Message-Id: <7.0.1.0.1.20090715193731.01a75230@xtra.co.nz>
Date: Wed, 15 Jul 2009 22:35:14 +1200
To: rsgb_lf_group@blacksheep.org
From: Brenda Hayes <bjhayes@xtra.co.nz>
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Subject: LF: Microwave oven transformers
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  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