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Marko

Tue Sept 26 2006, 12:19AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hm, ballasting the thing properly seems to be one of problems.

guessing he can't get such a ballast, would something like capacitive primary-side ballast work?

Tim Koene

Tue Sept 26 2006, 08:01AM

Registered Member #37 Joined: Sun Feb 05 2006, 05:35PM
Location:
Posts: 13

I have a few 8a and 16a variable inductor style variacs, not the new galvanically separated two coil thingamajigs. Those could be used as a variable inductor. Oh and 4KW is a rough estimate. The PT is like This One only about 3 times as large. And maybe 60"might be a bit large, think i'll make it 40-50"then.

I will get on those capacitor readings today. Thanks for all the replies.

(Hiya Steve Conner, my time cone is in storage for the time being ;) )

Hazmatt_(The Underdog)

Tue Sept 26 2006, 11:49PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I'm finishing my simulations for my 1KW Tesla Coil for CSULB right now, and I must say that its going to give us over 6 foot sparks, which is plenty for demonstrations.

Here is the major question I would ask myself: How much power do I really need?

High power comes at a large price because of mobility, breakers, huge sparks, power controllers, safety, and of course grounding.

* A 4kw sgtc can't easily be demonstrated in a lecture room or hall because of breakers, grounding, etc. and the shear weight of the beast!! Must be close to 400 lbs of equipment ...maybe more if you have multiple controllers for the input.

* Ballasting is a big problem. Large inductors with large sliding armatures increase weight and size of the control pannel and its components. You may consider using an SCR on-time phase controller. This is my plan for my 5kva step down for my PT.

*Grounding, do I need to say more? AT 1KVA I'm probably looking at 30 Amps RF into the ground. 4KVA well....that's enough to drive a pretty nice size motor.

*36KV input! Man....that cap will have to withstand huge voltage reversals. You may need 100KV+ ratings on your capacitor. Also, my Cx bank composed of doorknobs had LARGE flashovers from plate-plate at a pewny 12KVAC input.

*The ringdown voltage and reversals are going to kick your input voltage WAY up there. Your oscillating voltage could be up there at 160,000V or more!! This is a big dielectric problem for all components, not just primary or Cx!!

I would get my hands on PSpice or something and start a simulation right away and see what it gives you.

Just some thoughts.

Matt

Steve Ward

Wed Sept 27 2006, 02:21AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

*36KV input! Man....that cap will have to withstand huge voltage reversals.

Just as much reversal as any other TC... I dont see your point.

*The ringdown voltage and reversals are going to kick your input voltage WAY up there. Your oscillating voltage could be up there at 160,000V or more!! This is a big dielectric problem for all components, not just primary or Cx!!

I dont know where you are pulling these figures from, and why you seem to think that 36kVAC input is any more "troublesome" with respect to the possibility of the capacitor voltage increasing beyond the supply rating. This TC will play by the same rules of electrical circuits as any other TC.

The tank cap should be rated for 2*Vpk (due to a near 100% voltage reversal), which in this case is about 100kV. MMCs (using film/foil PP caps) can usually really push the rating, but, if this is for a professional system, its best to use a proper rating.

I suggest using inductive ballasting to the effect of making your tank capacitor "LTR" (larger than resonant). You can figure out the resonant cap value by (mathematically) reflecting the primary inductive ballast to the secondary side (multiply by the turns ratio) and solving the equation for resonance at 50hz. Its been found that its pretty safe to then to use 1.5X this value to avoid the possibility of the capacitor voltage ringing up over many mains cycles.

The 50kV charging voltage presents some interesting issues with spark gap design. You would likely want to run a rotary gap at this power level. I would definately suggest using a series static gap as well (with forced air across it). Perhaps use large rounded electrodes for the rotary gap to prevent "long" sparks from forming between the electrodes. The series gap should help with this too.

You may consider using an SCR on-time phase controller. This is my plan for my 5kva step down for my PT.

Have you considered the amount of "noise" that will put back into the line?

HV Enthusiast

Wed Sept 27 2006, 03:11AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

I believe he is stating that the voltage reversals in this system which utilizes a 36kVrms transformer are considerably higher than that of the typical 14.4kVrms powered Tesla coil.

Personally, i would do everything i could to get rid of the 36kVrms transformer. That higher voltage just adds lots of complexities to the system and its components. I remember in several of Richard Hull's Tesla coil tapes that he was using 36kVrms potential transformers for some coils and if i remember correctly, stated that the higher voltage posed many unique problems which required specialized solutions which were not straightforward or obvious.

Tim already stated he had full funding for the project, so i'm sure he would be able to procure an inexpensive pole transformer. Dr. Resonance would sure be able to ship him one if one couldn't be found anywhere.

Believe me, the change from 35kV to 14.4kV will make MANY things much simpler.

Hazmatt_(The Underdog)

Wed Sept 27 2006, 03:26AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I just took a look at my simulation profile and my ringdown voltage is about 20kv pk-pk from a 9kvac transformer.

36KVAC would then suggest that the peak ringdown voltages would be in excess of 72kvac, which is not as bad as I had thought, but its still a big insulation problem for convienent materials like wood. The primary might arc to itself or corona badly at these voltages.

I did mean the magnitude of ringdown voltage is very large in comparison to my system.

I'm going to stop being helpful because it just ain't worth it.

Matt

Tim Koene

Wed Sept 27 2006, 08:52AM

Registered Member #37 Joined: Sun Feb 05 2006, 05:35PM
Location:
Posts: 13

Hi everyone,

I have been doing some research on the potential transformer in question, the exact ratings were missing because the faceplatge was screwed off. It turns out that it is a 3 phase line potential transformer and it has an output rated of 36kv/sqrt(3) which is closer to 20KV, which should pose considerably less problems.

Furthermore I would like to stress the point that I am aiming for about 1-1.5 metres of streamer, so a pole pig would be a bit overkill. Maybe when this medium sized coil is done and approved they want a large >>10KW setup.

I have some nicely designed forced air single electrode sparkgaps, (Which I could put in series) or I could make a nice 6-10 piece series gap. I would like to avoid rotary design unless really necessary. It adds complexity in the building, possible service when it breaks down and does not add to the understanding of students of the inherent simplicity of the tesla coil.

Next week I will probably start building a test setup. Thanks for all your input!

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