[ARC5] BIG Variacs and popping breakers.

[Kenneth G. Gordon]

On 10 Dec 2016 at 23:08, J Mcvey wrote:

> I have a few  "benchtop" ten amp and 20 amp variacs (in a case with a big 
> knob,etc) . I have never had an unloaded one pop a breaker-not even on a power 
> strip.

OK. Maybe I've just been "lucky".

> Never thought about it much until it was mentioned here, but  wouldn't it just 
> present an inductive load when unloaded?

I dunno. I haven't thought much about it.

> A saturated core WOULD look like a near short ,though. I believe that They gap 
> transformers to avoid this condition. Are you sure the thing doesn't have shorted 
> winding or something?

Well, that is what I first thought. 

I looked very hard to find one and didn't. I suppose it is possible that I missed it. Maybe if 
there is one, it is internal. 

I used this same Variac when I worked at the University of Idaho, it would pop breakers 
there too. Which is probably why it was surplussed. 

Even leaving it on for extended periods...like a week at a time...it never got hot, which in my 
experience is the first indication of a shorted turn.

Maybe this one is missing the gap? 

It is a commercial Variac, mounted in a steel box with a meter and switch and a big black 
knob. There is a short piece of heavy cable leading out of a knockout with a cable clamp to 
an external 4" square steel box with two duplex outlets in it. All the wiring has been checked 
and found to be correct.

Also, it happens no matter where the output is set: whether to zero or 132 VAC out. Loaded 
or not. Same action. Instant trip of the line breaker when it is turned on...most of the time.

I dunno. In any case, my "step-start" fixed the problem....whatever it is/was. Hasn't 
happened since.

Ken W7EKB

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