On Mar 20, 8:53*am, "N_Cook" wrote:
Something to do while watching some less engaging TV. Unwound another large
toroidal mains transformer. Suspecting doubled up winding of the primary and
then for UK use joining opposite ends and so relying on 2 thicknesses of
lacquer to resist high voltage. Of course somewhere near the middle at some
point it fails catastrophically. Yes, burnt spot weld buried in the middle
of the primary. Anyone know what this duff winding technique is called ?
(reduces the number of shuttle passes by 2 must be the reason). Is there a
way of testing an unknown, but good, transformer for this winding pattern?

Not that I know of.

How come this wiring procedure is not outlawed?

Most of the transformers wound in this way don't burn out?

More generally, someone in production told me that a "goodness" test for a
mains transformer is an open secondaries, no load, monitoring of the primary
current is useful, any truth/rationale in that. ?

It tells you the inductance of the transformer. It doesn't tell you
how much heat is being dissipated in the windings and the core - for
that you have load the seconaries and watch how fast the output
voltage falls off with incresing load.

Since the permeability of the core declines with increasing
temperature, the inductance falls away as the transformer heats up, so
that magnetising current - and the power dissipation - goes up as the
transoformer gets warmer.

--
Bill Sloman, Nijmegen