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Mot psu update
All day yesterday I worked on my Mot power supply--I thought some of you
might be interested in the results. Note: this is a long post, so it probably
won't be interesting to the majority of you who aren't thinking about going
this
direction (i.e. the less skilled, budget coiler direction)
Since series-ing Mot's has come up again after Herwig's post, I'd like to
add that it is possible to arrange the second stage so that neither core-coil
breakdown or saturation occurs.
First, a note of tentative explanation on why core-coil breakdown and
saturation occur in the second stage. For the first problem it seems to be a
dual issue of insulation problems and improper loading. No matter what
arrangement of Mots you come up with, even the most sturdy of transformers,
epoxy impregnated or not, will break down if there is not more than a
half-centimetre of distance between the secondary and the core. So either
encapsulate your later stages or find ones that have heavy paper and/or mica
insulation *and* an air gap between the exposed windings and the core. These
will last.
Second, be very aware of where the "ground path" is, and how and in
what way
your transformers are being loaded. Under no-load conditions, the full voltage
of the second stage *wants* to get to ground. And it will, if it has its way.
To avoid this, always raise your transformers on isulators of some type, and
preferrably have some dielectric material (wax, epoxy, what have you) which you
can introduce between the cores and the detached ground wires.
Finally, don't arc the transformer output directly to the first
transformer's ground. The distance between the detached ground wire, the core,
the surface you are working on, and the cores of the other transformers is *too
short*. You will notice heavy coil-core breakdown in this event. It is
generally safer if you feel you need to test the output to arc across the
center
tap, to the oppositely phased output of the other set (if you are using a
center-tapped configuration--and it really is best to, for all practical
purposes). Always connect the grounds of the two first stage Mots together.
Otherwise you will again encounter coil-core breakdown.
On to saturation. I am still unclear as to why the second stages
saturate, and the third certainly do-- but it seems to make common sense that
the more you bottom-load the secondary, the greater the danger for saturation.
To avoid this, I constructed a 2 Mot second stage with the primaries and
secondaries in series. This is effectively a 220 volt rated Mot, and hence is
suitable for any over-voltage condition aside from insulation issues. The
drawback is that the added inductance of the second primary in series
dramatically reduces the output, while it nevertheless reduces the risk of
saturation to zero.
To eliminate this problem entirely, I knocked out the shunts in the second
stage Mots. What a difference! Across the array of 6 Mots I can draw a foot
long hemispherical arc that seems to be about an inch in diameter. All the
Mots
consume about the same amount of power, and are only slightly warm after use.
No coil-core breakdown was observed.
I experienced more than the usual difficulties with the doubler circuit.
First of all, always discharge your capacitors. Always discharge your
capacitors. *Always*. I got bitten more times yesterday than I would have
liked ; )
I used a total of 16 1 uf 2kv oven caps, arranged in series-parallel for
2uf on either side of the array, and 8 very large, high current oven diodes.
After fixing some phasing problems (you would think Mots were all wound in the
same direction, wouldn't you?) I discovered that the cases of the capacitors
were arcing to each other--which was temporarily fixed by electrical tape ; ).
I couldn't measure the voltage of the output, but since it arced in air at
nearly an inch it must have been over the 16kv I imagined. I'm not sure why a
simple doubler would provide 2.5 vrms+ As I recall the one-stage center-tapped
design puts out 12kv--this must be for the same reason.
After getting to this stage I arranged another array of 6 mots, another 16
capacitors, and another 4 diodes, side by side with the other one. The aim was
to provide the ~20kv at full-wave rectification. I had some serious problems
with this, I think due to capacitor charging problems. The plan was to have 2
oppositely phased legs on each side of the center tap, which charged and
discharged two capacitor banks alternately, their outputs being connected
together. I foresaw a problem with this when I designed it, because it looked
like the two banks would simply short each other. To solve this problem I
thought about rectifying each output, preventing the cap banks from discharging
into the ground path of the other leg.
In practice this is exactly what happend when I simply connected the two
doubler outputs--one of my transformers severely arced to the core, carbonizing
the insulation--probably ruining it. Oh well-- I have 8 or 10 more. . . When I
rectified each output, using 2 strings of 5 oven diodes (for a ~10kv rating
each, respectively) the output was stable and very vigorous, emitting snappy
static arcs of a centimetre and a half to any decently-sized metal object. I
can't wait to see the arc distance of both legs! I only had one leg of the
full-wave array working, because I had a limited number of diodes. But I just
had to arc it to ground, to see what would happen. I was suprised to get what
seemed like an AC arc. What happened to the direct current? Immediately
afterwards the output was reduced, either as a result of my now *severly*
arcing
transformer, or one or both of the diode strings failing from the over
voltage/over current from the "direct hit". At this point I had to quit, since
it was extremely late, and I had to arrange everything in the attic as it had
been before my parents left ; ))
I hate living at home. >: [
When I get more diodes (MMD!) and a Tesla coil to feed with this psu, I'll
update you guys again. All I have to say is, 20kv-at-300ma! yaay! At a price
of $0! Well, theoretically, at least. ; )
--Tired Mike in
Wet Oregon