DoN. Nichols wrote:
According to Grant Erwin :
I'm puzzled. For sure my wall voltage is connected properly. My
output voltage is showing about 95V (the item is in use at present)
with the voltage set a tad above 80%.
I'm not seeing how hooking up the wall voltage to the slider doesn't
get me a stepUP transformer.
It *does* -- within limits. The problem comes when you have too
few turns and the magnetic structure saturates, thus drawing lots of
current. Thus popping the breaker at 30%, but continuing to operate
(perhaps drawing a bit of excess current) at 80%.
I physically looked at the connections too. Unless the external terminals
don't match the wiring on the nameplate, the wall voltage (117VAC) is
connected across the input side. One side is directly connected (that's
the hot lead) and the other side (the white lead) is connected through a
fuse to the other input terminal. That terminal is also connected to the
output, as is the slider.
Now, *this* is wrong, at the least. You have your fuse in
series with the neutral rather than the hot, so if the fuse pops, the
whole circuit is floating at full line voltage, including both sides of
the load. The fuse should be in series with the black lead, and yes,
the white should continue on to the load as the neutral for the load
too.
Yup. The fuse should be on the black wire.
I would advise opening the housing and making sure that it is
correct, too. I know that I got (for free) an enclosed Powerstat which
was terribly mis-wired. It took a while to get it right, because of
a too-crowded wiring area.
On mine, the wires go to very clearly marked terminals on the front. It's
pretty easy to see how it's wired.
Now -- it *might* be possible that a damaged slider is bridging
multiple turns (more than two), but it still should be as likely to pop
the breaker at almost any setting of the knob -- unless the winding is
damaged by serious overcurrent at some point. I would strongly advise
you to open the case and see just how it is wired inside.
I will. After I'm done using it. It'll be early next week.
I think my problem is with my breaker tripping too low or too quickly.
I'm contemplating adding one of those temp-varistors to the input,
the kind which have resistance about 1 ohm when cold, but .01 ohms
when warm. Those are supposed to work OK in damping variac magnetizing
inrush current.
What kind of breaker is it? Normally, most home power
distribution panels have a combination of a thermal breaker, which will
trip within a few minutes at a certain level of overcurrent, and a
magnetic trip breaker which will trip almost instantly with severe
overcurrent.
It's a Square D QO 20 amp breaker.
Or is this breaker in the autotransformer's housing along with
the fuse? BTW -- is this a genuine General Radio Variac, or is it some
other brand? This can determine how the terminals are marked. I'll
just call it an autotransformer from here on until you assure me that
the brand is General Radio -- the only ones who made the Variac.
It's almost certainly another brand, I'm just using 'variac' like kleenex,
to mean a generic autotransformer.
For that matter -- a 20A autotransformer is fairly large,
whether 120 VAC or 240 VAC, and those that I have seen have had an
enclosure (cage) with the terminal board of the actual autotransformer
exposed through a gap in the cage. If that is the case, then the
terminals which you see on the outside are all that you will have.
Typically, the wiper will be the middle of the numbered terminals -- or
on at least some GR Variacs, there is an illustration of the winding,
with the voltages marked for each, and the wiper as an arrow pointing to
the inside of the curve of the winding.
An exception to the above would be in the case of a
center-tapped winding, with 120V being the center tap, and 240V being
full CW, so you can use it as either a step-up or 0-full line voltage
autotransformer.
Hmm ... a possible source for the miswiring. Typically, these
are supplied so they can either mount on a panel, with the shaft coming
through the base of the autotransformer, through the panel, and then to
the knob, with the scale screwed to the panel. However, when used as a
bench autotransformer, the shaft stops at the base, and extends up
through the disc which carries the wiper, through the cage, to the knob,
with the scale screwed to the cage. In this case, the sense of fully
CCW and fully CW interchange. So the fuse, if permanently mounted and
connected to one end of the winding would be at the CW end of the
winding (full voltage) in one condition, and at the CCW end of the
winding (zero voltage) in the other condition.
Don, I haven't yet heard anyone say anything (besides the fuse being on
the wrong power line) that would indicate to me there is any miswiring.
People keep telling me the wall AC must be connected to the wiper. Well,
remember when I plug this in with it set to 100% and the breaker doesn't
pop, then I can turn it all the way down to zero and it still doesn't
pop. And if the wall power were connected that would surely pop the breaker.
So wall power isn't connected to the slider. Furthermore, when I turn the
dial to turn down the voltage, the voltage goes down. It all works just
like it should, in fact. Nothing is getting warm, everything's fine. Except
it has a heck of a magnetizing current. As I google around, I'm not the only
one who's had this problem. It appears to be worse when the slider is in a
low position, but maybe that was poor test technique.
I wish I knew more about Square D QO panel breakers. I think I read somewhere
that if one pops, it will have a tendency to pop again for some time, meaning
that once it pops everything I do after that may have been irrelevant.
A guy at my partner Karen's work place recommended I put a thermistor in
series with the input power, perhaps one like this:
http://www.ametherm.com/Data%20Sheets/MS32%205R020.pdf
Grant