With all the questions about transformers on here, I thought I'd add this and
would be interested in comments.
About 10 years ago, a friend was building a twin 4CX1000A amplifier. A company
(which I believe was involved in printing) near where I worked went bust. As
their offices were being cleared, I found in the skip a large piece of equipment
with a transformer marked 'Prime-Arc 2600 V 3.15 A'. This looked an ideal
transformer for the amplifier, so with the help of someone else, I removed the
transformer and got it home. I can't recall the exact size, but it was probably
about 10" x 10" x 10".
Power was applied to the 240V primary and the secondary voltage was about 2.8 kV
or so (can't recall exactly), but it seemed reasonable for a 2.6 kV
transformer.
A box was welded to house the power supply and all seemed okay. There was
something over 3 kV off load (again I can't recall the exact values). All seemed
okay, until we tried to use it. At that point we discovered that the short
circuit current of this transformer was about 20 mA, despite the fact the plate
on it said it was a 2.6 kV 3.15 A transformer. It seemed quite happy to run into
a short, without getting hot or blowing our fuses.
One might have concluded the transformer had a fault. However, when we tried to
get a transformer wound, with a similar voltage at 2.6 A (less than the 3.15 A
of this transformer), it became apparent that nobody could make a transformer of
that rating that would fit in the box we had made. Hence a new power supply box
was welded up, to take the larger transformer we had wound.
So why did a physically large transformer, with a rating plate of 2.6 kV at 3.15
A, have a short circuit current of about 20 mA ? It seems unlikely it was
faulty, since a transformer of lower spec could not be made quite as small. The
fact the equipment I found this in was disposed of when a company went bust,
rather than thrown out since it was not working, again makes me think the
transformer had not developed a fault.
The equipment could well have had some form of arc lamp in it, so perhaps that
is where the name 'Prime-Arc' came from.
The transformer has long since been thrown away, so I can't measure resistances
of it. However, it was a very expensive mistake in terms of time welding up
power supply boxes and mounting all the components, only to discover the
transformer was useless.
--
Dr. David Kirkby PhD,
email: drkirkby@ntlworld.com
web page: http://www.david-kirkby.co.uk
Amateur radio callsign: G8WRB
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