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The Pig Uncovered


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From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Sunday, February 15, 1998 11:23 PM
To:  Tesla List
Subject:  Re: The Pig Uncovered

Hi Greg,

> 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.

All excellent questions deserving good answers. First of all, they 
wind these things with a sufficiently large wire diameter to stay 
within a specified temperature limit, even with non-sinusoidal 
current draw at full RMS load. Anyone looked at the current 
waveforms of a microwave oven lately? ;)
     To aid in reducing resistance in the windings, heavy wire with 
as few turns as possible is the goal. But few turns requires a high 
induction factor in the core or the magnetizing current is going to 
be a significant fraction of the full load current - highly 
undesirable as it means the transformer would sit there wasting power 
in copper losses just to magnetize the core. So, a large core volume 
is required and the fewer the turns, the larger the core mass has to 
be. Additionally, the windings have to fit so to maintain core 
volume, core dimensions must be adjusted to allow the copper + 
insulation to fit on the core.
    It is obvious that with no cooling oil, the thing would overheat 
if run to its spec with the copper mass they have put in the 
windings. Oil costs a lot less than copper so a large amount of oil 
to carry the heat away is mandated. This also requires radiating area 
in the surface of the container. 
    Transformer design is an interesting area. You can see the 
various tradeoffs occurring both in the engineering and the cost. 
It appears that cost governs how the final product turns out. I 
design transformers for switchmode power supplies on occasions. The 
challenge is fun and involves producing the smallest transformer for 
required throughput that you can get away with. That includes keeping 
temperature rise to an acceptable level to avoid messing up the 
properties of the core.

Malcolm