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MOT (microwave oven transformer) cooling.
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Ian.M:
I agree that 160V is the beginning of hard saturation.  Assuming your nominal supply voltage is 220V, you still need some margin for supply voltage variation.   10% overvoltage is a reasonable safety margin for most developed countries but if you are in a 3rd world s--thole*, you may need more margin.   Lets run the numbers for 10%.   Max input voltage is 220V * 1.1 = 242V.  242V - 160V = 82V.  82V / 0.8V/turn = 103 turns.

That's about half the turns of your original estimate, and if the primary fills the bobbin and is a close fit in the core window, is unlikely to be practical to add as an overwind on top of the primary, so its going to be a total PITA to have to build a reinforced insulation bobbin for the extra turns  as you cant simply 3D print two half bobbins and glue them as it wont take the heat, hence my comment earlier to use presspahn board, fishpaper and linen tape (which must be bare linen or with a thermosetting adhesive) and phenolic adhesive.

*  If you are in North America in a property with split phase 120V/240V, and old wiring, backstab outlet connections, or aluminum wiring, even 20% margin wont prevent a 'Weller style' meltdown from a bad neutral so for <expletive>'s sake fuse the primary properly!
bsfeechannel:
Around here, 220V installations are allowed to go from 202V to 231V. Up until very recently we were serviced by a subsidiary of AES Corporation. I can only hope they were not based in one of those 3d world sitholes (I learned that spelling with AvE).

Anyway, I think we are running out of options.

I use my MOT together with the variac as a poor man's high voltage source. I never go past 100V on the primary, since the high voltage winding has a 1:10 turns ratio and none of my meters can handle more than 1kV.
davelectronic:
This is very interesting data and ideas there. Unfortunately I don't have the split core transformer anymore. Going back a few years, I measured the primary current with no load on the secondary. That was right on 3 Amps with the 240 ish input. A lot of wasted energy. What I'm unsure about, is that current "wattless" a bit like an AC induction motor spinning with no load ? I kind of think the motor scenario and the transformer are not the same, identically. What seems to be the go to option must be primary ballast in series with the primary, so it's not saturated all the time. And adding extra primary winding is another great idea and option, but gets a bit trying on a closed core.
bsfeechannel:
The current is not "wattless", otherwise your transformer wouldn't be overheating, but part of it is "wattless". It has to be, because part of that energy is stored as a magnetic field in the core. However,  I'm not sure about the proportion exactly.

One thing crossed my mind while I was taking a nap. Normally we remove the MOT's magnetic shunts. What these shunts do is to provide an embedded ballast by increasing the leakage inductance. It acts as a current limiter and as such ends up reducing the voltage seen by the primary according to the model below.



I'm not sure how effective this embedded ballast is, but perhaps it helps reduce the number of turns to be added to the primary to take the core out of saturation. My graph doesn't show, but at 214V, I got 3.4A. However, my MOT is "shuntless". It would be nice if some MOT still intact could be measured for that current so that we could have an idea.
T3sl4co1l:
No, the shunt doesn't affect the primary side stats -- it's wound around the same core limb regardless.  The shunts do divert flux from the secondary, which means the secondary side core limb runs less Bpk.

The shunts are usually not the full core cross section, and not gapped much, which should tend to run them in saturation, drawing ~constant flux away from the secondary.  Which means the secondary regulation should still not be terrible, just a lower voltage.  Doesn't seem very useful, I guess it just happens to work out well enough for a magnetron.

The equivalent circuit is two ideal transformers sharing flux; the unlinked flux, is, of course, the leakage. :)

Tim
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