (it may be split on the core, but it is
electrically a single winding) that is both primary and secondary. For
example if you had a 400 turn winding and tapped it at 200 turns and
placed 120 volts across the tapped portion of the winding you would
obtain 240 volts across the entire winding. If it helps, a Variac is a
variable step down auto transformer (yes, some 120 volt Variacs can
output 140 volts so they also step up a bit).  Auto transformers are
usually used when low ratios (under 10:1) are required and where a
connection between the primary and secondary is not objectionable. There
are various pros and cons as to which type of unit is used. An auto
transformer will have lower copper and core losses, and a lower weight
for a given rating than a transformer.  However, this advantage
decreases as ratio increases, mostly for insulation reasons.  Here in
the US the most common line voltages are 7,200/12,470 and 13,800 volts
for residential distribution, which requires a higher ratio to obtain
240 volts, but most important is the lack of isolation between primary
and secondary, not desirable in residential service! You see autos used
in 2400v to 4160v systems sometimes. The core in your transformer is
small compared to the tank for insulation and heat dissipation reasons
and I doubt if it is 'potted'. I would leave it in the tank (and be sure
the tank is solidly grounded!). I built a wooden cart for mine and it
works fine. Not to belabor the point, but a 5 kVA unit can kill, be very
careful. Hope this helps!

Eric

Tesla List wrote:
> 
> ----------
> From:  Gregory R. Hunter [SMTP:ghunter-at-mail.enterprise-dot-net]
> Sent:  Sunday, February 15, 1998 2:25 PM
> To:  Tesla List
> Subject:  The Pig Uncovered
> 
> List,
> 
> I took the lid off of my pole transformer, just out of curiosity. In
> the UK, pole pigs are upright rectangular boxes, with the HV
> insulators on the top-front and angled up & out at about 45 degrees.
> My 5KVA, 11KV unit is so heavy, I use a 600 lb dolly to move it
> around, and it's still a struggle.  The thing must weigh 300 lbs.
> That's one thing I was curious about--why does a 5 kilowatt
> transformer weigh so much? I've got some notion as to the the
> required cross sectional area for a 5KVA transformer core, and this
> thing is just too heavy for its rating.  And the SIZE of it--it's
> 20"H x 18"W x 11"D. Nutty.  I've got 2 and 3 KVA autotransformers
> around here no bigger than shoeboxes (powering all my Yank
> appliances).   So I unbolted the lid and took a peek.  As I
> suspected, lotsa empty space in there.  The transformer itself
> occupies less than half of the box. It's under a bath of
> crystal-clear oil.  Why such a big box for such a small transformer?
> I think more than half the weight of the thing is box & oil.  I bet
> the transformer itself is no more than 100 lbs.  The core has a big
> hook eye welded on top.  I'm real tempted to hook a chain hoist on
> that dude and repot it.  If I mounted it in a smaller, lighter
> container I could probably carry it around by myself.
> 
> Greg
> 
> East Anglia, UK