[Prev][Next][Index][Thread]
Unused Winding Autotransformer Warning
A while back, a post warned that a variac, used as a
series current control, could develop an overvoltage
condition (due to the autotransformer effect) on the
"unused windings" and cause a burnout.
I thought about this and performed an experiment.
In this case, we're talking about a large variac used as
a series current control (alone) with one end used as
the input, the wiper used as the output and the unused
end, well, unused.
If the variac was to fry (I've fried several, including 1156's
and 1256's I'm proud to say), it would do so from windings
to case (read: console ground). The autotransformer effect
should only be between the wiper and the unused end (I
guess) not between unused end and ground. So...
I set the multimeter on High Value Hold and read the voltage
between unused end and ground and pulled between 50 and
100 amps in a variety of voltage and "current control" settings.
I did not read any voltage higher than the input voltage. This
suggests to me that the "unused winding autotransformer
effect" of a variac used as a series current control does not
contribute to it's demise in Tesla coil service. Or my
methods may be flawed.
Jeff W. Parisse
Director, kVA Effects
www.teslacoil-dot-com