Interesting discussion about 'Variacs', one brand known as Powerstats.
I wondered about the shorted turn where the wiper contacted, the carbon
theory makes most sense. In my workplace, the last of the old high power
amplifiers has a 30 kW filament power supply. It uses 12 pulse DC
(delta-Wye transformers with full wave rectification) to get 5.2 volts
at 6800 Amperes DC to light up the filament on the RCA/Burle/Photonis
7835 triode. As with all very large power tubes, there is a prescribed
ramp up/down time that the voltage must be applied. The power supply
uses the biggest stack of Powerstats that I have seen, running off 480
Volt 3 phase power. There are 9 units stacked on a common shaft, driven
by chain drive from a motor. Three are paralleled for each phase. The
primary current is something like 75 Amps per phase. As part of the
annual maintenance, the brushes have to be checked. If they wear too
wide, then they short more than 2 turns, and rapidly will burn up the
wire. If they are too thin, then they will break. So they are beveled
and have a small flat on the contact end, and the winding must be
cleaned and polished carefully (no steel wool!).
The replacement amplifiers use 950 Amp filament at 18.5 VDC, so it is
convenient to use a large switchmode power supply (Sorensen/Ametek) to
provide the DC with no moving parts (for ramping up).
73
John
K5PRO
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