All,

Regarding the above when did the first Thyristor drive Zs go
into service? John Dunn in his history of Commonwealth Engineering states
that the SRA were offered Thyristor drives for the 86 class which was early
80s but they did not want it preferring the very old technology camshaft
drive for which the manufacturer had to restart the production line. They
also offered them a German AC drive which was pretty impressive but so was
the price apparently.

The difference between standard SCRs and Gate Turn Off SCRs
is in the name. You can, with certain limitations, turn a standard SCR on
with the gate but it stays on until the line voltage reaches zero at the end
if the half cycle. In real life this happy circumstance does not always
happen without the assistance of additional circuitry.

Thyristors are being displaced in many applications
including variable AC drives by IGBTs which are a rather cunning combination
of two types of transistor technology. One wonders what the next big
development will be.

Geoff O.