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Hi Jim and all,
Jim Mora wrote:
Hi Bert, et al,
My newest gap is driven by a DC motor and has 8 equal spaced .5" tungsten
electrodes (one of several works in progress). The motor is very rugged but
will top out at 426 breaks calculated at full voltage rating DC (PWM).
Perfect! You should get close to 10 KW out using a 3H charging reactor
and a 0.1 uF tank cap.
I have an aircore choke that I use with my pig. It is thin wall 4" pvc wound
12 inches with milspec Teflon insulated 1000v silver coated 19 strand copper
held in place with a thin coat of epoxy. This was a recommendation from DC
Cox and he had one on top the Big Bruiser controller if I remember correctly
feeding out the "Big Red" HV line to the big gap and tank. I seem to
remember it was 100 turns.
I also have a stash of HV ceramic puck caps that screw together to send some
hash to ground.
** The charging inductor has always been the fly in the ointment on this
project ;-^)
Terry Fritz sold me a 20KV utility pole transient protector but it may it
may fire all the time though I think it is intended to handle lightning
transients. But I haven't looked at it in some time and never tested it.
I have six identical MOT's and others but didn't want to go down that road.
I will take the path of least resistance if it will be robust.
A typical MOT will have around 2H of secondary inductance with the
primary short-circuited (the mutual inductance), and about 35-40H with
the primary open-circuited. Unfortunately, both inductances are too high
to permit direct series stacking for the inductance and voltage standoff
you need. If you removed the existing windings and wound each core with
appropriately-insulated windings (somewhere around 220-270 turns/core)
you should get ~500 mH per unit. The insulation system would need to be
capable of handling 4.2 kV per unit - not too bad if you use 200C
double-build #21 or 22 AWG with Nomex (Aramid) or Mylar strips between
layers. If you can perfect the design of a single unit, making five more
would be a relatively simple task.
I did brake down that 10KVA dual C core transformer today, not easy! They
are big cores and I found a company that makes multi-sectional Bobbins in
selectable materials that fit it. I'll call their engineering department
tomorrow to see what they say.
HV transformers and chokes require close attention to field-grading to
prevent excessive dielectric stress, partial discharges, and insulation
failure. This is especially important because of the transient voltage
stresses seen in your application. These considerations typically lead
to thicker, multiple bobbins, special winding techniques, and winding
shapes. It will be interesting to see what their engineering folks have
to say.
As one example when using multilayer coils, the layer-to-layer winding
interconnection should be done as follows to more evenly distribute the
voltage stress across the windings between layers (use ASCII
non-proportional font to display properly). With Pie-shaped windings
(with fewer turns/layer), this is not as important.
oooooooooooooo Layer N+1
|_____________
|
oooooooooooooo Layer N
Let's review the 3H charge inductor and .1uf tank combination: Using
Richie's standard F-res first equation formula and assuming a low loss
inductor I get 291 hertz. How do you arrive at 600hz? Since I will have a
lower break rate, is increasing the inductance slightly reasonable? This
brings down the frequency of resonance even lower, yes?
I was referring to the maximum break rate (BPS). The maximum break rate
is 2X the resonant frequency (Fo) of the charging circuit. Using an Fo
of 291 Hz, the maximum break rate would be 582 BPS. The 2X factor is due
to the tank cap taking 1/2 cycle to resonantly recharge after each bang.
I rounded up the maximum BPS to 600 BPS since this is not a very "hard"
limit. Also, my numbers may differ slightly from Richie's since I use a
more exact spreadsheet model that takes into account the effects of
charging reactor resistance and DC link capacitance. I get about 293 Hz
for Fo versus 291 HZ using the lossless case on Richie's site - close
enough for government work. :)
Bert I really want to extend my appreciation for your help on this long side
lined project. This Is turning out to be a "learning opportunity".
Viva Lichtenberg discharges, shrunken quarters, and long streamers!
You're quite welcome. It's been an interesting task trying to make use
of your three-phase relic. :)