Patrick Turner wrote:

William Sommerwerck wrote:

The transformer will probably hum loudly and get hot, and perhaps
burst into flames after a while. Eventually, a fuse, if fitted, will blow,
and when the insulation burns off the wires, a short will blow the
mains fuse. This is the result of saturation.

But (as Red Green points out) that would at least as entertaining as your
average "Matlock" episode. Probably more so.

One doesn't leap with joy when one's house burns down,
or when you realise one hasn't renewed the fire insurance policy.....

Patrick Turner.

Reversing a transformer:
Sorry to continue this backward transformer thing. And noting the
discussions about transformer saturation!
But from a practical viewpoint we have used a Hammond transformer
for some 35-40 years to step up 115 volts to 230 mainly to
operate a heavy electric drill; a 1953 Wolf SD4C. But also on low
wattage electronics.
The transformer was originally intended to step down 230 volts
across two phases of a 230 volt delta system, to provide 115
volts for some equipment. So we are using it backwards. It weighs
about 15-20 lbs. and rated at 500 watts. (Seems conservative to
me).
It is a complete isolation transformer with separate input and
output windings. (I've avoided saying primary and secondary cos
there's nothing on the transformer which indicates. Just 115 volt
on one side and 230 volt the other.)
Any way we have stalled the drill, used it for many minutes at a
time and under some pretty low voltage (long extension cord)
conditions and never a moments trouble! Or over heating.
I realize that these are not the same voltage requirements as for
electronics.
But the message seems to be? If the transformer is big enough for
the job it will, taking transformer losses and the practicalities
of its construction, work well in both directions. After all
power companies step up and down, granted with much more
specialized equipment throughout their systems.
Non isolating transformers:
A discussion about non-isolating auto, buck and boost, etc.
transformers is another subject I guess; but I've done that,
before I could afford a Variac! For example:
A typical front door 'bell' transformer, provided it's 12 volt
secondary insulation and ampere (wire gauge) capacity is OK will
allow one to boost or decrease the 115/120 input by approximately
10%. Two such transformers by +/- 20%. The secondaries being
connected in series with the mains input etc. Was a great aid in
a rural area where 40+ years ago long low voltage power lines,
increasing electricity demands, weak tubes and so on might cause
a customer to explain "She worked OK up to about 8 o'clock and
then she just quit. And every time the fridge or the water well
pump cuts in the sound goes off for a minute or two!"
Aha! Low voltage problem?
Have fun. Terry.