It sounds as though you are applying power between one end of the winding and the wiper controlled by the knob.
The 20A Variac (if a genuine General Radio Variac, and not some other maker) is likely to have several taps, and may even have windings good for 240 VAC input.
You connect the input power to a pair of terminals according to the voltage you have to feed it and the markings on the terminal plate. (Typically a 0V at one end of the winding, and a 120V somewhere on the way up from there. There may be a 140V terminal even beyond that, which means that you can get overvoltage if you connect to the 120V terminal and run the wiper past that.
The *output* (load) should be connected between the 0V terminal and the wiper. Never connect the wiper to the input power.
With no load connected to the wiper, it should not pop the breaker at *any* setting of the wiper. I've used these things for many years -- Variacs, Powerstats, and other brands mixed in. *None* of them have ever exhibited the behaviour which you have observed, but then I never connected input power to the wiper either. I strongly believe that you have input power connected to the wiper. And if so, with the wiper set at 30%, and 120V input, you will probably have 360V (or more) on the high end of the coil -- for a moment before the breaker pops.
Well . . . How about it Grant ? You asked a question on RCM. You got some good answers, particularly those alluding to your having the variac connected improperly. When are you going to 'fess up and let us know what your problem was???
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.
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.
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.
It does, but the core and windings aren't designed for that amount of current, and will saturate. When that occurs, your input is looking at the DC resistance of the winding instead of the inductive reactance of the core.
Just for fun, hook the AC main across both sides of the variac, use the neutral as it is, and pull the AC OUT off of the slider, as follows:
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Do you still have the problem when using that configuration?
This part sounds wrong - the hot lead should be the fused lead, and the neutral should be both not fused, and connected directly to the load. Either way the described setup is connected is half-wrong, IME.
ie - hooked up as described, the fuse is in the wrong place, but the connection to the load is right.
A good bit of information, but a couple of things to beware of:
1) Since they are in the UK, the default voltage is 240 VAC, instead of the 120 VAC more common here.
2) Since they are a different manufacturer, the numbering of the terminals may differ from the General Radio Variacs, or the Powerstats (or other trade names) by other makers.
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%.
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.
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.
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.
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.
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.
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.
On mine, the wires go to very clearly marked terminals on the front. It's pretty easy to see how it's wired.
I will. After I'm done using it. It'll be early next week.
It's a Square D QO 20 amp breaker.
It's almost certainly another brand, I'm just using 'variac' like kleenex, to mean a generic autotransformer.
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:
O.K. But where is the "front"? Is it in a rectangular box or something similar?
O.K. You can even feel for damage to the winding by removing the power and rotating the knob -- feeling for any hitch.
O.K. I wonder if the 'Q' stands for "quick-blow"?
[ ... ]
O.K. What was what I was thinking -- and it can be important to list the actual brand to get good information about terminal numbers and such. The major other maker in the US was Superior Electric, which used the name Powerstat IIRC.
If you *know* GR equipment, the knob used on them is a sure giveaway as to the brand. :-_
[ ... ]
However -- the tripping at lower settings, but not at higher is not normal behavior for any autotransformer wired correctly.
I *hope* not. The wall AC should be connected to the ends of the winding, or to one end and an appropriate tap. If it is connected to the wiper, then that is exactly what will cause the problem which you are experiencing.
So -- did you really mean to type that above? If you put a "not" in front of the "be connected" then you are as you should be.
Let me try ASCII graphics and see whether this matches what you have (after moving the fuse, at least) -- View with a fixed pitch font as usual):
fuse (Hot)-----o~o------------3 3 3 3 3 3 LINE 3 Well,
Now *that* is new information. I don't remember reading that before. That would suggest that you don't have the line connected to the wiper, in spite of your sentence somewhere above.
Strange. Is that repeatable -- popping the breaker when it is first turned on -- only if you are at 30% or so?
Perhaps so.
That might do it -- or you might simply want to get a less touchy breaker.
"Quick Open" actually. They are very good breakers, unlikely to be the source of the problem.
I believe most breakers are designed to be fail safe where it they are stressed and going to drift out of calibration they will drift towards a lower trip point. Somewhere I recall reading that breakers are typically rated to be accurate for about three trips before starting to drift.
He needs to put a clamp on amp probe with peak recording on the input and see just what the peak is on startup. Either way, without a load on the output there should be virtually no peak on power up regardless of the variac setting.
On the test bench we normally had a light bulb in series with the variac which would light up and limit current to the unit being tested if it had a short. When we were confident the unit was ok we flipped a switch to bypass the light bulb.
As for the breaker, it's fairly unlikely a QO breaker will be "touchy", but for the $8 or so I'd pick up another and swap it out. If there is no change in the symptoms you just have a spare breaker on hand, always a good thing anyway.
I'll summarize: the variac (with no load) set to 30% output popped the 20A breaker (Square D QO) in the wall panel. When set to 100%, it didn't pop the breaker, and once magnetized, I could vary the dial all the way to zero normally. The variac is in use and appears to control voltage as expected. Many people have told me that the wall voltage must be wired to the slider, but it can't be and isn't. My conclusion is that for whatever random reason this variac has a greater magnetizing current with the slider in a low position than high. Admittedly, I didn't test this properly. To test it properly I'd use several circuits, and do a hundred or so tests repeated widely in time to ensure that the breakers weren't stressed from being recently popped.
The slider is the output and is never connected to the mains.
You have a transformer. The primary is across the mains. The secondary has a common line to the 'return white' main power line and the primary. The slider wire and a white from the return and common side is the load connection - put a light bulb there or whatnot.
Not this way: If power is applied - the black or white from the power panel to the wiper it will burn out windings (or blow circuits.) if the wiper is turned towards zero - as the wiper moves the hot to the neutral and begins a copper short - forget the transformer. Bad way bud.
I've used them for years at least 40 and have a half dozen in the shop. Had two on my train track. Have a large one in a power panel.
I'm sorry, but in *none* of the followups in this thread have I seen anyone saying that the wall power should be connected to the wiper.
I have seen *many* (and I am among those) who said that the wall power must *not* be connected to the wiper. Could you go back through and find the ones which suggest connecting the wall power to the wiper?
There have been some who asked whether the wall power *was* connected to the wiper, as that is the most logical reason for the breaker popping at low settings but not at high settings -- at least before you came back and said that once the power was turned on you could go through the full range with no subsequent popping of the breaker.
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