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


Just a WAG, but maybe the sliding contact has worn "flat" and is
touching too many turns of wire, creating a multiturn shorted winding
when set at the 30% position, thus causing the problem.

You didn't say whether your variac is a 0-100% kind or the kind which
can output more than 100% of the input voltage.

If it's the 100% kind, then maybe when it's set to 100% the contact arm
is no longer resting on the winding but is on a broader piece of metal
connected to the hot input.

More details please. If you plug it in at 100% and then lower it to 30%
do things stay OK?

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.