Here are some scope pictures from a transformer under various conditions.
The pictures show the primary current of the transformer.

Picture 1 is normal unloaded primary current with 120 VAC applied. You can
see the onset of core saturation.

Picture 2 is with 145 VAC applied to the primary. The core is well into
saturation.

Picture 3 is primary current with 120 VAC applied, but with a single
shorted turn of 21 gauge wire around the center leg.

Picture 4 is primary current with 120 VAC applied and with a single turn of
14 gauge wire around the center leg.

This test could help detect a shorted turn.

These are the what I got from my scope. The variac's readings might be off.
It looks like it might be +5 to 10 volts higher than what the file names
say. The variac does have an AC volt meter on it but it its 10V more than
what the dial says. My DMM reads 5V more than the dial.

Thanks,
Bob

On Thu, 15 Mar 2007 18:13:31 GMT, "Bob Jones"
wrote:


These are the what I got from my scope. The variac's readings might be off.
It looks like it might be +5 to 10 volts higher than what the file names
say. The variac does have an AC volt meter on it but it its 10V more than
what the dial says. My DMM reads 5V more than the dial.

Thanks,
Bob

You saw in the scope pix I posted that a single shorted turn overwhelms
the peaky current peaks caused by saturation. It still is possible that
you have a high resistance short, but that wouldn't produce much heat if it
were the case.

If you have some wire you could slip through a small space (if there is
any!) and get a turn around the center leg of the transformer, you could do
what I did and look at the primary current when you short a single turn.

Based on the scope pix, I don't think you have a shorted turn. I think
you have a cheap transformer.

In addition to running the core into saturation, it's probably a cheap
grade of iron, such as M19 instead of M6. This could help account for the
high current draw and the high temperature of the core even at no-load.