Hi guys,
Thank you for your interest.

At the moment I am detecting the zero crossing point by feeding the 3 phase into the input through high resistance chains. This works great until one of my welding machines goes 3000 miles away, and is run on a generator. Sometimes we get a meltdown, where the zero crossing is not detected correctly, and the thyristors are fired at the wrong times. This tends to give huge currents, instead of welding at 1500A you get more like 15000A. This is why the paper I attached is of so much interest to me.

Regards
Bob...