First coil, Medium DRSSTC

[AeraCura_]

Hi all,

After much consideration I'll soon begin construction of my first coil. I've purchased most of my parts but would encourage feedback on my proposed design.

Design details as follows:

• Bridge: 2x CM300DY-24H in full-bridge
• Bridge supply: 2x 6800uF at 350V, configured in series for voltage doubling 240V at 3400uF 630V
• Bridge rectifier: IXYS VUO110-12N07, 3~ rectifier choose due to same terminal height(30mm) as CM300 IGBTs
• Snubber caps: RBPS20591KR6GNZ
• Primary: 3/8" tubing with 1/2" spacing 10 turns taped 8.4 turns
• Secondary: 2000 turns of 28 AWG on 6.625" x 30" form
• MMC: 5s10p 942C20P15K-F for 0.3uF at 10KV
• Topload: 6.625" x 30" toroid
• Controller: Loneoceans UD2.7C
• Interrupter: TMaxElectronics MidiStick

JAVATC dump:


Overall I'm hoping this is an average build without many suprises, Ive basically designed for the most ununique setup possible as I hope to learn the basics before going bigger.

[acobaugh]

On paper sounds similar to my DR81 system: https://labs.cobaugh.io/hv/tesla/dr81/

With a bigger topload, MMC, and secondary, you could easily stretch that bridge even further down the road if you choose.

What peak current were you designing for?

I really think the limiting factor in your design will be the toroid size and distance from toroid to primary, if you want sparks longer than, say, 6 feet or so.

[Mads Barnkob]

Looks good and with plenty of head room on almost all components. You will have problems with heating in primary / secondary coil before IGBTs/MMC, so you are building it like a tank.

Sure it could easily drive a much large coil, but that will also cost a fair amount of additional money to get a bigger MMC.

Make sure to have enough extra primary coil turns to detune the primary up to maybe 20%? 10% is properly more reasonable...

Looking forward to see it come alive!

[AeraCura_]

Thanks for the info. I'm okay with my bridge being over spec'd, It's possible that I use this at annual fairs so having a coil I can dependably throw long on-times, hight BPS, or overall run times at is important.

I really think the limiting factor in your design will be the toroid size and distance from toroid to primary, if you want sparks longer than, say, 6 feet or so.

I was simply following a general rule that the bottom of the toroid is in the same plane as the top of the secondary and toroid major and minor diameter match the secondary. I've messed with these numbers in JavaTC and dont see much change in rF. However, JavaTC says nothing about spark length. Can you elaborate? Also any info on how to draw a correlation between parameter selection or what JavaTC spits out and spark length would be helpful. Is it really as simple as V = L di/dt. Decide on a spark length using the known kV/cm breakdown in air and spec secondary inductance and on time appropriately?

Make sure to have enough extra primary coil turns to detune the primary up to maybe 20%? 10% is properly more reasonable...

When you say detune do you mean drop or raise frequency? If I have 10 turns on the primary to produce rF of 38kHz and I tap at 5.8 turns to match the rF of my primary for 70kHz does that leave enough extra turns?

[AeraCura_]

8 months with no update, and its finished! ...mostly.

First the completed coil,





And a shot of soft switching,


After adjusting phase lead and feeling relatively confident that I would'nt break anything I decided to give this water filled pan all it had. Here it is glowing bright red after about 5 seconds. 


Unfortunaly I'll need to make a few modifications as the inner loops of my primary supports are melting, I knew this was a possibility as they're 3d-printed from ABS. I'm now considering some type of composite filament which has a higher melting point. I plan to borrow the FLIR thermal imaging camera from my work and scope how hot they actually get.

I also cannot correctly tap the tunning of my primary circuit due to differences in my calculated and actual resonant frequency. In my OP I calculated a secondary resonant frequency close to 70kHz loaded, I physically measured the resonant F of the secondary at 100kHz unloaded and 54kHz loaded. I guess my options at this point are to rewind my secondary, re-configure my mmc with more capacitance, or build a larger primary.


Despite not being able to tune correctly I was able to get a first light. Bolts were repeatedly reaching the ground which is 6ft from breakout. Bolts were thin and not very bright so I hope to see improvements there.

[davekni]

Unfortunaly I'll need to make a few modifications as the inner loops of my primary supports are melting, I knew this was a possibility as they're 3d-printed from ABS.

How about enlarging the inner support slots, then adding some insulating material?  Perhaps short sections of silicone rubber tubing round the copper pipe at the support locations, or even wraps of kapton tape.   Fiberglass would be better, but harder to hold in place.

[AeraCura_]

or even wraps of kapton tape.

I loved this idea so much, so I redesigned the primary supports with 2 more loops and had them laser cut out of wood. Results are spectacular!

[davekni]

I loved this idea so much, so I redesigned the primary supports with 2 more loops and had them laser cut out of wood. Results are spectacular!

I was actually suggesting thermal insulation to mitigate support melting, hoping you could make your ABS supports survive.  Electrical insulation for wood supports may work too.  I'm a bit concerned about long-term corona damage to the kapton, depending on how conductive your wood happens to be.  Likely fine in the short-term, and may last well enough into the future.  Thicker Kapton (more layers) will help longevity.

[AeraCura_]

I was actually suggesting thermal insulation to mitigate support melting, hoping you could make your ABS supports survive.  Electrical insulation for wood supports may work too.  I'm a bit concerned about long-term corona damage to the kapton, depending on how conductive your wood happens to be.  Likely fine in the short-term, and may last well enough into the future.  Thicker Kapton (more layers) will help longevity.

From my research the Kapton tape I purchased should be good for 7.5kV. Not sure how this translates to my application but I'm douting its necessity at this point as I see many examples of coils using wooden primary supports with no insulation. Time will tell.

I get occasional arcing from the primary to the secondary, from my research this seems to be known as the 'racing sparks phenomenon' which relates to coupling and tuning.

JavaTC spits out a coupling coefficient of 0.152 for my dimensions, seemingly too high.
I'll also admit that I spent very little time on tuning once I saw the results I was expecting so it could be a combination of issues.
I'll spend a bit more time getting the tuning down and experiment with raising the secondary.

Considering the arcing only happens with long on times and low frequencies does anyone know if this helps narrow in on the cause?

[davekni]

I'm no expert on racing sparks, but did have trouble with such.  Reduced coupling from 0.157 to 0.141 and added several more layers of urethane to my secondary.  Haven't had racing sparks since, but don't know how much of my fix is actually necessary.

[futurist]

Electrical insulation for wood supports may work too.  I'm a bit concerned about long-term corona damage to the kapton, depending on how conductive your wood happens to be.  Likely fine in the short-term, and may last well enough into the future.  Thicker Kapton (more layers) will help longevity.

I use laser cut wood primary supports without any isolation for both of my DRSSTCs and so far I didn't have any issues

[Mads Barnkob]

Great to see that you coil is finished! I would have loved to see more updates in the 8 months, but this is good as well

I like the folded busbar corners, great way to turn it without stretching or sawing/bolts, fewer bad connections in the future!

Clean switching waveforms!

I get occasional arcing from the primary to the secondary, from my research this seems to be known as the 'racing sparks phenomenon' which relates to coupling and tuning.

JavaTC spits out a coupling coefficient of 0.152 for my dimensions, seemingly too high.
I'll also admit that I spent very little time on tuning once I saw the results I was expecting so it could be a combination of issues.
I'll spend a bit more time getting the tuning down and experiment with raising the secondary.

Considering the arcing only happens with long on times and low frequencies does anyone know if this helps narrow in on the cause?

Too high coupling and badly tuned will give you racing sparks, but also damp weather conditions with high humidity, like in the start of the evening when its getter dark. A typical time to run a coil, but also with less than optimal conditions if you have condensation on your coil.

Long on-time will pull the coil more out of tune than shorter on-time, so this could be a reason for racing sparks at long on-time.

Low frequency interrupts could be because you have higher current discharges, from topload being able to get recharged between sparks. The high current discharges will result in a higher voltage wave front moving up the secondary coil and this is suspected to something like the "whiplash" problems described here: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/

Got any video of the coil running?

[AeraCura_]

Too high coupling and badly tuned will give you racing sparks, but also damp weather conditions with high humidity, like in the start of the evening when its getter dark. A typical time to run a coil, but also with less than optimal conditions if you have condensation on your coil.

Long on-time will pull the coil more out of tune than shorter on-time, so this could be a reason for racing sparks at long on-time.

Low frequency interrupts could be because you have higher current discharges, from topload being able to get recharged between sparks. The high current discharges will result in a higher voltage wave front moving up the secondary coil and this is suspected to something like the "whiplash" problems described here: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/

Got any video of the coil running?

Thanks for the explanation, It clear that I have some work to do.

Odd that my video isn't showing up. I switched over to my desktop and cannot see it either.
I've re-uploaded to Youtube.

I'll make a better video this weekend with the results of my tuning.