Electronics > Beginners
Power VA and secondary windings.
T3sl4co1l:
Good point, autoformer also gives lower impedance -- not much difference (though still in favor of auto.) for small changes, but would be worthwhile here. You'd use a 120:60V transformer (probably easy enough to find?) and wire them in series for 80V primary and 40V secondary, thus giving the requisite 80V to the larger transformer.
Tim
davelectronic:
--- Quote from: Gyro on December 15, 2019, 06:33:44 pm ---Ha, yes, it can be a pain. You can construct a long thin shuttle (basically a strip of wood with a notch at each end to hold the wire) to thread through the core.
Last time I wound a secondary with thick wire though, I ended up doing the winding while my wife ran backwards and forwards from the other corner of the room with the other end to avoid kinks! ;D
It's surprising how quickly the wire length builds up, make sure you measure correctly to ensure that you allow enough.
--- End quote ---
I did get the misses to do that with a MOT, i wanted as many turns on it as i could manage. She left me back earlier this year... maybe it was winding that transformer. The long thin shuttle is a great idea, as long as it fits through the centre, and built up winding slowly shrink the centre. But done carefully to factor in measurements, its doable with all but the heaviest gauge wire.
Gyro:
Sorry to hear that. :( I've always thought crappy MOTs were more trouble than they're worth!
It would be fun to try and mimic the way commercial automated toroid winders do it - a gapped ring shuttle that slips over the core and is then rotated and filled with wire. Once full, it can then wind the wire onto the core. Being a large diameter, it can hold loads of wire and still fit through a small centre hole and it avoids the inevitable wire folds that occur at the end of a 'stick' shuttle.
Of course, paying out the wire at a fixed tension while rotating the ring shuttle at a slower speed than you're winding would probably result in an almighty tangle! It might be doable though.
davelectronic:
Yes I've watched a few toroidal transformer winding videos on YouTube, fascinating how it works. I've got two MOT's laying around, one that's wound gives a solid 14.70 Volts. The other is stripped of its secondary, but unwound at the moment. They run very hot, the wound MOT i used silicone sheathed wire, and shrink wrapped the entire length that was wound on the core. I like the idea of picking up cheap toroidal transformers, and possibly rewinding the secondary with a usable voltage. Its a shame enamelled copper wire is so tough to wind on the core.
The transformer that i started this thread with, i might attempt to rewind the whole secondary with a slightly heavy gauge wire.
I would only go up one gauge for that, the idea of taps also sounds like something to try a some point in the future. Recently i purchased some isolation varnish for pcb and transformers / inductors etc. The data sheet for the varnish has a 27 page saftey process conformity of rules to follow. You would need less protective gear on a trip to Chernobyl, the mention of carcinogens and more in that product data sheet.
Jwillis:
MOTs are built to be as cheap as possible. Core area and copper area is kept as low as possible to minimize materials . Because of this they run very close to saturation resulting in high core losses so they run hot . The primary side has nowhere near the amount of copper needed for an ideal transformer resulting in high copper loses also resulting in extra heat. This is why there s a forced air fan inside the microwave ,just to keep the transformer and magnetron cool. If your just mucking about using the original primary is fine but expect things to get warm. If you want to rewind a proper transformer then both the primary and secondary need to be wound according to the core size and material used for the core .A typical MOT core will have at most 50-60 VA from a 500-700 watt microwave and maybe 80 -100 VA from a 1200W microwave oven.But only if the primary is wound correctly to minimize copper losses.About the only thing that's any good on a MOT transformer is the core it's self .
For your toroid of 100Va I'm guessing the outside diameter is around 4 inches. I'm guessing of course but your looking at around 60 -70 turns on the secondary which works out to around 7- 8 feet of wire.
Now you can rewind the secondary but you also need to consider the primary winding as well . As you load the secondary the primary will load as well. So the wire on the primary side also needs to able to handle the extra load. The current draw on the primary is proportional to the current draw on the secondary. Now the wire on the windings won't suddenly burst into flames if you reduce the voltage on the secondary side for more amps but it will get a bit warmer. After all your only talking about 2 extra amps. I've done this to a few myself because I couldn't find the correct secondary voltage.
Wire will handle much more current than it's rated for. Usually the rated current handling of wire is around 10 - 15 % the fusible rating . This keeps the transformer at spec for a class B which is around 25 degrees C . You can check the fusible rating of the wire online if you know the gauge.
If your only going to draw the original amount of current then I wouldn't worry at all. Ideally you only want to draw at most 80% of the rated current potential of the transformer at 20 degrees C ambient anyway . This allows a little overhead for ambient temperature changes without the need for active cooling.
Antek has very good quality transformers at reasonable prices but living in the UK the shipping may be prohibitive. I have a couple of these .One 800VA and a 1000VA and I can tell you they really kick out the current and stay really cool.
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