[AMPS] rheostat/autotransformers

[John Lyles]

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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