That one would be for a constant 120V line (lights) (motor...)
and the variable zero to 240 is the output for a 240 load.

On this one but mostly others one end is common and the 240 is max.
zero to max. if you hook the supply to the tap - then the
transformer works in 'auto' mode and generates more. Normally
it is 110 to 125 or the like and is used for that voltage only.

Martin

On 10/14/2012 7:03 AM, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2012-10-13, Jim Wilkins wrote:
...
It's a 240/280V 10A Variac. I used it with 120V on the center tap
to
check a motor I wanted to turn into a three-phase converter,
because I
don't have 240V on the bench........I checked
voltage vs current with resistive loads and found that the voltage
drooped badly above 6A.

And at 6 A out of the 240 V tap, you would be pulling 12 A into to
the 120 V input tap tap -- well above the nominal current, and
probably
saturating the transformer core.

If you had fused both input and output at the nominal 10 A, you
would have blown the input fuse, which would have given you a clue
what
your problem was. (Or an ammeter on both input and output as well
as a
voltmeter on the output.

Enjoy,
DoN.

I'm fairly sure of what happened but didn't scope the waveform for
clipping to prove it, so I just reported the observation that a
center-tapped 240V Variac won't deliver the nameplate current as a
voltage doubler.
jsw