First DRSSTC SKM100

[Saattvik24]

Hello everyone, I'm a 20 year old uni student and building a DRSSTC was one of my life goals and I'm so happy to have achieved it. Here are the specs of my coil.

Bridge - SKM100GB12T4 in Full Bridge
Bridge Supply - 0-260VAC, 35A Bridge Rectifier, 4700uF 400V EPCOS Electrolytic Capacitor
Primary Coil - 7 turns of 1/4 inch copper tube (tapped at 4.833 turns)
Secondary Coil - 110mm x 540mm, 1370 turns, 28SWG enameled copper wire
MMC - 3s3p of Cornell Dubilier 942C20P15K-F for 0.15uF at 6000VDC
Toroid -  510mm x 127mm
Driver - UD2.7C Thru Hole
OCD - 400A
Resonant Frequency - Around 150kHz

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There are still some issues and questions that I want to ask.

1) In the video I've attached I noticed the OCD triggering (the red light) more for small arc lengths than for long arc. Is this abnormal or is it because of the buildup current required to extend the arc which the brings the coil into tune because of the reduction of the resonant frequency? Should I leave it as is or does it require better tuning?


2) The primary connection lead(the thick red wire) passes by close to the driver enclosure(the yellow box) which led to it heating up after long runs. I thought this won't be the case if the enclosure is grounded. Is it true and its not grounded properly? Or should I just increase the distance between them?

3) I've been playing around with the onetesla midi interrupter and the omd files I found online but not everyone shares their midi files (which is understandable). So I wanted to know what's the process behind making midi files tesla coil compatible or how to compose my own music for it? What are the softwares used and what are the general guidelines to follow while doing so.

4) In the primary coil section of JavaTC, I don't understand what does it mean by the ribbon height and thickness. What is this ribbon?

[flyingperson23]

1. Based on your arc length it looks like you could probably tune it better. You should be able to get at least 2x the secondary height for arc length. I'd also suggest increasing coupling (raise primary/lower secondary), the recommend coupling is for a SGTC. Try for closer to 0.16

2. Grounding isn't the problem. You should probably move your CT and have a more direct path from the bridge to the MMC, preferably a path that doesn't involve circling around the driver box.

3. Any midi should work. For the onetesla interrupter, stick to 1 or 2 notes at a time, and try to avoid super high notes. There's a bunch of midis here https://highvoltageforum.net/index.php?topic=118.0
If you want a more advanced interrupter, check out the syntherrupter or tmax's midistick. Both of those connect directly to a computer and act as a midi output, I use the midistick and play files through anvil studio which isn't great but it works.

4. Some people make primaries with copper ribbon in a spiral, that's what that's for. You're using pipe, not ribbon, so you can leave those slots blank.

[Benjamin Lockhart]

I have compiled all the midis I could find over quite a few years, and some I edited.
Here you go.
All MIDI Files.zip

[davekni]

1) In the video I've attached I noticed the OCD triggering (the red light) more for small arc lengths than for long arc. Is this abnormal or is it because of the buildup current required to extend the arc which the brings the coil into tune because of the reduction of the resonant frequency? Should I leave it as is or does it require better tuning?

Notice in your video that the short arcs occur on lower notes.  Arc path from previous note cycle (previous enable pulse) has longer to cool before next enable pulse.  Cooler path requires more voltage to ionize air again.  Primary current hits OCD before secondary voltage has been high enough for long enough to grow arc longer.  Yes, you are correct: Once arc grows somewhat long, secondary frequency drops to better match primary frequency, allowing more energy transfer to secondary without exceeding primary current limit, further extending arc length.
Tuning primary frequency slightly higher will increase arc length for low notes, but likely reduce arc length for higher notes as secondary frequency drops below primary due to arc capacitance.  You can test to see which behavior you prefer.  No one "right" answer.
Your coil has a couple similarities to my DRSSTC: Secondary impedance is below typical 50k, and coupling factor is on the low side due to tall skinny secondary design.  Both tend to exacerbate above tuning tradeoff.  If mechanical construction allows, you could try raising primary slightly to increase coupling.  Higher coupling transfers more energy to secondary for given primary current and detuning.  If coupling is too high, secondary racing sparks become a problem.  I had to drop from my original 0.147 to 0.141 due to racing sparks (primary lowered from 50mm to 25mm above bottom of secondary).
Without remaking secondary, only option to increase impedance is smaller top load.  But that itself reduces performance, so not recommended.

2) The primary connection lead(the thick red wire) passes by close to the driver enclosure(the yellow box) which led to it heating up after long runs. I thought this won't be the case if the enclosure is grounded. Is it true and its not grounded properly? Or should I just increase the distance between them?

Likely issue is induction heating, not grounding or other electric field issue.  Steel (ferromagnetic) housing heats very efficiently due to local AC magnetic field.  Either move cable away from enclosure or change to an aluminum (or copper) enclosure.  Or add an aluminum partial enclosure around that portion of steel enclosure.
Did the entire enclosure heat up or just part near wire?  Looks to be far enough from primary coil, but induction heating from primary coil's magnetic field is also a possibility given how easy it is to heat steel.

Great accomplishment!  Thank you for sharing.

[Saattvik24]

I've increased the coupling to approximately 0.141 and increased the OCD to 450A and got longer sparks. Max spark length so far - 1.2m. I want to increase the OCD to 500A or maybe even 600A but I think my MMC might not be up to the task, as its only 3s3p. I didn't notice them getting hot during runs, only a little warm. So are they capable of handling the higher current? I've attached the MMC Calculator output for it (I used the Primary Inductance-Ldc value that I got from JavaTC). The actual RMS Current is higher than the rated value.

[davekni]

I've increased the coupling to approximately 0.141 and increased the OCD to 450A and got longer sparks. Max spark length so far - 1.2m. I want to increase the OCD to 500A or maybe even 600A but I think my MMC might not be up to the task, as its only 3s3p. I didn't notice them getting hot during runs, only a little warm. So are they capable of handling the higher current?

MMC survival depends on several factors.  Given TC use doesn't require continuous use for years with low failure rate, RMS current can likely be higher than rated RMS current, likely by 2x.  Depends on cooling too.  Fans over MMC will increase RMS current capability, as does spreading out caps to improve cooling air exposure.
However, SKM100GB12T4 may not survive 600A.  My personal experience is more with TO247 style packages than with bricks.  2x rated peak current is often roughly what is required to fry IGBTs.

[flyingperson23]

There was a post somewhere where a SKM50 survived 700A. SKM100GB12T4 appears to be rated for 300A pulsed, so it might be fine with 600A but I wouldn't be surprised if it wasn't. If you want to push limits like that, it might be worth adding a thermal switch to your heatsink to disable the driver above a certain temp.

[Mads Barnkob]

Temperature rise at 11 degree Celsius is border line to problems in longer runs without good forced air cooling.

I would place SKM100s around 400-600A depending on resonant frequency, do the loss calculations! https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/ or scroll down to table of proved IGBT speeds/current.

[Saattvik24]

I think I'll stick with 450A or 500A at most, cause the components for this build were provided to me by my professor and I don't think he'll give me more budget to buy new IGBT's if I blow them up lol. Will just try to tune it a bit more, 1.5m arc length should be possible I guess.

Now I'll focus on building a better interrupter, as the OneTesla interrupter can only play 2 notes at once which is inadequate, as I will sometimes be displaying my coil and want to play good music

Also I want to say a big thank you to this community for helping me!

[flyingperson23]

Do you know what on time the interrupter is running at? You generally want higher on time for larger coils as each cycle takes longer. Onetesla coils are much smaller than yours, so if your interrupter is running at low on time it will limit your arc length.

[Saattvik24]

I think by default the maximum on-time of a OneTesla interrupter is set to 100us but it can be changed in the code. I thought I can't increase it any more because I'm already hitting the current limit. Am I wrong? Will higher on times prevent it from hitting OCD too early by allowing it to get better into tune with longer arcs?

[Benjamin Lockhart]

The older onetesla interrupter that I have goes to 150us. No you're not wrong, if you're hitting the OCD increasing the pulse width further won't do anything because the OCD is cutting the pulses short once the limit is reached.

[Saattvik24]

While tuning my primary tap, I didn't quite know about the upper/lower pole operation (still don't understand it perfectly ) so I went ahead and tried to match the primary frequency to match the secondary frequency exactly (both measured with only one coil in place at a time). But the results were not that good so I tuned it experimentally. I ended up decreasing the number of turns (4.833 turns currently) until I achieved good performance. Now I know this must mean I'm running at the upper pole.

So I wanted to ask what is usually better? Running at the upper pole or the lower pole?

Or is it so that for musical operation upper pole is better and for normal operation lower pole is better for longer arcs?

[flyingperson23]

When measuring secondary resonant frequency, make sure you add a wire to simulate arc loading

[davekni]

So I wanted to ask what is usually better? Running at the upper pole or the lower pole?

QCW coils usually run at upper pole frequency.  Normal DRSSTCs are a bit more complex to describe.  Upper and lower pole frequencies describe steady state.  For short enable pulses, DRSSTCs don't reach steady state.  Currents are amplitude-modulated as energy sloshes back and forth between primary and secondary, a mix of upper and lower pole frequencies.   For long on times (typically for higher impedance coils), operation will settle to one pole.  Though that pole can change as arc loading changes secondary frequency.  Once secondary frequency drops below primary, operation changes to upper pole for remainder of enable pulse duration.

Typical DRSSTC tuning is with primary frequency below secondary (below unloaded secondary frequency).  Improves performance as arc capacitance reduces secondary frequency to match primary.  Matching frequencies with simulated arc loading (wire) is one way to get there.  More common is to just try a specific amount of detuning such as primary 10% below secondary.