Hi Tim, one thing to keep in mind about variacs is that most domestic
versions are rated for continuous duty. This means that a 30A variac is
rated to control 30A 24/7.
If you want to run them at a higher current than they are rated for, you
can do this, provided that you limit the duty cycle of the variac (the
limiting factor is the amount of heat that builds up). The manufacturers
actually plan for this, and provide a duty cycle chart for the units.
As an example, here is the duty cycle chart for GE Volt-Pac variacs (other
manufacturer use a similar curve):
http://i.imgur.com/2fOYa7N.jpg
As you can see, you can run a variac at double the rated current, provided
that you limit the duration to less than ten minutes. In Tesla coiling,
this usually isn't a problem, because we generally limit the run time to
less than a minute or two (I personally use a 25A, 240V variac at 60A with
no problems whatsoever, and actually built the power supply with this in
mind).
Jon
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