Greg's MOT New Stack (long post--sorry!)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gregory Hunter by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ghunter31014-at-yahoo-dot-com>

Dear List,

I finally quit putting it off and built a 4-pack MOT
stack. And what precipitated this momentous event, you
ask? Well, my voltage doubled twin MOT supply had
developed a gremlin. At 1080 or 1700VA, the diode
string held up fine, but at 3200VA the 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 secondary
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. Greg like!

I've posted one picture at:

http://hot-streamer-dot-com/greg/4pack.jpg

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.

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.

Sorry, no spark photos yet. I'll get to it when I can.
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.

Regards,


=====
Gregory R. Hunter

http://hot-streamer-dot-com/greg

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