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Re: [TCML] MOTkenstein
Hi Scott,
I think Greg's comments pretty much wrap it up. If you
really enjoyed the challenge of the task that you propose,
then go for it. Otherwise, your time and money would be
better spend on just getting a surplus used pig and being
done with it. As several others have already pointed out,
unless you increase the cross sectional area of the core,
you will not increase the VA rating of the transformer by
pairing two MOT cores lengthwise. MOTs do rock pret-
ty well for their size but still pail by far in perfromance in
comparison to even the smallest pole pig or PT. As Greg
pointed out, MOTs are designed to run on the ragged
edge of their power handling limits. OTOH, pole pigs and
PTs are very conservatively rated and can put out several
times more power than what their nameplate rating would
suggest and stay cool doing it, assuming the duty cycle is
kept reasonably low.
--
David Rieben
-------------- Original message --------------
From: G Hunter <dogbrain_39560@xxxxxxxxx>
> The underlying subject of your email is really power transformer design. I know
> just enough about the subject to know it is mostly over my head!
>
> The power handling capability of a MOT, or any E I iron-core power transformer,
> is governed by the cross-sectional area of the center core leg, not by its
> length. Therefore, pressing two MOT "E" cores together will give you much more
> core metal, but will not increase the power handling ability of the core.
> You'll still be limited to one MOTs worth of VA. That's not necessarily a bad
> thing. A single MOT still processes far more VA than any NST.
>
> Also, you are constrained by a "turns per volt" rule for your windings, which is
> related to the cross-sectional area of the center leg and the magnetic path
> length around the outer legs. In other words, you can't wind just any old
> number of primary and secondary turns, and expect it to work based only on
> primary/secondary turns ratio. It just isn't that simple. If it was, all
> step-up transformers would feature one-turn primaries!
>
> If you study a common MOT from a USA oven, you'll see roughly 130 primary turns.
> Even this is bare-minimum, and probably violates textbook turns-per-volt
> formulas. This is evidenced by the large primary current draw when the
> secondary is open-circuit. A properly designed transformer draws only a tiny
> primary current when idling in this manner. In other words, when the secondary
> is open-circuit, the primary should "feel" almost like an open-circuit too.
> Contrary to this ideal, MOTs can suck several AMPS (!) from a wall outlet even
> in the no-load condition. MOTs are victims of crappy design in order to save a
> few pennies worth of copper and iron. I suppose on the huge scale that cheap
> ovens are manufactured, the pennies matter. Too bad.
>
> I'm not trying to turn you off of transformer design & fabrication. Other
> coilers have done it successfully. One guy I read about even rewound a MOT for
> 8kv output, but it was a tedious, time-consuming task for him. You might be
> able to pull it off too, IF you do your homework first. A good starting point
> is the "transformer design" article in Wikipedia. This will at least outline
> the basic rules for you.
>
> Best of Luck,
>
> Greg
>
>
> --- On Thu, 7/24/08, Scott Bogard wrote:
>
> > From: Scott Bogard
> > Subject: [TCML] MOTkenstein
> > To: tesla@xxxxxxxxxx
> > Date: Thursday, July 24, 2008, 9:51 PM
> > Here's a random idea,
> > I saw "How it's Made" the other day, and
> > they showed distribution
> > transformers. If I understood correctly, the primary was
> > sheet aluminum
> > separated by thick paper, wrapped like a spiral around the
> > whole core,
> > instead of a cylindrical winding of wire. I'm not sure
> > if I understood
> > correctly but it seems to me that would work for a homemade
> > rig as
> > well. Here is my idea, take two identical MOT cores, and
> > cut them into
> > two "E" shapes. Then create a wooden mold the
> > same size as the center
> > leg of the "E." Wind ten or twenty layers of
> > sheet aluminum (or copper)
> > separated by thick cardboard (to be soaked in oil) and
> > leave two tabs
> > sticking out, one at the center one at the edge. Then wind
> > 600 or 1200
> > turns of magnet wire separating each layer of windings by
> > several layers
> > of paper, grounding the bottom winding to the core. When
> > this is done
> > (assuming everything fits) the wooden core would be removed
> > and the two
> > "E's" would be put together creating a really
> > tall MOT core, with sheet
> > primary underneath wire secondary. The core will be
> > grounded to the
> > white wire, and a tabs of sheet aluminum will be the 240 V
> > inputs.
> > Seems like a place to start for those interested in
> > building their own
> > transformers, should be good to 14.4 kV when soaked in oil,
> > I just don't
> > know what the efficiency will be like, but if logic serves
> > me, it should
> > be good the the same wattage as two MOTS (about 2kVA.)
> > I'd like to hear
> > some thoughts on this before I start building, or before I
> > purchase a
> > pig, as I am sick of using MOTs which burn out every time I
> > say
> > something nasty to them (probably because of my roughly
> > resonant sized
> > cap, which I would like to keep, as using 14.4 kV instead
> > of 8 should
> > make it roughly LTR for my system.) Comments would be
> > appreciated.
> >
> > Scott Bogard.
> >
> >
> > _________________________________________________________________
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