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[TCML] Re: Variac troubles

Hi Jay,

Try running them off 120 volts to see if they operate correctly.

Although I wasn't able to find the specs for your specific variacs, Staco does show what may be a similar version (5011-3Y). Deciphering the part number suggests that your ganged variacs may be actually be configured to operate off 120 volts. Staco's variacs often have a variety of terminals/taps that allow them to be configured for operation in various single-phase and 3-phase modes and ganged for higher currents.

The chokes are likely "paralleling chokes". These are are actually small transformers that are used to balance slight output voltage differences when multiple variacs are connect4ed together to increase output current and eliminate unwanted circulating currents between paralleled variacs.

Once you remove the covers, you should be able to rewire them to operate of 240 volts by changing jumper settings. The Staco web site has lots of information that you may find useful.

Good luck!


On Sun, Aug 8, 2021, 7:39 AM jhowson4 <jhowson4@xxxxxxxxxxx> wrote:

All, I would like to pic your brains. I acquired a stacco 5010-3Y, its
labeled as a 240V 3 phase unit set at 0-280 out. 50A per core. Only 3
terminals per core. Clearly marked. I'm running it on a 30A breaker at the
moment. With no load you'd think that would be fine. Sounds great, except I
can't get the darn thing to operate. Pops the breaker every time. Makes a
hellacious 60hz hum in the process on all the cores. This is with it
connected to nothing on the output, just applying voltage. Logically I
checked for anomalous resistances, everything showing proper
isolation.Visual inspection doesn't show anything noticable, Brushes and
contact surface looks good. One of the cleaner units i own..  Inrush
current seemed to be the next logical thing to try, got a 35A CW rated
varistor 2 to.02ohms (i only wanted to run these at 30A for the time
being)  which promptly exploded.  A relay shorting a high power 6ohm
resistor was the next step. Powered on at 240v with series resistor for
about 5 seconds then kicked on the relay to remove the resistor, bzzzzzzz
pop goes the breaker. No magic smoke or smells of burnt enamel.
Unfortunatly I havnt been able to observe the actual current its drawing
yet. But I will set up a scope in the near future to record it. I've
brought many variacs back into service, as have many of you. But this is
the only one thats been a real head scratcher. Next step is to pull the top
plate and see if anything is going on in the middle with the connections,
but alas Its not like I'm reading any shorts. There are 3 little inductors
or something that seem to be inside each core. I assumed they were on each
input/output to act as a current limiter of some sort.  But ill have a
better look when I pull the cover and go over a core with a fine toothed
comb.Anyone have any ideas of what I could be looking for or potential
causes. What ever the matter is, it seems to effect all 3 cores. Unclear if
the effect is equal on each core, but the result is the same. -Jay
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