Anon01 wrote ...

Hello guys. I'm upgrading parts of my good old SGTC from the 6th grade. I've already ordered the MMC parts, a motor for the SRSG (Help later), and I am constructing a 6MOT stack PSU for it. Although, this setup has a 4" secondary, with 4-6kW input. At this point, I have 1/2 the parts needed for a new complete SGTC, using a 8" secondary. Or, should I make a 8" secondary and just raise it a bit

Specs of the current coil:
12000V 60mA NST
Vacuum static spark gap using a large shop vac and 1" copper pipe
Primary coil is .375" copper tubing with .5" spacing braced to 6 sections of 1/2" PVC via zipties, overall of 14 or so turns. 5.5" ID
Secondary is 26" of 20AWG wire(Huge, I know) around 4.25" PVC(Thinwall) quad-epoxy-varnished.
Topload I believe is 10" Alu duct, with an OD of 22"
If I remember correctly the ResFq is/was 265kHz or so
Caps I used were three buckets of salt water caps, totaling about 9-10nF

Specs of upgraded coil:
~8000V 500mA MOT stack (6 wired at 3 MOTs in series per leg of 240, 80V/MOT, totaling 8kV with series secondaries. Immersed in non-detergent motor(Help with other types of oil) oil with an oil exchanger once it starts to get hot, ballasting is going to involve MOTs as well. Need help here, either separated cores or just shorted secondaries. I have a set that will ballast it down to 40A total at 240V, still too much, resulting in 9.5kW)
SRSG at ?BPS(Help please)
Primary coil is going to be the same
Secondary is going to be the same, although I'm considering making a 8" secondary, using a cone to hold it in the same place, making it a lower ResFq, bigger/more caps needed, etc. Original reason for posting, then it grew into this. :)
Topload is going to stay the same.
I have 51 940C30S47K-Fs, 3000VDC 47nF planning on making a MMC of 17 strings with 3 caps per string, so I have a massive amount of capasitance/voltages possible.
Lowest amount I can plan on using is 2 strings of 6, totaling in 15.67nF @ 18kV, tuned at 7-8 turns.
Although, I can do 8 strings of 6 getting 62.6nF @ 18kV, tuned at 3-4 turns. Is this too little? What is a good amount of turns for good coupling?

Alright, for the total questions for those who didn't read all of it ;)

Should I make a whole new coil because of 4" coilform?
What is a good number of BPS for the suggested system?
How to make the MMC, because VAC is 750 and VDC is 3000?
Good type of oil to use for MOT insulation+cooling?
Recommended type of ballasting with modded MOTs/etc?
And just general criticism/comments is fine.

Thanks,
Sam.

Kristian wrote ...

Sam,

A good rule of thumb for designing a secondary is between 4 to 1 or 5 to 1 height to diameter ratio and between 800 and 1200 turns in the given length. If the secondary coil form is 8.5 od then you would want at least 34" of winding. 20 gauge wire will put you at about 977 turns which is right where you want to be.

The dimension of your top load doesn't make sense to me. 10" diameter aluminum duct at 22" od? To describe the toroid dimensions I'll use minor diameter = duct diameter and major diameter = over all od of the toroid. A good starting point for toroid dimensions is minor diameter = the diameter of your secondary and major diameter of .75 - 1 x the length of your secondary winding. Going by the above suggested secondary dimensions, your toroid should be minor = 8.5 diameter duct around a 17" disk to give you major = 34" od and about 33pf. Finding 8.5" aluminum duct will be difficult. I would recommend using a stack of two toroids. A 6" minor, 34" major stacked on top of a 4" minor, 28" major would equal the recommended 33pf. Another benefit of using the stacked toroid design is that the smaller toroid creates an e-field under the bigger one. This will "push" the sparks outward instead of them wanting to go downward, towards the primary.

I'm not quite sure what you are saying about your transformer stack. Are you running a 240V circuit in series to the primaries on each leg for an input of 80V/mot? If this is the case, I calculate your output to be 8400V at 333mA for 2.8KVA. If you want to get to around 8000V at 500mA, why not use a 4 mot stack with 240V in series to the primaries and the secondaries in series? This will give you an output of about 8400V at 500mA for about 4.2KVA.


As far as primary capacitance, the resonant value will be dependent upon the inductance of your transformers PLUS the inductance of your ballast, so remember to add that into the equation when determining the resonant cap value. I have never used mots for a ballast so I can't offer any experienced advice on that subject.

The DC voltage rating of the MMC should be at least 2x the transformer output. This is due to voltage stress on the dielectric of the capacitors. If the voltage rating of the cap bank is too low, the dielectric will fail and the cap will eventually short out. Cooling the bank will not offer much protection against this.

If you remove the inner turns of your primary, shoot for about 1.25" clearance between the two coils. If you need to adjust the coupling, you can do so by moving the secondary up or down in relation to the primary height. You may want to use the copper from the inside turns to add to the outside turns, if more inductance is necessary (determined by cap size and secondary frequency). Coupling and primary inductance are two separate things.

The break rate of a synchronous rotary will be a multiple of the input frequency, usually 2x input frequency, so 120BPS at 60Hz. To achieve this at rotation rate of 1800RPM you need 4 flying electrodes. 1/25 hp is not going to turn a very large rotor, and 1/8" diameter electrodes are not going to handle very much input power without getting ripping hot. With that motor, I think you would be limited to a design that would handle less than 2KVA.

If you are going to use a synchronous rotary gap, give yourself more headroom on the resonant value, say at least 2x larger than resonant. This is due to the fact that the gap is firing at a set rate determined by the speed of the rotor, not by the distance of the electrodes as in a static gap. A static gap will fire when the voltage in the cap reaches the breakdown voltage of the gap setting, limiting the charging time of the capacitor. A rotary gap allows the capacitor to charge for a set amount of time, and cannot discharge until the electrodes are aligned. This will put added stress on them especially if the gap misses a firing, allowing the cap to charge for twice the amount of time.

The LTR cap value starts to get impractical at higher power inputs. To get around this, the guys that use pigs or PT's usually run around 2x Smaller than resonant with an asynchronous rotary running at much higher break rates, around 300 - 400BPS. This is the route I would also take for a mot powered rotary gap coil.

Have you tried JavaTC,

Anon01 wrote ...

Kristian,

I really appreciate the advice, I really do, but this is what I have to work with, the only thing that has answered any of my previous question is the BPM rate, and the MMC rating, which was also elaborated upon. A string of six is over 2*Vin. (3000*6 = 18000) :\ Back to the spark gap, I was going for a Terry Blake style gap. I have all the equipment needed to do such, and he uses 3/16" electrodes with 7.5kW through it. Safely issues resolved, using snap rings.

The secondary, I've seen many sites that say large(10-24) coils would be around 3:1 or so. Anyways, the faraday cage I have built for it is made for the height is it at anyways.

About the power supply, I am contacting a good friend of mine that does the local G&E cleanup. The original MOT stack was indeed 3 per leg of 240, with 80V across each primary. Reason being is that I wouldn't have any real insulation or heating problems like a 4 stack, it has active cooling anyways. I have a really nice variable 1-10kW PFC cap I can use that would be nice in this setup, to negate large phase problems etc. It's estimated that I could get up to 1A + depend on how long I want to run it under cooling conditions etc with the 6MOT setup. :\ I'm trying to do this (edit: relatively) cheap and with the materials I already have.

The toroid, I am going to have to deal with. I have already spent about.... $80 in caps in this, and planning on 50-70 for the wire. I still am wanting to sell my 833As or in trade for this wire. :)

I indeed appreciate the effort put into that post, and the advice given but it has addressed mostly subjects that have been resolved, although the LSR on the caps is a new subject I completely forgot about. I'll have to consider that. If you could elaborate on the designated questions/subjects it will be great,

Sincerely, thank you,
Sam