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Large DRSSTC Tesla Coil - Colorado Springs

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Landondesi

Mon Jan 11 2016, 02:53AM

Registered Member #58250 Joined: Wed Jan 06 2016, 05:38AM
Location: Colorado Springs, CO
Posts: 4

Hello,

I am part of a Makerspace in Colorado Springs, we are building our own large coil near Tesla's old laboratory in Colorado. We've begun construction as of a few days ago. At this point I have a couple of questions.

One, we have a nice HP signal gen and numbers of scopes: today my self and two other experienced electrical engineers were a bit perplexed by how to properly test our secondary for calculated resonance. Many sites suggest simply connecting the signal gen open ground to the secondary and then using the scope probe as an antenna to read back the emission.... In all reality this seems a bit loose, is there a better method?

Secondly, are there adverse affects to the coils operation by using larger secondary winding conductors? Most coilers seem to be affixed near the 30-22 awg range.

Graham Armitage

Mon Jan 11 2016, 03:41AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Sounds like you have a fun project going on. As for the secondary measurement, I have used the method you described and it simply works. It's easy and I have found it very reliable. You must keep the coil away from other objects as objects close by can have a huge impact on measured frequency.

On SG coils I have used larger wire and it worked well. I typically work backwards from the power output -> resonant frequency-> physical coil dimensions -> number of turns and lastly the AWG that gives that many turns. Hope this helps.

Landondesi

Mon Jan 11 2016, 08:01PM

Registered Member #58250 Joined: Wed Jan 06 2016, 05:38AM
Location: Colorado Springs, CO
Posts: 4

Graham ,

Thanks for the reply! Everything rung out as expected today and the secondary is now complete at 54" tall x 10.75" in diameter. This week we will have the acrylic base, drive circuitry and primary done.... So close to live testing!

Mads Barnkob

Tue Jan 12 2016, 09:39AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Landondesi, welcome to the forum.

You came here with some specific questions to the problem you have in your head right now, which makes perfectly sense to ask for you as you know all about your own plans and build.

For us to be able to help you, describe all the specifications of your coil for us to have an idea of how to help you. We might give you an okay answer isolated around wire size for a secondary coil, but it might be totally inadequate for your bridge or primary or some other dimension that is way out of best practise.

Uspring

Tue Jan 12 2016, 11:38AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Thicker wire implies less turns and therefore less inductance. For a given value of primary amp-turns you'll get a lower secondary voltage. For a big coil like yours, you probably have lots of primary amps, so that might not be an issue. You can also increase the number of primary turns to increase amp-turns, but that will increase the necessary burst length to achieve a given current.

Generally I'd prefer a larger secondary inductance. That also implies a lower secondary Q under arc loading, making tuning less problematic. On the other hand there is certainly a lower limit to wire diameter in order to keep heating in the secondary at acceptable levels.

As Mads said, this is really a systems issue and cannot be addressed unless more of the coil specs are known.

Landondesi

Tue Jan 12 2016, 04:40PM

Registered Member #58250 Joined: Wed Jan 06 2016, 05:38AM
Location: Colorado Springs, CO
Posts: 4

I've been doing research for some time prior to construction, most parameters are known well at this point. The answer being yes, we are shooting for a high amperage primary as the secondary is quite large and using the 18awg wire.

The specs thus far :

Secondary - 53.9 inches in height , 10.75 inches diameter, 1227 turns of 18awg , resonant at 106KHz

Toroid - Estimated 39pF , 36" Major Diameter, 8" Minor Diameter (Constructed of 7-8 rings) of 1/2" copper tubing aligned in arcrylic discs

Primary - 1/4" Copper Tubing, 1/4" spacing, 6" inner radius, Approximately 15-16 turns, Tapped near 9-10

Coupling - Undetermined as of yet, we are laser cutting some 1/4" acrylic with adjustable nylon mounting hardware to be able to adjust coupling , current plan is to have the pancake centered on plane with the bottom turn of the secondary and then have -+1" of travel from there

Final output drivers (Don't Laugh, this project is not self funded, we are working in conjunction with The Space Foundation and a few other donations) Currently I have a full H-Bridge of 600v 100a IXFB110N60P3 mosfets running 8 total with parallel pairs.... IGBT's were not yet in the budget, however we have people working on donations for these and my pre-drive + protection diodes , etc. have been designed to easily "upgrade" to them when available.

DC Bus Voltage - I have an industrial transformer, 240-480 single phase, Hoping to see AC in around 460-470 , so I'm anticipating that after the bridge rec I'll have stable DC at 500-550VDC

MMC - Looking at 3 parallel strings of 4 series 940C30P15K-F to produce a final cap rating of .115 uf , 12000 vdc

Pre-Drive / Audio Mod / Etc.. --- We are using a very basic PWM controller on board the coil which will be coupled to an atmel micro for precision control of Freq, Amplitude, Etc. This micro is housed isolated in an aluminum enclosure with two unique BOSA fiber modules 1550nm/1300nm duplex units. One channel to pass serial data for remote monitoring and control , the other channel to pass audio signal. I have a 50' duplex MM fiber for safe remote operation. Also utilizing current transformer on primary for feedback and over current protection.

Not sure what I may have forgotten to mention here but have to run to an appointment for a bit.....

Mads Barnkob

Tue Jan 12 2016, 10:33PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

A fast review of your setup.

Your secondary wire is a tad to the thick side, not a real probem, will properly work just fine with some 1000A+ pumping.

Topload sounds fairly sized too, corresponds with secondary dimensions.

Primary coil seems fine.

Using the suggested coupling from JavaTC works perfect for DRSSTCs.

Spare yourself a lot of trouble, just do not use MOSFETs in a DRSSTC. IGBT losses goes up linearly with primary current, in MOSFETs it goes up squared. You do the math and will quickly find out why you are never going to switch 1000A at 550VDC on MOSFETs.

Keep DC bus around 400VDC for 600V IGBTs and 800VDC for 1200V IGBTs.

Your MMC seems to be made for a SGTC, you need some more capacitance to get some good bright sparks, aim for 0.4 - 0.8 uF in a large DRSSTC. You also need to have it be able to withstand peak current, rms current, voltage across it etc. Try a MMC calculator

Uspring

Wed Jan 13 2016, 01:51PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Mads Barnkob wrote:

Using the suggested coupling from JavaTC works perfect for DRSSTCs.

Any idea what's the rationale behind this? I've always thought, that for DRSSTCs the largest coupling possible is best, keeping it low enough, though, to avoid flashovers and other undesirables.

Your MMC seems to be made for a SGTC, you need some more capacitance to get some good bright sparks, aim for 0.4 - 0.8 uF in a large DRSSTC.

I've tried to work out some scaling law for coils of similar power input but differing secondary inductances and bridges. With some approximations I found, that.

Cprimary * Rsource ~ Lsec

Rsource being the ratio of bus voltage and max primary current. The idea is to match the bridges output capabilities to the arc load. Or put in another way, the arc load should limit the primary current to the max the bridge can handle.
For a low Lsec, this implies lower primary C.

Landondesi

Mon Feb 08 2016, 05:52AM

Registered Member #58250 Joined: Wed Jan 06 2016, 05:38AM
Location: Colorado Springs, CO
Posts: 4

Figured I'd post some updates on the build. We are near completion and have done a few test runs with way too small of NST's laying around the shop. So far the coil holds up well with that little power (12kv @ 60ma) 30" max streamer length , we did score an old 32kv @ 250ma Raytheon transformer from a retired AF engineer out here in Colorado

The DRSS circuits are nearly complete and will be final tested this week, so that's exciting.

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