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Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
I have a new page on my site with the title "Failures". This one is what I hoped would be a better rolled cap and below is an extract from the article. I was after a homemade rolled cap that would be cheap and reliable enough to use instead of the expensive MMC's for a Tesla coil. The background is that a standard rolled capacitor under oil is cheap but messy and fails readily. I have had many fail.
Typically they are made of two layers of aluminum foil separated by cheap polyethylene sheeting and rolled up then immersed in transformer oil. Electrodes are attached at one end.
Recently I developed a better understanding due to two things. Firstly an autopsy of the caps showed that they failed on the innermost tightest turns, the furthest away from the electrode wires. Secondly that this could be understood in terms of the operation of a spiral line generator where voltage is multiplied towards the end of the turns by many times.
So I went on to make a prototype (1/10 size approx). Not only were the leads to be at opposite ends but there were 4 equal capacitors in series as well by overlapping foils. As current flows with each foil layer would be in opposite directions I had hoped that this would cancel out inductive effects.
The photo shows the basic idea in a 1/10 prototype and the next one shows the full size one reading 27nF.
It was a complete failure and quite unable to generate sparks in a TC situation. I presume the self inductance is high but I am having trouble measuring it. How do I do that. Is there an easy fix, or is the whole idea doomed?
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Hi TDU, I need to comment a bit...
I think you overestimate the problems of the blumlein effect. In such a crap cap design (no offence mine are the same!) there are much more critical factors than a slight rise of the voltage. Of course the probability of breakthrough is much higher where the voltage is highest but the real determinant of survival is in the used dielectric and the overall ozone stress. I'm pushing my caps to their limits as you can see here and I have no failures in the TC use yet. I think it is because those caps are based on PP-CP which is one of the most stable polymers you can get. I think your polyethylene dielectric gets very quickly eaten by the ozone regardless the way you roll the cap.
Now, it seems like you are trying to increase a longevity of patience in the terminal stadium of cancer by rolling him in a warmer blanket
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Mates wrote ...
I think you overestimate the problems of the blumlein effect...
I can get 5 times voltage multiplication with 30 turns in a spiral line generator and the only difference in a rolled cap is that the spark gap has the primary in series.
Mates wrote ...
...but the real determinant of survival is in the used dielectric and the overall ozone stress. I'm pushing my caps to their limits as you can see here and I have no failures in the TC use yet.
The cap corona is impressive, however my problem with the old caps was failure under oil. These caps have produced 8 foot sparks so they were stressed significantly. Ozone is not an issue unless there are air voids. Anyway, I don't believe that ozone is the major determinant of cap failure or else using Nitrogen or an inert gas would fix the problem. You just have to see woodburn fractals to see how corona creeps along in an increasing carbonising track. My new cap did not breakdown, it just presumably had so much inductance that it didn't work, when I predicted otherwise.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Tesladownunder wrote ...
however my problem with the old caps was failure under oil. These caps have produced 8 foot sparks so they were stressed significantly. Ozone is not an issue unless there are air voids. TDU
Are you really so sure you had no air bubbles left in your caps? Did you make any corona tests (Like sealing the cap inside a glass tube and try a night run?) It is really not easy to get rid of all the bubbles. My caps also died under oil - because the oil can make things much worse in case small bubbles persist (oil+ozone=nasty acids)
Also, according to the voltage multiplication - a rolled cap is not a blumlein generator (desptie it can share partially the same voltage-rise effect). I've never experienced an increase in the voltage after I connected a cap to the HV source (measured as break-down distance of a spark gap). It was always the oposite - there was rather a voltage drop caused by the corrona losses and I always had to decrease the distance of my spark gap electrodes. The reason why your most innerpart was always destroyed could originate also in the bigger physical stress on the dielectic (remember, it is the most bended part).
TDU, I'm not saying you are not right, I'm only trying to see things more from the Ockham's razor view. I would blaim the blumlein effect after I would be sure it is not the ozone or the bending problem...BTW: Did you check the literature - this "blumlein thing" should be well discussed in case it plays a significant role in general hv cap designs...
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Somewhere I read that manufacturers of similar hv caps also use a layer of thin kraft paper between the foil and plastic (if the foil width is more than a certain rule-of-thumb amount)...IIRC the purpose is to wick the oil into the spaces it may not otherwise get into.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Mates wrote ...
Are you really so sure you had no air bubbles left in your caps? .....
No but my caps are loose wound and kept upright. They were not vacuum pumped.
Mates wrote ...
Also, according to the voltage multiplication - a rolled cap is not a blumlein generator . I've never experienced an increase in the voltage ...
You misunderstand the spiral line generator. The voltage develops along the length of each aluminum sheet. But thats not my concern as commercial caps I have dismantled seem to have electrodes taken from opposite ends anyway like microwave oven caps if I recall.
Mates wrote ...
The reason why your most innerpart was always destroyed could originate also in the bigger physical stress on the dielectic (remember, it is the most bended part).
Possible but I wound the last one on a former (not shown) to avoid this.
My real concern is where I have gone wrong with the inductance of the setup.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Tesladownunder wrote ...
No but my caps are loose wound and kept upright. They were not vacuum pumped.
TDU
I'm sure here is your problem... Loose wounding and upright position is NEVER enough for air bubbles removing. That's my personal experience. In case you would use transparent casing for the caps (that's what I did) you would see the blue corona during operation. I tried a lot to solve this problem, oils and liquids of different viskosity, even things like vacuum cleaner - nothing really helped. You need a strong vacuum pump to make a good oil based cap.
And one more small thing: I have always thought that the voltage potencial in blumlein is more or less constant between the parallel conductive plates. That's the biggest advantage of this device, simillar like in Marx or CW the needs for insulation are constant within the whole device. Or you trying to say that the mass of the insulation layer is increasing ? It does not make too much sense to me, I saw very compact blumleins with output voltage of 150KV - I think the device would have to be much bigger in case the insulation layer would have to stand 150KV. It would be also very impractical for construction - at 150KV you need more than 1cm of PP layer !
Registered Member #1538
Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28
Although my experience might not be very helpful at all, because your understanding off how to build “homemade†caps is probably way beyond mine, I will try to point out my observations.
For my first sgtc I constructed some rolled caps. I wanted to keep it clean and portable, so putting it into oil was not an option for me. Of course I had the same problems with corona discharges. But because I had a LOT of plastic foils on hand (a foil recycling company nearby) I made some experiments. The foil I came up with is 0,17mm thick and can withstand at least 25kV DC (and there was something printed on it, but it worked best). For the final Cap I had to use 5 layers to prevent breakdown and leave 20cm with no aluminium foil at the edges. So the creeping distance would be 40cm. There was no corona visible anymore. However, I fond that adding tabbing points about every meter to reduce series inductance increased performance significantly (Note that these tabs go sidewards out of the cap). But it lowered the creeping distance to 20cm at some spots.
This Cap has been running for maybe 5 hours during the last 3 years. The only real problem, besides high losses and a little too much inductance, is the weight: 21,5kg for a length of 1,10m and a diameter of 17cm. But it is a 120nF cap, and its cost was about 2 USD for the alu foil.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Mates wrote ...
... In case you would use transparent casing for the caps (that's what I did) you would see the blue corona during operation. ...
Thats an interesting idea although I suspect it is not just showing bubbles. There are electrolytic effects that will light up like this. I did make a transparent flate plate cap under motor oil many years ago and it lit up like a christmas tree in beautiful yellow but not due to bubbles.
Mates wrote ...
And one more small thing: I have always thought that the voltage potencial in blumlein is more or less constant between the parallel conductive plates. ...
That is true and wouldn't explain the failure at the ends but I can't help thinking it is a bad thing to generate huge voltages at one end of the cap with respect to the electodes
Timo wrote ...
For my first sgtc I constructed some rolled caps. I wanted to keep it clean and portable, so putting it into oil was not an option for me. .... For the final Cap I had to use 5 layers to prevent breakdown and leave 20cm with no aluminium foil at the edges. .... I found that adding tabbing points about every meter to reduce series inductance increased performance significantly (Note that these tabs go sidewards out of the cap). This Cap has been running for maybe 5 hours during the last 3 years. The only real problem, besides high losses and a little too much inductance, is the weight: 21,5kg for a length of 1,10m and a diameter of 17cm. But it is a 120nF cap, and its cost was about 2 USD for the alu foil.
I think that your rolled cap without oil is a success and I agree with the periodic tabs. I think that making a cheap rolled cap without oil that lasts will need a lot of total dielectric weight so you end up with a big cap but the raw materials are cheap. I still have a hankering to get this right but using a MMC approach rather than single dielectric.
Registered Member #58
Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317
whatever happened to my plate cap to take place of an MMC.... buy a great amount of plastic dinner plates at a grocery store, and glue/place a circle of foildown with a tab sticking up, alternate the sides that the tabs go on and you can make a huge stack... here it is discharging on youtube.
Edit just in case anyone wanted to take a look my old thread is here
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Anti-Blumlein rolled cap failure
