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LF: Microwave oven transformers

To: [email protected]
Subject: LF: Microwave oven transformers
From: Brenda Hayes <[email protected]>
Date: Wed, 15 Jul 2009 22:35:14 +1200
Reply-to: [email protected]
Sender: [email protected]
 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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