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I am working on a quad MOT (microwave oven transformer) tesla coil.

We tore apart 5 microwaves and got some MOTs. We first tested each MOT individually. They did not seem to have any problems. We then hooked up 2 MOTS together with the primary coils in parallel and the secondary coils in series. Once we got the "polarity" or phase correct in all of them, we went for 3 then all 4 MOTs. 

We had some working arcs with all four MOTs and seemed good, so we soldered and heat-shrank everything. 

We came in the next day, and fired it up, but problems started to occur.  When we plugged it in, quickly two of the MOTs had internal arcing, and we're really bright and smoke came out. We tested each MOT individually, and everything seemed to be okay, and the MOTs were all arcing, but when we added some MOTs together, the same weird internal bright flash happened. Even weirder, the flash seemed located in the PRIMARY, which I would not think arcing would happen there. If we felt where it arced, this powdery black stuff got on our fingers. 

In addition to that, some combinations of MOTs together still worked, but there was low current "purple lightning" small arcing coming off of one MOTs secondary.

Now, we have tried so much, some MOTs are almost completely failed, and the arcs are degrading. If we can get anything to work, we hear a large hissing like an uninsulated area where corona is discharging.

We can get more MOTs easily, but we want to make sure we don't screw up this time and get out 8,000 volt PSU for the tesla coil. 

Do we have to dunk the MOTS in oil? I have seen quad MOT tesla coils on the internet, so I know it has to be possible with out our weird failures.

And please avoid suggesting a NST (neon sign transformer) as they are too expensive for the low current output and wouldn't be suitable for the size of tesla coil we are working on.

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I'm sure you figured it out by now, but you need to limit current to your power supply by attaching two reactors (modified MOT's) as inductive ballast. The arcing you experienced was most likely in the outer two MOT's as they are not build to handle more than 3000 V. Oil submersion seems to be necessary at least for the outer two transformers. Go to Steffan's Tesla Coil Page. There I have instructions on how to hook up the reactors. I have been testing without oil submersion and haven't had any problems so far.

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