 Rewinding secondary on MOT for 50V supply
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GeordieBoy
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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
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Microwave Oven transformers are the absolute crappiest transformers there are, (well okay except maybe for that Nigerian Welding transformer made with cardboard laminations and boot laces!)
They are only designed to operate at the rated power for the duration of a typical cooking cycle with an acceptable temperature rise with forced air cooling from the internal fan.
Compared to an ideal transformer, just about everything is wrong. They have too little iron for the power rating (to minimise manufacturing cost, storage space and shipping weight.) As a result the flux density is way too high so the core overheats. They have too little copper (cost reduction) to keep magnetising current and 3rd harmonic distortion under control, so they draw a large ugly off-load line-current waveform. And to add insult to injury they leave an air gap in the core by not interleaving lams and then run a stick welder down the sides and short all the laminations together! (Of course the addition of the shunts wrecks the load regulation but that is necessary for its intended purpose.)
If you want to *TRY* to make an MOT into a half-decent transformer, derate the power handling to about 50%, knock out the shunts, and wind on enough extra primary turns that the magnetising current is reduced and starts to look vaguely sinusoidal!
It might interest you to know that a friend who worked for a world-leading manufacturer of microwave ovens said that transformer fires on the production line were a frequent occurance during product's soak tests. You wonder how they can make an entire oven for $45... now you know.
-Richie,
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