4-Pack MOT Stack
I finally quit putting it off and built a 4-pack MOT stack. And what precipitated this momentous event, you ask? Well, my twin MOT power supply with voltage doublers had developed a gremlin. At 1080 or 1700VA, the diode string held up fine, but at 3200VA t he diodes failed within minutes. After replacing the diode string for the third time in as many months, I decided on radical surgery. I laboriously converted from half-wave to a full-wave doubler circuit. I thought placing the MOT caps across the diodes would result in superior filtering & spike protection. When I fired up the full-wave doubled MOT twins for the first time, the diode string failed within seconds! In a fit of frustration I declared: no more SILICON in my disruptive TC power supplies--EVER! So I got busy on a MOT stack.
I had some spare MOTs lying around, so I mounted two similar units on a scrap of 1/2" high-density fiberboard cut to fit a .30 cal ammo can. The MOTs were not modified in any way. I used short screws so they wouldn't poke thru the fiberboard and make contact with the can. I put two layers of cardboard in the bottom of the can as well. It only needs to hold off 4kvac, and I figured it would insulate well enough once it was soaked with oil. 4kvac supplied by my original MOT twins is brought in through PVC pipe bushings on the lid. The 8kvac output exits through similar bushings made a bit taller by use of a PVC coupler (to better hold off the higher potential). Once the wiring was done, I filled the ammo can with non-detergent motor oil just high enough to cover the windings.
I fumbled with the phasing for half an hour before I finally figured out what I was doing wrong. I wired the 4-pack for 120vac operation. I ballasted the stack at 17A using a single small MOT with it's second ary shorted to its core. Performance was pretty dismal. The 4" x 24" junk box coil put out a few anemic streamers, but nothing to brag about. My 9kv/180ma NST farm turns in a much better performance. A quick test of the 4-pack output with an ammeter revealed I was only getting about 140ma. Even though the single MOT ballast was sucking over 2KVA from the wall outlet, only about 1100VA was making it to the TC. It wasn't too hard to figure out where the lost kilowatt was going. The ballast MOT was hot enough to iron shirt collars! I quickly re-evaluated the usefulness of my single ballast MOT.
Impatient to get the 4-pack working at its full potential, I plugged it straight into a 120v/20A outlet. The breaker held for about a second--but what a second! The junkbox coil erupted with streamers like a Chinese sparkle fountain. It was an intense experience. Obviously, 120vac operation was out of the question. I had a 240vac/20A plug in my junk box. I wasted no time rewiring for 240v. This was pretty spectacular too , but the breaker only held for about 5 seconds. I was also alarmed at how hot the power cord got on this shorty run. I really needed a ballast, and I just happened to have one on hand. MOT caps! I had 10 spare .97uF/2200wvac MOT caps from USA manufacturing. I quickly cemented them to scraps of vinyl I-beam (the stuff that joins 2 vinyl windows together to make a picture window), giving me two rows of 5 caps. I wired them in series and placed one string in each leg between the two boxes of MOTs. The capacitive reactance formula predicted 27360 Ohms of reactance. That would limit the 4-pack to 310ma (assuming 8500vac), or 2640VA. First light with the capacitive ballasted 4-pack was very satisfying--nice intense streamers and no hot power cord. I jumpered back to 8 series MOT caps for 3300VA, and the junkbox coil responded positively, with plenty of thick, intense streamers.
I know it looks kind of cheesy, but this is just a temporary lashup for proof of concept. I'll make it look better later (I did clean it up--have a look). This is hardly pioneering work. The 4-pack MOT stack is a well-worn trail. I do like my idea of packaging the MOTs in pairs. I can pick up a brace of MOTs in an ammo box and carry it around by its handle. On the other hand, 4 MOTs in one container, plus oil, would be a hernia maker and a back-breaker. Likewise, I thought the MOT cap ballast was lighter, cheaper, and easier to build than any inductive ballast. Before I thought about using the MOT caps, I was contemplating a 500 ft spool of #12 THHN with a bundle of rebar for the core. I like the caps better. Based on only a few minutes of run time, I think the 4-pack is a pretty effective power supply. I really need more spark time for a proper evaluation though.