In a message dated 3/11/2009 3:50:21 A.M. Eastern Daylight Time,
g8gsq@eltac.co.uk writes:
Microwave oven transformers can be very different one from
another, even when the original appliance had the same nominal rating.
A lot have quite high resistance windings for the power rating, so
will give poor regulation and run hot on continuous load. Most
also have too few turns on the primary for continuous
energisation, and get very hot from that (in most ovens they are
fan cooled, even though on for only short periods). You can add
extra primary turns in the space from the heater winding and
shunts to help with this, but that increases the resistance.
Measure some and use the Duncan Amps PSU Designer to see what
performance they will give.
All I've toyed with can be used with the secondary disconnected
from the frame - one out of eight or nine needed a bit of kapton
tape for insulation on a hipot test.
Steve
_______________________________________________________
Dittos from all I have learned about MOTs about the only thing they will
do correctly is run a microwave oven. They have too few turns in the
primary
so the transformer runs hot and lossy, the magnetic shunts limit output
current
and the welds along the edges of the laminations act like a big shorted turn,
so the transformer runs hot and lossy. The transformer pulls several
hundred watts just idling with no load. Did I mention they run hot and lossy?
They were built with one thing in mind - maximum cheapness.
It is a shame because they look so tempting to use. But for all the work
and aggravation of trying to rebuild them, grinding off the welds across the
laminations, knocking out the magnetic shunts, possibly adding turns to the
primary, you might be much better off to just buy some laminations and
build your own transformer. Plus you will learn something by building
your own transformer and it is fun!
73 Todd WD4NGG
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