Reply to Peter Chadwick:
Some of them use a motorized stack of variacs, 9 total, ganged with a
driveshaft and chain drive. Three are in parallel per phase of 480
VAC, to handle the 75 amps per phase. The thing goes up in 90 seconds
or so, then there is a 5 min. timer before HV can be energized.
Others use smaller motorized variac stacks. My best filament system
(I am proud of it) is using saturable reactor with a ramped DC supply
on the control winding. It is good for a 250 kW tube filament,
roughly 5 KVA at single phase 480 VAC 60 Hz.
J
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