Re: Calling all transformer gurus

BobW wrote:


"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

You should probably knock out the magnetic shunts, if you want to make good
welds. It is possible to make sort of welds with relatively low currents
if you don't clamp the electrodes hard, so that the resistance of the joint
is high and the I^2*R loss in the joint is higher. But when the electrodes
are deliberately not clamped hard, the resistance is unpredictable and so
is the weld quality. Also you can't then do a second weld close to the
first one, because the current will preferentially flow through the first
weld. If you want reliable welds you need much higher clamping force and
clean metal that is being joined, and then you need much more current
because of the low (but predictable) resistance, in the range of 5000 Amps
and more. To get this much current you'll have to do all you can to reduce
the leakage inductance, e.g. knocking out the shunts, and also keep the
secondary wiring very short and stout to keep the resistance low. If
limiting the primary current requirement is something that has been
concerning you, I think you probably need a bigger electrical supply with a
higher rated circuit breaker.

Chris


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