On Sun, 7 Aug 2005 17:44:36 +0000 (UTC), Christopher Tidy
wrote:

snip
Thanks for the advice. Out of curiousity, why is the primary
traditionally wound closest to the core? This transformer is tempting,
but my concern is that the "official" voltage in the UK is 415 V, so I
will be supplying a somewhat excessive voltage to the motors. What do
you think?

I asume that you are more comfortable with 480 vac than I am. I hate
480.

I'm pretty careful. I only ever once had a shock, and that was off a
capacitor which had held a charge for a while. After that I was very
wary of capacitors.

Chris

It's more than tradition. The winding closest to the core has
the shortest mean turn length so the copper loss of an inner winding
is less than that of an identically wound outer winding.

Best efficiency occurs if the winding handling the highest
power (the primary) is the inner winding.

Because this results in the lowest total copper loss it also
yields the best voltage regulation.

Power transformers are efficient devices and it's usually OK to
use an outer winding as a primary and accept a pretty minor drop in
efficiency.

However it reverses the design corrections for the voltage drop
resulting from winding resistance. This will alter the apparent full
load voltage ratio. For example:-

With 1% copper loss in an inner winding and 1 1/2% copper loss in an
outer winding a 1:1 full load rated transformer would need 2 1/2%
extra turns on the secondary to compensate for the voltage drop.

If this outer winding is now used as a primary the OPEN CIRCUIT
output voltage is now only 97 1/2 %. The full load voltage drop will
reduce this to 95% so that the nominally 1:1 transformer is now only
1:0.95

Perhaps a more useful way of assessing this is to allow for a
voltage error of twice the rated no load to full load voltage drop

Jim