PlainBill wrote in message
news
On Fri, 20 Mar 2009 07:53:39 -0000, "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
?
How come this wiring procedure is not outlawed ?
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. ?

Way back in my dark past I worked in a plant where we wound our own
transformers. Indeed, the doubling of the primaries is standard
practice. Note that if your primary voltage is 240VAC the voltage
between the adjacent wires will be approximately 180V maximum.

Of course, these were using 'I and E' laminates. We tested the wound
bobbin for the correct number of turns and leakage between the wires
of the paired windings. After the laminations were added the entire
transformer was vacuum impregnated with varnish. Not much chance for
movement between the windings after that!

PlainBill


In the ideal yes, but this Tx and another similar there is no interlayer
insulation between each of the 3 or 4 primary "layers".
Compact winding only on the inside diameter, not the outside , where
windings
could easily slip a "layer". So worst case voltage could be 240/0.707 , peak
to peak, without any mains spikes.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/