On Thursday, February 12, 2015 at 9:59:58 PM UTC-8, 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 think he was confused. Look at a transformer-style soldering gun,
the hairpin secondary doesn't have ANY helical orientation at all,
and it transforms down to high current at low volts just fine.
Ditto for feedthrough AC current meter transformers.

Output windings on such high-ratio transformers don't couple capacitively
to the primary (the low resistance and inductance completely swamp
tiny capacitive currents), and it's hard to imagine any importance of flux
coupling defects at low (50 to 60 Hz) frequency with soft-iron cores.