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


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?



The answer is to reduce the primary turns, to reduce the flux in the core. Wind one in the secondary space.

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 !



yes, you're right, you would expect a low to effectively zero primary current, with no secondary current ! It should be a big inductor - and it isn't. I wonder if its wired too close to the limits,and you're seeing saturation effects in the iron.

I need to think about this ;-)

There's too much leakage inductance, and the thing's drawing too much current. MEASURE the current now, and then remove the shunts and try again.

You are going to force me to have to revise my transformer theory notes from University - so old, i think I wrote them on parchment......


Maybe I should do an 'ible on that.



Mine started out as a 3KVA transformer - lots of space for a heavy secondary.

Remember my work is largely commercial. I don't /can't post it.

I've just found a great "free" (well free-ish) source - the 110V transformers used for building sites. They are a.) Very big b.) easy to remove c.) easy to strip. d.) easy to rewind.

Tempted to do an Instructable on it - but one never does when one does it for the the first time.

I had to make a 55-0-55 in, to 12V @ 70 A out - DC - there's a HUGE rectifier in the case too.


Essentially, you have short circuited the transformer, magnetically AND electrically.


6 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.

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.