I have used both rheostats and autotransformers for filament and HV
variation control - always degrading regulation in the process
(unless it is the control element for a closed loop regulator!). All
of these will decrease the voltage regulation, making it more sloppy,
if load current varies. The rheostat will probably be worse,while the
series resistance of the autotransformer should be lower, and the IR
drop less. By the way, Variac and Powerstat are particular trade
names for variable autotransfomers. Not sure which one works best,
but Warner Electric still carries an extensive Powerstat line, so I
tend to use a lot of them. Biggest one I work with is a 9 stacker,
each pancake is the size of an American stop sign, octagonal, and
three are paralleled for current in each phase of a 3 phase supply.
Its chain driven from a motor with about a 6-8 minute time for zero
to maximum setting. It is used to ramp up the filaments on the 7835
superpower triodes. (We also have a more modern SCR type supply which
is so much nicer).
In recent work (last summer) I used a saturable reactor to vary a 480
VAC line from 10% to 90% driving the 5 kW filament transformers for
250 kW tetrodes - again for the slow ramp up and down. While the
saturable reactor goes back to 1915 (Alexanderson at GE used them in
his 100 KHz alternators used for early wireless, with a microphone),
it certainly is a beautiful device to control high power with,
absolutely no moving or wiping parts. And it fails safe, cannot fail
shorted like an SCR/triac. I sure love retro-technology like that. No
silicon chips, no worry about transients.
I have used rheostats in some of the commerical FM broadcast rigs, as
they were inexpensive (competitive pressure) and they do work well if
they are run within ratings. I used them on the filament primary for
4CX3500,7500 tetrodes. With a small value of 100 Watt rheostat, I
could get the variation needed to cut back the filaments up to 5% in
voltage, to practice life extention techniques. But never used them
for day to day adjustment from zero to max. A variable
autotransformer is much better, as far as ruggedness and mechanical
integrity for that. A filament is a constant load resistance,
virtually, so that the loss of regulation of a series resistance
(rheostat) is not a problem as long as you account for it in your
transformer output voltage. Variac is not needed here unless you
really want to go from low to high voltage with the twist of a knob.
However, it is also prone to premature failure if the Variac (generic
use of the name now) brushes stay parked in one setting all the time.
Contact resistance seems to build, and eventually sparking and
meltdown canb occur, at the one or two windings being bridged by the
graphite brush. We try and move the brushes occasionally (more than
once a year) to clean off whatever it is that bothers them. I spoke
with an engineer at Superior/Warner Electric about that, and he
agreed that is a concern, especially when running near the rating,
say at 18 Amperes on a 25 Ampere model.
Remember that in an autotransformer, only the portion of the winding
from the AC input hot terminal to the load has to withstand the full
load current, and the rest of the winding carries the magnetizing
current of the transformer. Hence, for a given KVa rating, an
autotransformer is much lighter than a comparable tapped 2 winding
transformer.
In a rheostat, part of the winding does not heat, while the rest
does. Calculate the loss for the wiper at both extremes, to make sure
you don't exceed the manufacturers ratings, and stay about 1/2 of
rated power, when mounted in a close spaced enclosure.
Are rheostats recommended for AC line control to HV power supplies
too? Only for the type feeding a constant load, not something like
SSB amplifiers. Otherwise, regulation will degrade. Here a Powerstat
or Variac is recommended, if you have to use variable HV for some
reason. Or an SCR, GTO, or IGBT controlled regulator/controller.
73
K5PRO
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