Why does my MOT get so hot when nothing is connected?


I have a rewound MOT (230V input and 9-50V output, different custom windings)
and I'm wondering why the transformer gets so hot when no load is even connected?

Is it normal that MOT's get so hot quickly because the primary impedance is just so low?
and how do they fix this problem when the MOT is still inside the microwave oven?
Because there, it does need to be able to run for hours.

I've read about ballasting the MOT, but I thought that was only for current limiting, when you Are attaching loads on the secondary?


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From http://wiki.4hv.org/index.php/Microwave_oven_transformer

"The transformer is designed to be as cheap to manufacture as possible, with no regard for efficiency. This is because it is the manufacturer who pays for the copper and iron, but the user who pays for the energy consumed. Thus the iron area is minimised which results in the core being taken well into saturation with result high core losses. The copper area is also minimised, resulting in high copper losses. The heat that these generate is handled by forced air cooling, usually by the same fan that is required to cool the magnetron. The core saturation is not part of the non-ideal classification, it is merely as a result of the economics of manufacture."

....which is what I said...well the bit about saturation effects.....

Also, what is "hot" in use ? These things might be wound to happily work at 150 C !

iceng4 years ago
Shorted turns draw more current be it primary or secondary.

That impact on the primary has got to be the cause of your MOT Heat :(

Yeah, look for a shorted turn somewhere.
Re-design4 years ago
Did you rewind the primary?  what's the resistance of the primary?

I think you got a short either in the primary or the secondary.