On 2/12/2015 10:30 PM, Phil Allison wrote:
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?


** A spot welding transformer is almost the direct opposite of a microwave oven tranny - big step-down instead of step-up and the lowest possible leakage reactance instead of heaps.

To help with the latter, it could easily be important to wind with the same sense. Also winding multiple, identical secondaries and connecting them in parallel is the way to go plus using other techniques like interleaving and use of strip conductors for the secondary.

Getting 3V at 200Amps is non trivial.


... Phil

Problems I discovered with these welders a
600W ain't nearly enough power to weld anything substantial.

Very thin stuff like battery tabs require very accurate energy
delivery. The difference between no weld and blasting a hole
thru everything is a small pressure difference holding the weldment
together.

If you don't turn it on/off at zero crossings, the saturation
state of the core can make a great difference in the next weld.

I gave up trying to weld with a MOT.

CD welders deliver a known energy to the weld and are very much
less dependent on contact resistance.