MOT question

I separated 2 cores by grinding out the weld, and put 2 primaries on one core to make an isolation transformer for working on old radios like 5 tube ac/dc sets with "hot" chassis. I feed it from a large Variac

I'd be interested in hearing more about this. How do you keep the resistance of joints from concentrating the heat in a small space with perhaps catastrophic results?

If you mean build a DC supply for your inductive furnace, probably. But you still need the high frequency generator to produce the heat.

Older microwave transformers are designed to work more like current sources. There are magnetic shunts that decrease the coupling between primary and secondary. You can/should remove those shunts, but you're still left with the fact that the vendor has no incentive to make a core that achieves the tight coupling that you want in a typical transformer application.

I'll second the saturation issue. I built a battery tab spot welder around a two-turn secondary. The weld consistency was horrible, so I gave up. After I acquired my first digital storage scope and looked at the waveforms, it was obvious that there was lots of hysteresis in the core and the transformer was saturating. I switched to multiple full-cycle pulses with start and stop at line zero crossing, the weld consistency got much better.

Did you visualize which way the eddy currents would go? The welds that hold the laminations together are fairly shallow and I believe the direction is orthogonal to the direction of the eddy currents.

Dan

I think inductive furnaces generally just rectify the line voltage and do not use a transformer for the power supply.

Dan

You connect one end of the transformer secondary to one end of where the pi pe is exposed to the weather, and the other end to the other end of the exp osed pipe. Will work bost with iron pipe, but can work with copper pipe. Probably best to keep the magnetic shunt in the transformer.

Dan

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I got 2 KV from a power supply that could recharge a 200 uF bank in 100 mS, so it was a serious zap.

Enjoy, DoN.

Sure, but I'm not sure you need it. The input voltages vary all over, I see some at 12 Volts, 15 volts and

180 volts and one that just rectifies the line voltage for either 155 volts or 310 volts. Just pick a design that doesn't use a transformer.

The last time I was looking at one it was a kit, but the design was very forgiving of the load, meaning he could change the work coil and still get power to the piece being heated. So he could heat different size pieces.

Click on some of these, I'm sure you will find some neat designs and videos of them in action.

Mikek

Yes, that's a little more "real" than my measly 800v source.

I pulled apart two of my 3 spare ovens over the weekend. One had a nice little xfmr, the other a plastic-cored, plastic-spooled coil of

8ga wire and semi-external ferrite rect. bar. I haven't pulled it farther apart to see WTH the thing is, but any secondary winding is hiding.

I hope the 3rd has a matching xfmr to the little one.

I was just looking for a way to use this salvaged MOT , and I think I found one . I need a multi-voltage power supply suitable for driving a small stepper motor and other low-voltage DC devices . This was about a 900 watt oven , I figger I can rewind multiple secondaries

Ahhh ... the latest MO technology: inverters. Just picking one up tells all - the missing 5 lbs of steel and copper is obvious.

Has anyone tried to modify a ballast from a street lamp? I picked up five of them the other day, when a crew was rebuilding the lights in a parking lot.

Negatory here. Are they replacing them with LEDs and drivers?

On the local scene: I pulled the last MOT out of the largest microwave oven and was really surprised.

I may not need a pair of these guys after all. I have .8 x .94 holes after getting the secondary winding out of the way. After hacksawing most of the way through the first half, I chiseled the rest plus the other side. It went much more quickly and smoothly. I'll do that to remove xfmr windings in the future. The best chisel turned out to be an old SnatchOn gasket scraper.

I can't really tell from the picture, but it looks like one of the nice ones I used to pull from the old faux-woodgrain ovens. The biggest ones I ever harvested, two of them, were from old Sharp ovens, with a mechanical bell and an analog timer. The trannies from those ovens had laminations that were bolted together, instead of just having a weld seam to hold them together. Big beefy suckers, and heavy as hell.

Jon

I didn't know you could easily grind out the welded parts and not mess up the laminations. Will have to try this.

I think the welding is electro beam welding. I have cut out the welds on a MOT with a hacksaw

Dan.

I looked at the one I have laying here and it looks like a few minutes with a dremel or maybe the bandsaw and it'll come apart . If done carefully the remaining weld will keep the laminations together . This one looks like it was just TIG fused or maybe as you said electro beam or laser .

Microwave transformers are designed to do ONE thing very well and CHEAP. Rescue microwave transformers have a couple of uses. Existing high voltage winding where you don't need tight coupling. New secondary of few turns of big wire where you don't need tight coupling. Most other applications are a fantasy of using the wrong part for the application. Once you grind it apart, how the hell are you gonna get it back together?

in general, working with electronics is good if you have 20/20 vision. If you know anyone in the field, you might want to invite them over and let them help.

I have a transformer core from an old tombstone welder

Probably 5 or 6 times bigger than a big MOT. Been storing it for years, it needs a project, but I don't know what. It would make a heck of a spot welder. Mikek