Re: Calling all transformer gurus


"Bert Hickman" <bert-hickman@xxxxxxxxxxx> wrote in message
news:2_idnTkYHemK0QXUnZ2dnUVZ_oninZ2d@xxxxxxxxxxxxxxx
BobW wrote:
I bought a surplus microwave oven transformer with the intention of
turning it into a spot welder. The trick is to replace the secondary with
a couple turns of THICK wire in order to get a couple of volts at a bunch
of amps.

I've removed the high voltage secondary (what a pita!) and I'm seeing
about 5A flow with 115V applied at the primary. This seems very high, to
me. A friend of mine suggested that it might have a shorted winding in
the primary.

It does get fairly warm after about 10 minutes of running.

When I add a secondary winding, it produces about 1V for every wind in
the secondary.

It seems to me that even with a shorted primary winding you'll still see
the primary current at (nearly) 90 degrees out-of-phase with the voltage
when there's no secondary load attached. I haven't looked at the
voltage/current phase, however.

So, the question is, how can I distinguish between a shorted primary
winding and merely a low primary inductance?

Thanks.

Bob

Your transformer is perfectly fine... for a microwave oven transformer.
These are very cheaply made transformers - they use just enough copper and
iron to allow them to do the task, but not enough to allow them to run
efficiently, especially under no load, as are operating with the core in
partial saturation. They are designed to be cheap, not efficient. However,
you could add more turns to the primary or drive it at a lower voltage to
reduce no-load current via a Variac or a buck transformer.

These transformers also use magnetic shunts between the primary and
secondary windings. These may be left in to help limit short circuit
current, or removed for higher output current and to give you a bit larger
winding window.

Bert
--

Thanks, Bert. I've done a little more reading on these transformers and I
think you're exactly right.

I only plan to weld very thin material so I'm going to leave the shunts in
as this will, as you say, provide built-in current limiting without the need
for an external ballast.

Bob
--
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