[ARC5] BIG Variacs and popping breakers.

[Kenneth G. Gordon]

On 10 Dec 2016 at 15:12, George Babits wrote:

> Hi Ken,
> 
> I have a pair of 20 amp 120 volt variacs.  These were 2/3rds of a 3 phase 
> variac for ??

George: There were literally hundreds of those used in mines in Idaho. I'll bet you have part 
of one of those. I worked at the University of Idaho for over 30 years. One of our 
departments was for Mining. We had several of those 3 phase jobs, and found others at old 
mines in the state. Most of those had a big "wheel" like knob on one end and a long shaft 
connecting the three variacs together. They generally weighed at least 100 lbs per section.

Yours is actually pretty small in comparison with those.

Most mines were a long ways from the main line, and often had very substandard wiring to 
them. These large variacs were used to control the mining equipment.

>   Anyway, my plan was to gang them back together (I have 
> everything but the spacers) and run them on 240 volts since my whole shack 
> is set up to run on 240.  Did I understand you correctly that these will 
> probably pop the 20 amp circuit breaker when I turn them on?

Well, it depends on just how "good" they are. The problem with really good ones is that 
there is no back-emf from the core without a load. Therefore, they show a dead-short to the 
power line until they get "saturated" or whatever it is that is called when the magnetic field 
builds up. 

My large 20 amp Variac would always pop a breaker whenever it was turned on unless I got 
lucky and hit a "crossing" when the AC sine wave crossed the zero line. When I first used it, 
once I got it turned on, I wouldn't turn it off, but would just turn it down. I didn't like that from 
the safety standpoint.

That is why I installed a sort of "step-start" in it. I have not had any problems with it since. 

I use that one on my bench because it is rated for 20 amps. I have a bunch of the smaller 
ones rated at 3 amps, and I have one tiny one rated for 1 amp or less.

My "step-start" is simply a big honking resistor in series with the hot side of the AC input 
line, and a big time-delay relay which shorts the resistor out after a few seconds. Turning 
the Variac off then resets the TD relay.

>  Why would that 
> happen if there is no load on them?

Until the magnetic field builds up, the coil is essentially a short circuit.

>  I usually switch on and then turn on 
> whatever radios I want to use.  I guess if I did a step start I would need 
> one for each side?

I don't know, but I think one on either side would do it. After all, they are connected in series 
across the 220 V line.

> Running everything on 240 makes it pretty hard to set up a bucking 
> transformer.  I would guess it takes two identical ones; one on each side of 
> neutral.

Yes. I would think so.

Ken W7EKB

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