On Oct 11, 12:20 am, clare at snyder.on.ca wrote:
On Wed, 10 Oct 2007 08:29:49 -0700, Grant Erwin

wrote:
I have a 20 amp variac. Yesterday I tried plugging it into a 20A circuit with
its output voltage set to 30% of input voltage, and it (consistently) popped
the breaker. When I set it to 100% of output voltage, however, it didn't pop
the breaker. Obviously the magnetizing current is higher with the variac turned
down. The question is why? What's going on here?

I believe the variac and my electrical circuits are both in good working
condition, in other words I don't believe this is faulty behavior.

Grant Erwin
Kirkland, Washington

Will happen if you hook the sucker up backwards. The "primary" goes on
the 2 ends. The "secondary" is one end to the wiper.

I think this is very likely what is going on. If you have the primary
wired through the wiper, the current drawn will increase dramatically
as you turn the variac down to low values; at low values, it means
that the primary is essentially a short (although possibly not the
kind of short you were looking for).

Open the variac and check the wiring as soon as you can, because it
might overheat and destroy the insulation.

Best wishes,

Chris