On 10/13/2012 04:29 AM, 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. .

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.

Hmmm, maybe some type of inductive feedback is causing that? I have
never had any trouble running my variacs at a high current, with no
significant voltage droop. This is with running a resistive or an
inductive (transformer) load, but not with a motor.

You probably already know this, but it should be mentioned that a variac
can be, and is actually designed to, run at a higher current than is
specified by the nameplate, when you run it on a duty cycle (like a
welder). The limit here is the heat generated by the windings because
of the overcurrent, not any saturation in the core.

This duty cycle is normally provided on a chart, but is generally
similar among different models. I'm too lazy to pull the picture from
the .pdf file, but if you look at "Curve 3" under "Overload Capacity" on
page three, you will find it:

http://www.weschler.com/_upload/site...ac_catalog.pdf

It should also be mentioned that the weak link in any variac is the
wiper brush, and any overcurrent capability assumes that the wiper brush
is in satisfactory condition.

Jon