Rich Grise wrote:

On Wed, 10 Oct 2007 12:05:24 -0700, Grant Erwin wrote:

Don Foreman wrote:

On Wed, 10 Oct 2007 08:29:49 -0700, Grant Erwin


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.

Magnetizing current doesn't depend on the position of the slider. Sounds
like a short.

Many people have suggested to me that there is a short.

If you think of the primary side as having terminals L1 and L2, let's for
the sake of argument say that there are 100 coils between L1 and L2,
numbered 1, 2, 3, ... 99, 100. The output terminals are connected between
S (for slider) and L2. As you vary the control, S should physically
contact one of the coils e.g. coil number 50.

Let's imagine that it is instead contacting coils 50 through 54. This
would be the short that people tell me I probably have, right? As I see
it, that would be equivalent to having a primary with 95 coils, and I
*still* don't see where a large increase in magnetizing current would
happen.

I'm sure the problem is in my thinking - please show me where!



Are you absolutely sure you've got it wired right? If you connect the
mains to, say, L2 and the wiper, that would do what you say (and probably
cause collateral damage, because it would be in a step-up configuration.)

I'm working with the thing right now, unfortunately. But I can confirm that
when I move the slider up to 100% the current goes up, and when I move the
slider down, it goes down. Nor does the current at 100% exceed the current
if I just plug the load (a car battery charger) into the wall directly, so
I really doubt it's stepping anything up.

I'm trying to take digital pictures right now.

Grant