Don Foreman 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


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!

GWE