[R-390] Variacs and solas: additional thoughts.
Barry Hauser
barry at hausernet.com
Sun May 1 13:27:23 EDT 2005
Hi Roy --
Many thanks for all the detail. Despite Ohms Law and various other well
established principles, there seems to be remaining variance in opinions
about the appropriate, safe and effective use of variacs.
Very often, some list members tire easily of certain threads, even when
substantive. Oddly, it seems the complainants are those who rarely, if
ever, contribute anything. Even among the "regular crew" it has become
customary to trot out the dead horse lamentations.
One of the reasons Seabiscut & friends get dug up and dragged to the paddock
again and again, is that, despite the length of the thread, there is
something short of a final resolution of a question sometime before it
peters out. Archives are difficult to search -- one of the reasons W. Li
created and periodically updates his "Pearls of Wisdom". Further, it's
difficult to come up with totally new material, and, if that were a
requirement (avoidus oderiferous moribund equinus), we might as well shut
down this list because the only new news would be the latest provocation on
the e-place.
Another phenomenon: Even the most experienced vary in opinions on a number
of things. Often, this is due to differences in input or assumptions.
Given a complete and common baseline, there would be a good deal more
agreement, and less ambiguity facing the modern pilgrim.
Anyway, persuant to the foregoing ....
Here's some more <insert groan here> on variacs, AKA autotransformers ...
as followup to the questions Roy posed:
Roy wrote:
> Here are some questions to investigate:
>
> Variacs:
> 1) What particular makes and models of "variacs" are set up for
> overvoltage as made?
I have quite a few, both "plug 'n play" in enclosures, plus raw ones. Most
all of them I've found are set up with the "extra taps" -- at least one, but
usually two -- even if they aren't brought out to a switch arrangement. At
minimum the taps, at about 10 and 2 o'clock, usually at least are brought
out to the terminal board on the variac. I've got Staco's, GenRad Variacs,
Powerstats, etc. All pretty similar. If the enclosed type has a "140" volt
switch position, it's easy to defeat it for safety purposes if you're sure
you'll never want to step up.
> 2) Do small variacs behave differently than bigger ones? (I doubt it.)
Don't know for sure but also doubt it.
> 3) Are fuses or circuit breakers normally in the input as made?
Most I've noticed are on one side of the input. I have some variac's that
are part of multipurpose power supplies and may have additional fusing.
Possibly relevant: Autotransformers are not isolated. They are similar to
a wirewound pot, except the core is an iron doughnut which provides the
electrical characteristics of the creature. (Someone else can explain the
theory.) It is not the same as a true transformer with isolated primary and
secondary windings. Not sure, but I think the AC and/or DC resistance is
pretty low. So, there might not be much additional protection from a fuse
added to the load side. I haven't thought it through, but there might be
some hazard to a load side fuse blowing while a supply side one remained
intact under certain circumstances.
> 4) What errors are found in the dial readings due to line voltages being
> higher than the unit was made for?
I, for one, never pay much attention to the dial readings as they amost all
read low. My AC runs about 126 volts most of the time, except during
Summer, exceeding the nominal 115-120 assumed by those dials. (Older ones
may have assumed 110 VAC). In addition, the rotor may not have been
calibrated right on if it's mounted with a set screw that allows for
rotating it relative to the shaft. The carbon wiper can wear in such a way
to cause changes. Always use an accurate AC voltmeter with them.
> 5) Are the voltmeters found on variacs at all accurate?
Some are, but should be checked against a known good voltmeter. If it
tracks accurately around the critical ranges, then you can rely on the
internal voltmeter. Assume nothing.
I've posted on this before, but some more on safety: If you acquire a used
variac, or even an NOS one, open it up and examine it carefully. If there
is a buildup of carbon/graphite fallout from the wiper, clean it up
thoroughly. (NOS variacs may have been played with/twiddled). The buildup
may be confined to a small portion of the "race" due to tweaking over a
limited range. It doesn't take many bridged windings to start a meltdown.
Use a nylon detailing brush or old toothbrush - -no solvent and no
abrasives. The contacts are made up of the edges of the windings which have
been ground down, flattened and often thinly plated. If the carbon fallout
fills too many gaps it can cause dramatic failure -- with plenty of smoke to
go with it. The wiper normally straddles 2 or 3 windings - to avoid
intermittents when you adjust it. That's OK. However, if the wiper seems
to be crumbling or leaving a heavy track right after you cleaned it up, time
for a new wiper.
Check for prior damage -- prior overheating or burnout. There will ususally
be a blackened stripe around the doughnut. This may be partially hidden by
the terminal board. Not to be confused with the black potting that most
have partway up the doughnut, near the wiper edge.
As I mentioned above, despite all the dead horse thrashings, there is
remaining disagreement on the appropriate use of variacs. I suspect if all
the operating assumptions were pulled together in one place, there wouldn't
be much disagreement at all. Here are some factoids from personal
experience, but mainly gleaned from this list over the years:
1. "Bringing it up slowly on a variac" may often be false security. If a
tube rectifier is involved, B+ won't start up until about 90 volts or
whatever and suddenly. You can temporarily sub out tube rectifiers with
silicon rectifiers to do this. If so, ideally B+ should be monitored.
2. Same -- "bringing up slowly" means different things -- over what time
frame. Some will say very slowly, as in hours or even days. This may be
questionable. Even if you take care of the rectifier situation, the
operation of the filaments possibly heating up things at partial voltage
while the B+ is too low may be a bad thing -- much the same as using the
standby switch on the R-390's. I don't recall exactly, but there were posts
on this -- possible damage to some tubes - -I don't recall.
3. Not particularly effective as a way to reform electrolytics in place,
even if the rectifier aspect is dealt with. Not a good way to detect bad
electrolytics. Whether to reform or replace is arguable, however, reforming
is best done with a separate power supply or capacitor tester. Disconnect
the electrolytics and go about the test procedure with the cap checker --one
that tests caps at operating voltages. However, start the leakage test at a
lower voltage -- e.g. 50, switch to monitor leakage and make sure it's going
down, then step up, allow some time, re-check leakage which should drop down
to negligible/acceptable level if you're going to try to use them. Some
will say this has little merit and it's best to replace them outright, and
that makes sense, but, let's face it, the realities are that many of us
can't resist the impetus to go forward and fire 'em up. Even if the caps
are "acceptable", it's best to reform/precharge them to avoid excessive
stress on transformers after a "long sleep". Further, if the leakage is bad
enough, you will know not to proceed.
But using a variac to somehow avoid transformer failure and cap explosions,
etc. is false security. In that sense, I agree with Les. If that's what
you're going to do, don't get one.
4. You can use a variac to bring up a transformerless AC/DC unit (like a
5-tube table radio or tube Transoceanic) if they have selenium rectifiers,
"as is", I would think -- but be aware that variacs provide no isolation
whatsoever. Use an isolation transformer in combination -- in front of the
variac. There may be some value in a slow start to long-unused/unknown
solid state equipment. With these, the old cap checkers don't have a
voltage setting low enough.
5. Variacs are useful for checking regulation of B+ and short term for
dropping line voltage down to 110-115. Long term, permanent installation
would be better handled with a bucking transformer. (Unless your voltage
varies seasonally.) You can also check if the overvoltage condition you
have seems critical or not by comparing voltages at various test points at
full line voltage (eg. 127) vs dropped to 115/120 to determine if there
really is a need for a bucking transformer -- I suppose.
6. If you feel more comfortable with the idea, by all means, add a load
fuse. I usually plug a good surge suppressor or outlet strip into the
variac, plug the equipment under test into it along with a hookup (line cord
with banana plugs) into a DVM. Of course, the circuit breakers in those
things are typically set to trip at 15 Amps, so probably not that much of a
help, if any. Best to add a fuse at a lower rating if you feel the need.
Again, I'm not so sure it makes a difference with variacs which may be why
they typically have fuses or breakers on the input side only.
7. There is the notion that stepping the voltage up slowly may give one the
opportunity to pull the plug sooner if some crackling, arcing or smoke
occurs partway up the scale. This sounds like it makes some sense and might
help, depending on the exact circumstances and might limit "collateral
damage". However, it presumes that you are watching closely full time and
have the chassis situated so that you can hear and see any developments
immediately and you're quick on the trigger finger. To me, that would
require removal from any cabinet and up-ending the chassis so you can see,
smell, hear as quickly as possible. And then, there's always the
quintessential question ... "Do you feel lucky?".
After reading Roy's info on Sola constant voltage transformers, I have some
questions and concerns about them. I've got a 20 amp unit supplying a
circuit with computer equipment on it. It runs hot and noisy even heavily
loaded. No apparentl problems in 8 years, but I seriously wonder whether it
was worth the bother. However, I'll pursue that off list with Roy.
Anyhow, that's my story and I'm stickin' to it -- until further notice.
Barry
>
> Constant Voltage Transformers:
> 1) Do small ones behave in a way similar to large ones? (I have examples
> from 60 volt-amps up to one Kilowatt.)
> 2) What peak voltages come out of the harmonic neutralized type, and what
> from the non-netutralized ones, as a function of loading.
> 3) Does harmonic content in the output lead to high voltages in rectifier
> power suppllies? Are choke input plate supplies affected in the same way?
> 4) What are the overload characteristics of these things? Are they the
> same for small and bigger units?
> 5) Where above rated output do these things collapse, and do all such
> transformers behave this way?
>
>
> Sooo many projects, sooo little time!
>
> Roy
>
>
>
>
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254, Fax: 301-948-6213
> roy.morgan at nist.gov --
>
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