Re: Calling all transformer gurus
- From: "Paul Hovnanian P.E." <paul@xxxxxxxxxxxxx>
- Date: Sun, 15 Feb 2009 20:47:01 -0800
Archimedes' Lever wrote:
On Sun, 15 Feb 2009 18:36:17 -0800, "Paul Hovnanian P.E."
<paul@xxxxxxxxxxxxx> wrote:
Bert Hickman wrote:
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.
Adding some turns might provide an added benefit: You can bring out some
taps and fine tune the output power by selecting the appropriate one.
Take apart entire transformer. Examine core construction. If core has
shorting welds on it, forget it.
Better off going to an industrial liquidator or such and buying an
actual AC transformer, and winding your welding winding onto that
transformer.They are meant to convert AC line voltage and frequency to
other AC voltages with maximum efficiency. A microwave oven transformer
is not made for any such purpose.
But they're cheap.
--
Paul Hovnanian mailto:Paul@xxxxxxxxxxxxx
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Matter cannot be created or destroyed, nor can it be returned without a
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