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Messages - Phoenix

Pages: [1] 2 3 4
1
 Yes you are right, it should reduce stray inductance a bit, but stray inductance is no problem here. Copper Disks in this size are also way harder to get and cost a lot more then simple busbar.

2
Stray inductance does not really matter here, my tuning point will just be a little different (less turns). 3p3s is not possible, I will get to much capacitance for the MMC.


3
Connecting GTO Snubbers directly head to tail is a bad idea, because most of the heat losses in the capacitor get dissipated by the terminals. The thermal impedance is very low between the screw terminals and the inside of the capacitor, this is also stated in the DS. So if you screw them head to tail, the capacitors will heat up each other even more. I designed my MMC like that on purpose so there is copper busbar in between the capacitors which acts like additional thermal mass and as a heatsink.

Celem even states that screwing their power capacitors head to tail is an absolute no go.

Greetings
Phoenix

4
Hello Mads

Sorry for my late reply, it has been a long time and I have been quite busy lately  ;)

Thank you for always updating your design guides, they provide very useful information.

So I was finally able to get proper capacitors for my MMC.



The MMC can theoretically do 9kA Peak, 12kV and 225ARMS at 85°C. But if the MMC ist still colder, it could handle up to 350ARMS. These caps are very robust, they have a peak dc voltage rating of 2400V and they can do up to 3500VDC for 10s/day, which equates to around 8 minutes of DRSSTC runtime per day. Of course the 9kA are only theoretical because of the voltage rise, but my planned 3kA are no problem. With my MMC design, I also ensured proper current sharing between the caps.

I also found a good deal on way better Bricks, I will be using very modern FF1400R12IP4 Modules from Infineon. They can do 1400A Hardswitching, but are way faster and better in every aspect than my old 800A modules from the pre 2000 ages. These modules will laugh if I throw 3000A Softswitching in a DRSSTC at them  ;D

The bus layout also gets way easier with this modules, they are halfbridge modules. Here is the DS: https://www.google.com/url?sa=t&source=web&cd=1&ved=2ahUKEwjaxPiEyqzlAhWRJVAKHbQ6BZgQFjAAegQIBhAC&url=https%3A%2F%2Fwww.infineon.com%2Fdgdl%2FInfineon-FF1400R12IP4-DS-v02_04-EN.pdf%3FfileId%3Ddb3a30431f848401011feb7645573e9f&usg=AOvVaw1uqJS7NSpx5lVx8c3jLFJh

Greetings
Phoenix

5
Hello

Are you only using 4 turns for your GDT? I would use way more turns, the GDT of my DRSSTC has 14 turns for example. Your second core should be fine, I am using the same N30 material.

Greetings
Phoenix

6
Transformer (Ferrite Core) / Re: Large Ferrite Core with IGBT-Brick Driver
« on: September 23, 2019, 06:43:39 AM »
Hello

Thank you for your replies  :)

@rikkitikkitavi

Yes you are right, a larger wheel would make it easier to adjust it faster. But it is not too hard to turn even with the smaller knob.

@LongLiveRocknRoll

I did not draw a schematic, the layout of the driver is the same as before. I just added a three phase full bridge rectifier and electrolytic capacitors with a higher voltage rating.

@davekni

Yes, I am from Austria. As Mads already said, 3 phase power is very common here in Austria. You can also get main fuses with higher current rating if you pay more. I believe you can get 25A, 35A, 50A and 63A. We have 50A fuses at our house input.

I measured the power with a normal analog panel meter. Yes, the power factor is horrible because of the large capacitive load, but i believe that the inductance of the variac improves it a bit. Of course the variac will never correct it to a 1, but maybe from 0.6 to 0.8.

Greetings
Phoenix




7
The solid copper wire was better because it had about 1.8mm in diameter, which is more than your steel rod. A thinner wire leads to more corona losses because the curvature of the cross section is sharper. I am using a 4mm copper rod with a smooth surface on my drsstc. I also added two heat shrink tubes to decrease corona losses.

If you take a long time exposure with a camera of the steel rod, you will see corona glow all over it.

Greetings
Phoenix

8
Hello

Are you feeding a 0V/+V signal into your GDT? A GDT is a transformer and you can't run them with 0/12V. If you try to do that, the zero line shifts as we can see in the first scope shot. You need to supply a symmetrical waveform, for example -12V/+12V. If the area under the waveform isn't the same above and below the zero v line, it begins to shift due to core saturation.

Greetings
Phoenix

9
Quite shocking how fast the wood gets accelerated. I wouldn't have thought that it actually goes that fast.

If you calculate the kinetic energy of it, it has around 114 Joules! If it hits you with the edge where more energy gets concentrated on a smaller surface area, it could penetrate dangerously deep.

For comparison, a 9mm bullet fired from a handgun has around 400 Joules. Of course, the bullet concentrates the energy way more on a smaller surface area, but as I said, the edge could still penetrate quite a bit.

I would always wear thick clothes when working with a circular saw so the energy gets spread out in case of a kickback.

EDIT: Even though 20m/s quarters the energy, I still wouldn't like to get hit by it.

Greetings
Phoenix

10
Yes, you are right, there are quite a lot of things to consider when building a DRSSTC. You need to keep the stray inductance of your bus layout low, you have to make sure that no components get overloaded via calculations, you need to take care when winding the secondary to avoid flashovers, you need to keep the distance from the primary coil to the secondary coil high enough to prevent flashovers between them, ....

Here you can see the primary coil I designed with a computer software called "Geogebra". The red lines are the primary supports and by measuring the points via the software where the spiral crosses them, I get the distance and I know where to drill without any annoying calculations and possible mistakes. The x and y scales are in mm.



Greetings
Phoenix


11
Solid State Tesla Coils (SSTC) / Re: Flat Secondary Coil on PCB?
« on: May 01, 2019, 08:17:27 PM »
No, i do not have a video about this coil. But if you look at the image from the second link, you can see that the primary is on the same PCB as the secondary. There are 2 bigger traces after the secondary coil.

The description says that this is a class E SSTC. I think you could use any normal Audio PLL Class E SSTC schematic and simply hook this coil up instead of a classical resonator: http://uzzors2k.4hv.org/projectfiles/classesstc1/classE_SSTC.GIF

Maybe you need to adjust some component values according to your resonant frequency, but the basic principle is the same.

Greetings
Phoenix

13
Thank you for your replies  :)

I really didn't knew that RS components sells such large heatsinks, but they are quite expensive as you said, oneKone.

My IGBT Bricks are way larger than CM300 Bricks and they have an area of about 182cm² each, so a few small M6 holes shouldn't really be a problem like you said, Mads.

I think I could still close the holes with some metal epoxy (JB Weld). I just need to be careful that nothing protrudes from the hole, because then the baseplate wouldn't make perfect contact. With the epoxy there also would be no need for grinding, because I can simply wipe away the excess before the whole thing cures.

Greetings
Phoenix

14
Hello

As you may know, it is very hard to find new large heatsinks without any holes. I am trying to find a heatsink for my large DRSSTC with at least 30x30cm dimensions, but all the offers I can find have holes in them.

Is it a big problem if there are for example a few M5 and M6 holes under the baseplates of the IGBTs? I personally do not really think that this is a big issue, because the heat will get spreaded by the highly thermally conductive baseplate, but I would like to hear your thoughts.

And do you know where I could get such a heatsink?

With best regards
Phoenix

15
Hello Laci  :)

I would also use more caps in parallel to get the 0.2uF 6400V MMC. Because you are planning to use freewheeling, you really need a high RMS current MMC, if you want to run moderate BPS values. As you said correctly, more caps in parallel decrease your ESR value. Low ESR is very important in such an application, because with more ESR there are more losses generated in the capacitors (P=ESR*IRMS^2).

Because you are building the same MMC like I did, just with less capacitors, I can really recommend you a very good design for equal current sharing between the capacitors. I am also using Wima FKP1 1600V 100nF capacitors (4 in series 15 strings parallel for 375nF at 6400V). Here you can see the MMC of my DRSSTC I:



As you can see, the connections from the copper rails to the inverter/primary are not on the same side. This ensures good current sharing.

Greetings
Phoenix


16
Hello Laci

Your calculations about the MMC's reactance at 110kHz are correct, but there is a very important thing that happens in an LC circuit: The reactance of the capacitor and the reactance of the inductor cancel each other out. If the LC circuit gets exited with the exact resonance frequency, the impedance will be 0 Ohms, but you will still have the ESR of the Primary and the MMC limit your current. The impedance will be 0 because with each cycle the voltage increases and so does the current. Theoretically, the LC circuit would ring up infinitely.

If you use more capacitance, you will have a faster current rise and you could get more peak current with the same ontime and number of primary cycles. The more time you give your DRSSTC to ring up, the more peak current you will get. If you use less capacitance it will just take more cycles to reach the same current than with more capacitance. Of course you need to make sure that the voltage rating of your MMC gets not exceeded during the ringup. The calculation about the maximum peak current you can allow with the MMC is correct.

I think the Wima FKP1 are a really good choice for your coil. I have also used these capacitors for my DRSSTC I. But I do not know how much RMS current your chosen capacitors can handle, Wima doesn't provide datasheets with a lot of information. Have you already tried the MMC calculator from Mads? It is really good for doing the calculations.

EDIT:

I just calculated your primary inductance and I think lowering your primary inductance using less turns and increasing your MMC capacitance to get to the 110 kHz would be a good idea. If you use 4 Wima FKP1 1600V 100nF caps in series and 8 of these strings in parallel, you get 200nF at 6400V. You could go up to 880A with this MMC.

Here you can buy Wima FKP1 capacitors new and for a very good price: https://www.reichelt.at/pulse-capacitor-fkp1-1600-100n-p7733.html?GROUPID=7175&START=0&OFFSET=16&SID=94XE7uy6wQAT4AAEAgW0ke260e2724a5569dec4ba647f3dc371a5&LANGUAGE=EN&&r=1


Greetings
Phoenix

17
Hello Laci and Bambinz

Thank you for your compliments and the nice replies  :)

@Laci

I have yet to find a heatsink that large, but there are quite frequent ebay offers for such heatsinks.

I think you can't steal techniques from me, i have seen your thread and you definitely know how to build a DRSSTC  :)

@Bambinz

It really means a lot to me to hear something like that from an electronic Engineer   :D

I know the components may look intemidating, but it really isn't more difficult to build a large DRSSTC than a medium DRSSTC. Everything just needs to get scaled up, the basic working principle stays the same. But as you already said, you have to be way more careful with a large DRSSTC. The arcs could strike further than you might expect and a crash of the fullbridge could cause a lot of shrapnell to fly around. 3600 Joules is definitely more than enough to really blow something up.

Greetings
Phoenix

18
The IGBT Bricks have finally arrived and they are way bigger then expected ;D

The SKM200 looks tiny next to them:



They are the same size like the CM600DU24NF Halfbridge Brick, but I need 4 of them, because this are single IGBTs.

19
Hello Mads

Thank you for your advice  :)

I have found a good ebay offer on new 10000µF 350V electrolytic capacitors, so I bought 6 of them to get 700V @ 15000µF for the inverter. This gives me around 3600 Joules, which is more than enough.

I also found a good offer for large IGBT Bricks, so I bought 4× FZ800R12KF4 from Eupec/Infineon. These Bricks are not based on the newest technologies, but i believe they are still suitable. According to my calculations, they should easily handle up to 3000A. I really like that these Bricks only have 55nF gate capacitance, which is really low for a 800A IGBT. This low capacitance enables me to use a GDT instead of a direct drive IC. Here you can download the data sheet: https://www.mouser.at/datasheet/2/196/fz800r12kf4-93487.pdf

Here you can see the electrolytic capacitors and the GTO Snubber Caps:


Here you can see the MMC. I will try to get more of these caps to increase my RMS and Peak current capability. Right now, i am limited to 2200A, because of the maximum peak current of the capacitors.


I have got another question about the RMS current capability of these large GTO snubber capacitors. According to the datasheet value, they can handle 100A RMS. But according to my own hotspot temperature calculations, they should be able to handle around 180A RMS. I think the capacitors are only rated as 100A because of the terminals. I saw on your website that the capacitors in your large DRSSTC III are rated for 80A RMS. Did you run into any heating issues because of that?

Have you seen my comment about the IGBT calculations on your website? I think I found a mistake there, but I am not sure.

Greetings
Phoenix

 

20
Hello everyone  :)

About one year ago, I have finished my 160mm DRSSTC. Now, i am planning a 315mm DRSSTC. I have already ordered 6 large GTO Snubbers with 6µF and 3200V DC. I will put them in series to get 19200V DC @1µF. The capacitors can handle up to 2200A peak and 100 A RMS. Here is the ebay link: https://www.ebay.at/itm/Electronicon-6uF-660Vac-3200V-DC-1050V-AC-Cylindrical-Film-Capacitor/362555302076?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I got them for 35€ each, which is not too bad.

Here are my JAVA TC calculations: 


Secondary Diameter: 315mm
Wire diameter: 0.8mm
1800 Turns
158cm winding length

Primary inner diameter: 385mm
outer diameter: 865mm
15mm Copper tube
15mm turn spacing
8 turns in total
tapped at around 4.8 Turns including spark loading

Topload:
40cm Aluflextube
150cm diameter

I am planning on using a fullbridge made out of cm600du-24nf Igbts. The coil is going to run on a DIY 3 Phase variac that can deliver up to 30kW for one minute. This will give me up to 660V DC Bus voltage. What do you think about this specs?

Here is another question: Is there a big difference in terms of performance between DC Link and electrolytic capacitors? I know that you need less dc bus energy if you use dc link capacitors. How much less energy do i need?

Greetings
Phoenix 

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