David Lesher writes:

Someone one mentioned they were F-R, and a casual look seemed to confirm
that, so I never questioned it. A F-R is also current limited; short the
output and it delivers rated current, period..

They look similar. Both transformers have magnetic shunts, and there's
a big capacitor near the transformer. But the circuits are different.

The FR transformer puts the capacitor across the secondary winding, or
two secondary windings in series, and the windings plus capacitor
resonate at the designed line frequency. This causes the voltage to
rise above what you'd expect from the turns ratio alone, but the voltage
is limited by the portion of the core that the secondary is wound on
going into saturation. Sometimes the secondary voltage waveform looks
pretty square because of this peak clipping, but sometimes there's a
third winding that (somehow) reduces the second harmonic and gives
something closer to a sine wave.

There's a magnetic shunt between the primary and secondary windings so
that the primary current doesn't go through the roof when the iron in
the secondary saturates. It limits the shorted output current to about
*twice* the rated current, not equal to the rated current.

Still, the FR transformer runs hot with no load, dissipating about 20%
of its full output rating as heat.

In comparison, the capacitor in the microwave is wired as part of a
voltage doubler; it doesn't resonate with the transformer secondary.
The transformer iron is not designed to saturate (though, as an
intermittent-duty transformer that is fan-cooled in use, it is
apparently designed to operate close to saturation to minimize the
amount of iron).

Dave