On 2012-10-13, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...

[ ... ]

But essentially, you divide the nominal current by the ratio
between maximum and input tap to get the safe output current.

Essentially, make sure that neither the input current or the
output current exceeds the nominal 10A -- check *both* to be sure.
(This is why a fuse on each. :-)

[ ... ]

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. .

O.K. So you were using it as a voltage doubler. This means
that the input current would also be doubled in the other direction
relative to the load current.

The 240V output wouldn't keep the 240V motor spinning. 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.

--
Remove oil spill source from e-mail
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---