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Blitzorn

Wed May 27 2009, 09:16PM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

okay i was just in touch with my source for my transformer, and it turns out that he hasn't actually got a 12kV 30mA NST, but he sent us two 7.5kV 30mA NSTs instead on rush deivery, and was very apologetic about the entire thing. In some ways this is a plus, i get my transformer sooner, i get a spare, and he's willing to break me a deal for a 12kV once he gets it in stock again. As i understand it, moving down in voltage is not a huge problem, it just means i will need a bigger capacitor and as such need to tap elsewhere on the primary coil. I'll run the numbers this afternoon and see what i get, but does anyone know of any problems caused or necessary adjustments to be made when changing transformer power?

Herr Zapp

Wed May 27 2009, 11:36PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

There are a few things you need to be aware of when shifting from a 12/30 to a 7.5/60 power supply:

1. The resonant capacitor value will change to .02uF, so your target value for a "larger-than-resonant" tank capacitor will increase to around .03uF. This is significant if you are planning to use salt-water bottle caps.

2. The lower voltage will make spark gap adjustment quite a bit more critical, since the "total" gap distance will be less than it would have been with a 12kV NST.

3. The higher output current will make spark gap cooling more critical; you definitely will need to have plenty of air movement for cooling. Since the lower voltage will force you to use fewer gap segments, each segment will be dissipating more heat.

4. The "good" news is that the pair of 7500v NST's, with the correct value tank cap, can deliver a slightly larger "bang size" (energy pulse) at every gap firing. This may yield slightly better performance than you could get with the 12/30. However, the lower firing voltage will make spark gap adjustment more critical, and the gap will be more sensitive to the oxidation that occurs on the surface of the copper gap electrodes.

Regards,
Herr Zapp

StevenCaton

Thu May 28 2009, 12:48AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Blitzorn, you do know how to connect the two transformers in parallel right?

If the transformers are indentical, you could probably get away with connecting the AC hot to one transformers primary terminals, and the AC neutral to the other primary terminal (and then do the exact same way for the other transformer)
However, this is how to do it if one has two transformers that are not identical, but of the same amperage and voltage specs. (I recommend you do it this way)

First, get an extension cord and hook it up to one transformer, then take the conductors from the SAME extension cord and hook up the other transformer. So when you plug this cord in, both transformers will be receiving 117VAC into their primaries from one extension cord. (two primaries in parallel basically) Let's assume your first transformer has two High Voltage leads labeled A and B, and that your second transformer has two High Voltage leads called C and D. Plug the two transformers in and see if A will arc to C. If A connects to C with no arcing, then they should be connected together. If A does produce arcing to C, then A is supposed to connect to D. Once you've connected two HV terminals successfully, then the other two will also connect successfully.

Hope this made sense.

Blitzorn

Thu May 28 2009, 01:00AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Herr Zapp-

1. you say 7.5/60 although i'm pretty certain they're 7.5/30... are you suggesting running both of them on one coil? This is something i'd be more than happy to try but don't know how to wire. At 7.5/30 however, my calculator says that LTR cap should be 17.2...

2. if i had eight electrodes for a total of up to seven gaps, would it make sense to have them 1 credit card width apart?

3. Duly noted... i think i'm probably going to stick with my shopvac despite the obnoxious racket.

4. Will having higher circulation reduce corrosion, or is that a separate issue?

Herr Zapp

Thu May 28 2009, 02:25AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

Your "substitute" NST's are each rated at 30ma secondary current.

Since you will (supposedly) be getting two of them, it's only rational to use both of them, connecting their outputs in parallel so you effectively have a 7.5kV, 60ma power supply.

The only potential problem with this is that both NST's must be connected such that their outputs are "in-phase". Since the output of a transformer is AC, the voltage at the HV terminals will be alternating between +7.5kV and -7.5kV 60 times per second.

Assume that you have the HV output terminals of both transformers connected in parallel, and the input (prmiary) terminals are also connected in parallel. We want the "phase" (polarity) of the HV output voltage of both transformers to be same at any point in the AC sinewave, so we have a net 7.5kV across the HV terminals. If the transformers are "out-of-phase", the output of one will be swinging positive while the output of the second transformer is swinging negative, and the voltages will cancel each (no net output).

It's easy to "phase" the transformers, but you should probably first verify that each transformer is developing full output. (It seems that your transformer supplier may not be 100% reliable.)

Individually connect each transformer's primary terminals to an AC linecord. (if you have your variac figured out, use it for these tests.) Connect a piece of stiff, solid wire (#12 or #14 household wire, coat-hanger wire, etc) to each HV bushing, and form the wires so the free ends are about 3/16" to 1/4" apart. This is a spark gap that will provide visual evidence that the transformer is generating HV output. Apply line voltage (or slowly crank up the variac), and there should be a continuous, bright blue arc between the ends of the wires.

After verifying that both transformers are generating HV output, proceed with "phasing":

Connect the primary terminals of both NSTs in parallel, and to an AC linecord (or the variac).
Connect the HV terminals of both NSTs in parallel, and again connect the stiff wires to the HV terminals of one transformer.
Apply 120VAC to the NST primaries (again, use the variac if at all possible).

If there is no arcing present at the wire spark gap, reverse the wires to the primary OF ONE TRANSFORMER ONLY. Repeat the test, and there should now be a bright, healthy arc between the ends of the wires. Your transformers are now properly "phased".

Regards,
Herr Zapp

Blitzorn

Thu May 28 2009, 04:09AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

So maybe i am misunderstanding, but wouldn't having them both running off the same AC linecord reduce available amperage, especially with a long extension?

StevenCaton

Thu May 28 2009, 04:34AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

So maybe i am misunderstanding, but wouldn't having them both running off the same AC linecord reduce available amperage, especially with a long extension?

No, If one was to build a tesla coil with one single 7.5KV 30ma transformer, it would probably pull 3.5 amps on a 117VAC line. (thats a rough guess) If you had two transformers it would double your input power so lets assume that your coil now pulls 7 amps from the line.

So if you have a 117VAC line that is capable of delivering 20 amps (typical outlet in a garage) then it could power your tesla coil easily.

The copper losses in an AC linecord are insignificant right now, don't worry about that.

Do you feel like you understand this now, and do you think you would be able to properly phase two NST's. If not, keep asking because the clock is ticking.

Blitzorn

Thu May 28 2009, 05:07AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Yeah i'm pretty clear on it now, i was just doing a quick sanity check. Transformers will probably be here tomorrow afternoon so i'll give it a whack then.

Good news - Made good progress on spark gap and primary coil today!

Bad news - With this setup i'll need 50 SWC bottles... *sigh* it's gonna be a long weekend...

Herr Zapp

Thu May 28 2009, 05:11AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

A "normal power factor" 7.5/30 NST draws approximately 1.9A from a 120VAC line.

A "high power factor" 7.5/30 NST (with an internal power-factor-correction capacitor) draws approximately 1.1A from a 120VAC line.

As the high power factor NST are more expensive, and harder to find than the normal power factor NSTs, I'll assume that what you are getting will be the normal power factor units.

Regardless, ~4A draw from both transformers is not a significant load for a typical 15A or 20A, 120VAC circuit. If you use your shop vac on the same circuit, and it draws 10-12A, AND you power everything from a single long extension cord, then you might see some voltage drop.

Make sure the combined load of the NSTs and the shop vac doesn't exceed the rating of whatever branch circuit you plug it into, and if you run the coil a long distance from the outlets use two cords, one for the vac and one for the coil. If you are only using an 8-15' cord, then don't worry about anything.

I only mentioned the "long cord problem" because of a voltage drop problem someone had when running a BIG leaf blower cooled spark gap plus a bank of 4 NSTs off a single 50', small gage extension cord

Regards,
Herr Zapp

Blitzorn

Thu May 28 2009, 05:33AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Hahaha i can see how that would get messy!

No, chances are my extension cords are going to be on the order of 10-20', maybe more if i end up demoing outside. I'll just plug the shopvac into a different... um... how do i know it's a different circuit? Can it be the other plug on the outlet? a different outlet? a different breaker box?

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