On 2012-10-10, Pete C. wrote:

Ecnerwal wrote:

In article ,
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?

[ ... ]

If you want to make variable US-Spec 220V power with both sides varying
the same amount, you want 120-0-120 just like the service, which means
the "centers" (0 side of the winding) is tied to neutral, which is tied
to ground at the service entrance.

If you just need variable 220V power and the item is not using neutral,
and is properly insulated to use regular 220VAC, you can just use a
single variac and "0" will be both sides 120VAC away from ground
together, no volts between - just don't confuse "0" with "off" and get
bit.

I don't believe you can use a 120V rated lighting Variac on 240V singly.

You can't -- but there are 240 V rated ones with a center tap so
you can feed them from 120 V and still get the whole range.

But hook 240 V across a 120 V rated one (or even one of the
over-range ones which can go to 140 V) and you will probably fry the
fuse in series with the line input (if you put a proper rating on the
fuse, of course. :-)

BTW I remembered another thing to consider when wiring up these
(both single input voltage and the the double trick to get 240V
coverage from two 120 V ones).

Use *two* fuses per variable autotransformer -- one at the
input, and one at the wiper output. The first protects you from
too much current going in when you have an overrange one set to
produce more output voltage than the input, and the second
protects you when you are putting out less voltage (often much
less) than the input.

The current rating is based on what the wire in the winding will
handle, and depending on the setting, you get more current at
the input or at the output, so protect both.

Remember these are sections from a three phase lighting control, they do
0-120V, no over range like some test bench variacs and they were used
with three sections, one for each 120V to neutral phase of the 208/102
Wye service.

Indeed so. These won't have overvoltage ranges, but still need
fuses at both input and output to be truly safe.

Enjoy,
DoN.

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