Author Topic: My first DRSSTC on bricks  (Read 3253 times)

Offline Mads Barnkob

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Re: My first DRSSTC on bricks
« Reply #60 on: May 03, 2019, 07:49:06 PM »
I found this ebay listing with 0.1uF 2.5kV paper in oil capacitors:
https://www.ebay.com/itm/K41-1a-Paper-PIO-pulse-capacitor-0-1uF-10-2-5kV-Lot-of-2/251259264733?_trkparms=aid%3D888007%26algo%3DDISC.MBE%26ao%3D1%26asc%3D20131227121020%26meid%3Dc3967893df3249af835219ad5cd172cc%26pid%3D100009%26rk%3D1%26rkt%3D1%26mehot%3Dag%26sd%3D162844731444%26itm%3D251259264733&_trksid=p2047675.c100009.m1982

Does anybody have experience with this kind of capacitors?Are they any good for a MMC?

Another thing: in series connection the overall pulse resistance(dVdT) of the MMC gets multiplied by the amount of series capacitors? In this case the voltage gets shared by the series capacitors and they see a smaller voltage rise across them,so 1.8kV/us capacitors would already be good with 4 in series at 6kV across the MMC?
I just saw Futurist's DRSSTC1 and he used C4BS capacitors,which have around 1kV/us rating if I remember right and they worked fine for him...

I doubt old PIO capacitors will last long in a DRSSTC, unless it is some special Russian secret cold war indestructible super caps :) As mentioned, their losses at high frequency is most likely very bad, but check their datasheet if you can find one.

I think you are mistaken about the AC voltage rating needed for a resonant capacitor vs. dV/dt, dV/dt is simply a value derived from capacitance and the peak current it is rated to pass through it, and the pulse capacitors we use are almost always high peak current specced or low capacitance, so there is rarely a case of dV/dt being broken.

I wrote the math down used in my calculator, just beneath it: http://kaizerpowerelectronics.dk/calculators/mmc-calculator/

If you want to design your MMC correct, I give 3 alternatives to do so here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/mmc-tank-capacitor/
« Last Edit: May 03, 2019, 08:47:43 PM by Mads Barnkob »
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline Laci

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Re: My first DRSSTC on bricks
« Reply #61 on: May 04, 2019, 09:48:14 PM »
I did some more calculations and choose the KEMET R76TR31004030J. 4 in series, 8 in parallel gives 0.2uF at 6400V,so around 850A peak. They are cheap and common so I can update the MMC in the future if needed.Their RMS current rating is just under 10A around 100kHz.This gives around IRMS=80A for 8 in parallel.



I calculated a maximum of 100 bps at 800us for the 80A IRMS. What do you think about this choice?

Offline Laci

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Re: My first DRSSTC on bricks
« Reply #62 on: May 12, 2019, 09:49:02 PM »
After waiting a week for the components and another few days of work, I could finally make some better tests.It looks like with longer pulses at lower peak currents I get good results,just like with short pulses at higher currents.I tested midi notes up to 300A peak and transient up to 500 and the midi notes can already destroy 4A fuses.I'm a bit worried about huge sparks,because I blew up my SSTC2 bridge yesterday. I was running it from mains ground and cranked up the power to maximum.A secondary - primary flashover killed it...Stupid!...RIP 2pcs HGTG30N60A4D...

The MMC is the same that I planned, 0.2uF 6400V KEMET R76 series, around 80A IRMS. They are working,but didn't make any measurements yet.
I painted the metal enclosure blue,just like the MMC and added plenty of screw terminals.

For higher power running I want to make sure that it won't kill the bridge. I don't have strike rail and bus negative to earth capacitor.Adding these should hopefully save the bridge from flashovers. Another thing that might result in a fail is the IGBT losses.I have to calculate this...

The sparks already scary to me, I have to get used to them. :)
One bigger spark from the video, spark length is as much as secondary + topload length:












« Last Edit: May 12, 2019, 09:50:47 PM by Laci »

Offline Laci

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Re: My first DRSSTC on bricks
« Reply #63 on: June 15, 2019, 09:27:30 PM »
I posted here quite a while ago. Not because I didn't want, but I made small changes and I needed time. Some of the changes I made:
-cut the primary at 5.75 turns and added a strike rail
-added a 0.22uF 1000V MKT capacitor from the heatsink to the bus negative
-paralleled two 50A 1000V full bridge rectifiers after the single one blew up
-added a 10A circuit breaker and a 7.5kW contactor
-finished the UD3 by using the bus voltage sensing circuit and installing the LEDs on the metal enclosure

The parts used up all of the space under the coil, so wires are everywhere.
I wanted to upgrade the 30A current sensor to a 50A, but either it was factory defective or I overheated it while soldering and it doesn't work.

The cheapest communication was bluetooth for me, the android app can already control the UD3 without being able to play MIDI files. For that I got myself a USB-BLE dongle and tried to use it as a master device to connect to the slave HM-10 module on the UD3's side. Sadly the dongle doesn't want to automatically connect to the HM-10 after trying many settings...











While testing a big Tesla coil fan appeared. :)




Offline Laci

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Re: My first DRSSTC on bricks
« Reply #64 on: June 16, 2019, 09:00:31 PM »
I made a test a few minutes ago. Plenty of things confirmed to work, like temperature sensing, fan control and internal fuse.
After around 10 minutes of not continous run I smelled something melting or burning and turned the coil off, but I couldn't see any problems so that might be just coming from the neighbourhood.

The goal was to achieve sparks that can hit the floor. The breakout point is 110cm over it and at 800A, 800us I was hoping this to be possible. For some reason no overcurrent occours over 600A which should be displayed by the LED. Can something prevent the primary current from ringing up, or the UD3 goes too early in freewheeling/limits ths current? Maybe because my primary frequency is tuned too low(Fpri=120kHz Fsec=135kHz)?

Offline Laci

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Re: My first DRSSTC on bricks
« Reply #65 on: June 24, 2019, 09:26:47 PM »
After doing many measurements, the only thing I can think of that limits the spark length is inadequate tuning. The secondary Fres is 136kHz with the "bottom FG, scope antenna" measurement.
The primary is a bit more complicated. Right now the driving frequency is 120kHz which is the lower pole. I suppose this is a bit too much undertuned and some heavy spark loading would be required to lower the secondary's frequency to this level.
The upper pole is already over the secondary's frequency, so basically the secondary is between the primary poles.
Steve Ward mentioned in his DRSSTC log book:
Quote
I found something interesting with the tuning of this coil now. Originally the primary was tuned somewhere in the middle of the 2 modes. This worked well with very high coupling as normal, but I did see some limit to my spark length (about 24" max). I really liked the idea of tuning to the lower pole, because when the coil starts making streamers nearly 3X its length, its going to detune considerably! So I started tuning the primary lower. Instantly I noticed the primary current had a much nicer linear ring up, instead of the choppy looking current ring up when tuned in the middle. I had to reduce my coupling to about .15-.2 because arcs began racing up the side of the secondary, and jumping to the primary nearby. When tuned lower, it took more input voltage to get it to produce real streamers, but once streamers start forming, the result is like an explosion of streamers! Looking at the primary current, its forming a notch at the end of the burst (current rises and then returns back to 0). If I turned the
power even higher, a second "burst" began to form after the notch. The notch occurs at about 18 cycles or so... and I'm not sure what causes it, but I think its a sign that the coils need to be tuned better. So I slapped on a turn of 12 awg at the base of my primary to get more tuning room. Now it takes even higher input voltage (about 60-70% input) but the resulting sparks are even longer! Interestingly, primary current increases with voltage input, until long streamers are formed, then the primary current doesn't increase any more at all! Right now I'm running about 420A. But even still, as I'm getting about 30" sparks, the notch is occurring again at the end of the burst... need more primary L! Its easy to see the improvement that adding more primary inductance has. I'm tuned at least 1.5 turns lower than before (the coil only had 5 turns to start with!),
obviously these streamers really do detune things a lot.

One other benefit I see to running at the lower pole is that you are reducing switching losses. You now have fewer RF cycles per burst length, and also, any delays in the gate driver become less significant as each half-cycle is now longer. I think my primary circuit is running at about 170khz, the secondary Fr is something like 220khz. Hey, but it works great so far!


It looks like the UD3 locks to the pole where the current is the highest. As I decrease the primary turns, so the frequency goes up, the upper pole starts to overtake which is already higher than the secondary's frequency.





The problem is that I see no overcurrent over 400A, which I think is caused by the weak energy transfer caused by too much undertuning. What am I doing wrong here?  ::)

Offline Netzpfuscher

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Re: My first DRSSTC on bricks
« Reply #66 on: Today at 07:46:25 AM »
I think you have run the tune command with the secondary in place? Then you see the secondary resonance around 131 kHz in the graph. The secondary is sucking energy out of the primary.

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Re: My first DRSSTC on bricks
« Reply #66 on: Today at 07:46:25 AM »

 


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