Microwave power transformer

Hello James,

No idea, really, but with components like that it is often a race to the rock-bottom cost point. They might have chosen the cheapest core metals they could get by with, at the longest expected runtime. Minus the failures allowed to flow into warranty overhead.

Regards, Joerg

have

warm to

Snip Would I need to also cut out the welds on the core to lessen the

I earlier had suggested you add turns to the primary, but I don't think four extra turns gets you very much, I just counted 142 tuns on the primary of a microwave transformer I have. It might be a lot of work, but wind 200 tuns on the secondary side and test it. Then start backing off on turns until to core temp becomes a problem. Another thought, I think you can just wind 50 turns in the secondary window and wire it series adding to the primary, then check to see how it works. Then you could back off on turns as allowed. Mike

It may not be so bad, I am going to put a temp lead on it and see what it raises to. What I want this transformer for is for 15 to 20 amp dc regulated supply / battery charger. Maybe add variable voltage and current limit. The first transformer I rewound didn't have the mag shunt. I am using right now the

12 guage pvc coated multistrand wire in the secondary. I will redo it with magnet wire for perm. use. I saw an article while web hunting on using 2 of these transformers to make an arc welder, I may give that a try also. Hey thanks for your input on this, and I will let you know how much the temp rises under no load.

After 1 hour of no load this transformer temp went from 80 degrees (room temp) to 131 degrees. Maybe it would be best to use these for maybe a car battery charger so as not to run continuous.

I think I understand what you are saying. More turns = more impedance and less current = lower magnetic flux = less core saturation and less eddy heating. Correct ?

[More Snip]

I wouldn't add any more turns to the secondary until I get that primary figured out. I have an MOT:

That, if I guesstimate accurately, has 10 turns on the outside layer, and

15 layers, of which 7 are only 9 turns - they're in the valleys between layers, so there's 10 * 8 + 7 * 9 = 143 turns. It draws a horrendous amount of current - like, 2 or 3 amps, when just idling. I put the penny there just to see if it got eddy currents, but nothing. The wire is .040" diameter, which is real close to #18, so I've decided, whenever I can scrounge about 25 feet of #18 formvar magnet wire, I'm going to add 15 turns, in a pie, right up against the face of the winding loop, going through where the shunts are now. Oh, yeah, I'm going to knock out the shunts and measure the current again one of these days, but haven't got a round tuit yet. I'll probably do that when 25' or so of #18 magnet wire mysteriously shows up at my doorstep. ;-)

Then, I intend to use it to dick around with, winding various secondaries and such; maybe even adding and subtracting windings to/from the primary. ;-)

Cheers! Rich

I don't think 131F is dangerous for a transformer - it's too hot to touch, but I don't think it'll start any fires. You are talking F, aren't you? I wouldn't want to be in a room whose temperature is 80C!

But, don't forget, when under continuous duty in an oven, it's got forced-air cooling going all of the time that it's on.

Cheers! Rich

Pretty much, yeah. :-)

Cheers! Rich

Lower the volts per turn applied to it.

4 turns isn't enough to lower the flux swing enough to make it idle efficiently.

They are placed where they do very little harm.

and

gets

21.8

of

works.

Yes, that is my thinking. And to reiterate what I said above, you have removed the secondary, so you could add 20 to 50 turns in that window and put them in series with the primary and see if that is enough to get your core out of saturation.

Or you could see how much you need to lower the primary voltage until it runs cool.... then you can figure out how many more turns you need... but remember when you put it under load, the wire will then heat up too...

Mark

To add that many windings, I would also need to use a smaller wire and rewind the whole primary. The wire I have on the secondary fills all the space in the core now. I believe I understand enough now to go from bare core to custom transformer so I will experiment and see if it all works out. I will like you said, add windings to the core ( about 200 or so ) and measure the idle current is steps as I remove some windings. When the current begins to rise, then I will know the lowest number of turns to make the primary winding. That way it will not get the core into saturation and give me some headroom so when it draws secondary current. Thank you for your help. I have allways wanted to figure out these transformers.

A simple way to use MOTs for other uses is to take two similar units and wire the primaries in series. Then you can wire the secondaries in parallel for low voltage, high current outputs. This, of course, lowers the volts per turn by half.

Not to mention that if you cut them out, the transformer will fall apart. Cutting them is actually a good way to get new windings on, provided you weld them back after ensuring *tight* contact between the E and the I parts. Bolt through the holes especially on the I parts *before* cutting the welds, and leave the bolts in place until after it's re-welded, or you'll wind up with a gapped xformer.