OT, Thanks
That explains it very wel and had often wondered they solved the problems
involved.
As for the IGBT's, aggreed. I still have a copy somewhere of the GE Thyristor
Manual, which has an example of an ac in, ac out cycloconverter. A lot of thyristors,
each with it's own trigger circuit and commutation. Ground breaking devices in their
day, but later devices are so much more conveneient, if not quite so rugged :-)...