Hello all,
I have recently done some tests on a pair of identical transformers
removed from two identical ovens.
Perhaps unfortunately, I removed the magnetic shunts before doing any
no-load testing. After half an hour, with 235V applied to the primary
windings and no secondary load, the iron was far too hot for comfort..
At this stage, I measured the iron losses with an accurate wattmeter
that certainly takes account of the crummy power factor..
At 50 watts apiece, for no-load losses, I had visions of cooling fans etc.
My solution was to wire a double-wound 230/32V [150VA] transformer in
step-down "auto" style to feed the ex-microwave transformer
primaries. This step-down transformer showed negligible iron loss of
4W or so. With a new primary voltage of 200, the standing iron loss
dropped to around 30W per ex-microwave transformer. Naturally, the
secondary voltage has also dropped. I now have about
1600 - 0 - 1600V from the two transformers when the primaries are
correctly phased.
While the earthed ends of each HT winding could be lifted and FW
bridge rectification employed, I was reluctant to follow this path
due to insulation considerations.
There is another trick that I found useful for safely matching
non-identical transformers, provided they are much the same size:
Apply 230V to the HV secondary windings and compare output voltages -
usually 23/26 volts or so.
Watch where you put your fingers.
Kevin ZL4MD
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