Thread: BIG variacs
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Pete C. Pete C. is offline
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Default BIG variacs


Karl Townsend wrote:

Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

Karl


The neutral needs to connect to neutral I believe, otherwise you have a
variable inductor of sorts inline, rather than an autoformer. With the
pair connected and balanced it might work without an actual neutral
connection, but I think it's better to have the neutral. In their
original three phase configuration the neutrals would be connected to
the neutral of the 208/120 wye service.

From DoN. Nichols's post in the previous thread:

With a couple of caveats for US power systems.

1) Wire both Powerstats (these are that brand, not Variac, which
was General Radio's brand) with the CCW end of the winding
connected to neutral, and the CW ends of each to the two hot
lines

2) Wire the load to the two wipers.

With this wiring, your load will remain centered around ground,
and both sides will increase together.

Oh yes -- also be sure to turn both to fully CCW before you
tighten the setscrews for the common shaft to the knob.

Hmm ... another thing to watch out for. At least the General
Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps,
given when they were designed) and there was no attempt to insulate the
center of the rotor plate from the shaft, so you would have a short
between the two output sides. I forget whether Superior Electric (the
maker of the Powerstat) did the same or not. Check it out for
insulation if you are going to use a bare metal shaft. Perhaps a Delrin
shaft would be a better bet.

With other wirings, you would likely burn out one of the
variable autotransformers (Powerstats, Variacs) and/or have more
hazardous voltages.

Also, fuse both hots -- ideally with a shared circuit breaker.

Good Luck,
DoN.