On 13/02/15 16:59, isw wrote:
I recently came across a YouTube video where a guy rewound a microwave
transformer to make a spot welder.

In talking about it, he stated -- and repeated -- that it was very
important for the secondary winding to have the same "sense" as the
primary -- that is, both windings had to go around the core in the same
direction.

I know that matters if the transformer is handling very asymmetric
waveforms such as in a flyback configuration, but I have never heard
that it matters for plain old 60 Hz. sinusoids.

Does it? Or was the guy just confused?

I think he's confused.

I have a professionally-made spot welder, designed for the days when
orthodontists needed to weld stainless-steel brackets for people's
teeth. It uses a variac and bridge to charge an electrolytic capacitor,
which is dumped via a contacter into the primary of a transformer with a
secondary having a half-dozen fat turns (think, flattened water-pipe).
The secondary has a fat braid running to the contacts.

The contacts are brought together by pressing a foot pedal, and are
spring-loaded with a screw adjuster. When the preset force is reached, a
microswitch closes and activates the contacter.

To use it, adjust the variac to set the pulse energy, and adjust the
activation force, place your work between the contacts and press the
foot pedal. You get repeatable energy and force with both hands free to
position the work.

It is fairly easy to remove an MOT secondary since they tend to use an
EI core that's not interleaved. Grind away the weld line, remove the I
section, pull off the secondary and insert your new one made from a few
turns of fat copper. Weld the I-section back on the transformer. Not
sure if you need to remove the magnetic shunts... anyone know?

The rest is easy, but you'd use an adjustable HV supply not a variac
these days.

Clifford Heath.