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Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Hello!
I’m currently working on a big DRSSTC, it’s for a customer and not for myself. They want a dual coil, because they want the sparks to reach each other while they are standing on the top. They will use it mostly indoors, so I thought I should build a coil without RF ground, needing only the mains connection „plug and playâ€. There was 2 option, a bipolar coil or a ÊŽ/2 magnifier (or a 3rd, a dual coil with separated primaries and secondaries grounded to each other, but I’m not sure if that would work without RF ground). Sparklengh needed is a minimum of 3 meters between coils. With the IGBTs I already have, it’s not a problem. I don’t want to push the coil to the maximum efficiency, instead for maximum reliability. So every parameter should below the maximum ratings of the individual parts.
A few parameters and parts:
Power: 8kW 3 Phase Primary topology: H bridge IGBT: Mitsubishi CM600-HA-24H Primary capacitor: 625nF 16kVDC (80x Aerovox 2uF 1kVDC snubber caps) Primary impedance: 6,7 ohms Controller: Flexibrute 1.0 Universal DRSSTC Controller from Eastern Voltage Research Modulator: Advanced modulator with fiber optic cable Energy storage capacitors: 2x 3300uF 350VDC Powerlytics 36DX + 48uF PP film capacitor Bus voltage: 550VDC Primary frequency: 38kHz Extra resonator coils: 250mm PVC tube with 0,5mm copper wire 2540 turns.
I built many spark gap coils, a few SSTCs, and no DRSSTCs. So if you have any suggestions please don’t hesitate to share it with me . Pictures and additional informations will be added soon.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
out of my league but if you have a 1/2 wavelength system with no centre earth, if one topload arcs to anywhere other than the other topload (e.g. 'earth') then the centre will be at eht. I'd expect flashovers to the primary etc.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
others are more expierinced than I but can it be treated like a half wave cener tap so there is one drver unit and two teslas with the same resonant frequency that are grounded
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
To me it looks like you don't have enough capacitance on the DC bus, you'll surely be exceeding the max rms currents of the lytics. If you're going for reliability, use at least 4 of the caps.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
I'm aware of the flashovers. I'm going to use a lower coupling than other magnifier systems, + a few PE or PP sheets between coils, and a safety spark gap between the feedlines. I worry about the stray capacitances too. Or it may be enough,if the center is grounded to a metal sheet on the floor. I made a small spark gap model of this topology, I can test a few things on it this week I hope.
If you're going for reliability, use at least 4 of the caps.
Then I'll order 4 of them. These are the only parts missing right now.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
omegalabs wrote ...
I'm aware of the flashovers. I'm going to use a lower coupling than other magnifier systems, + a few PE or PP sheets between coils, and a safety spark gap between the feedlines. I worry about the stray capacitances too. Or it may be enough,if the center is grounded to a metal sheet on the floor. I made a small spark gap model of this topology, I can test a few things on it this week I hope.
If you're going for reliability, use at least 4 of the caps.
Then I'll order 4 of them. These are the only parts missing right now.
I wouldn't use those caps for bus capacitors at all. Any datasheet that doesn't spec their rated RMS current or ESR means that they are not intended to be used in a high current application. Those caps are most likely marketed at things like audio amplifiers and other application that have low RMS bus current. A another great indicator is the terminal size on the cap. Those capacitors will likely cook when presented with the kinds of currents you want to run.
I would recommend buying capacitor that are intended for inverter applications. Be prepared to pay at least double unless you buy from the surplus market.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Goodchild wrote ...
I wouldn't use those caps for bus capacitors at all. Any datasheet that doesn't spec their rated RMS current or ESR means that they are not intended to be used in a high current application. Those caps are most likely marketed at things like audio amplifiers and other application that have low RMS bus current. A another great indicator is the terminal size on the cap. Those capacitors will likely cook when presented with the kinds of currents you want to run.
I would recommend buying capacitor that are intended for inverter applications. Be prepared to pay at least double unless you buy from the surplus market.
Their datasheet doesn't give any more information than "high ripple current". So that means it's way less than 20A I probably need? I bought 4 used ones for 100$, so I'll buy new caps for the final version anyway, but I hope these will hold for at least some low power testings. Can you give me an exact type I should use? Right now I can't afford expensive capacitors.
I wouldn't use those caps for bus capacitors at all. Any datasheet that doesn't spec their rated RMS current or ESR means that they are not intended to be used in a high current application. Those caps are most likely marketed at things like audio amplifiers and other application that have low RMS bus current. A another great indicator is the terminal size on the cap. Those capacitors will likely cook when presented with the kinds of currents you want to run.
I would recommend buying capacitor that are intended for inverter applications. Be prepared to pay at least double unless you buy from the surplus market.
Their datasheet doesn't give any more information than "high ripple current". So that means it's way less than 20A I probably need? I bought 4 used ones for 100$, so I'll buy new caps for the final version anyway, but I hope these will hold for at least some low power testings. Can you give me an exact type I should use? Right now I can't afford expensive capacitors.
I agree with Goodchild, especially if you're building a coil for a customer. The caps you are using will probably work fine for a while, but I wouldn't place my bets on it for any sort of long-term reliability or even any sort of high duty cycle runs.
A quick search on ebay turned up a few good capacitors for reasonable prices. I know many folks have had great success with Rifa capacitors which are affordable if you are on a budget:
These are 3400uF 420VDC rated. Look at the size of their terminals compared to the ones you are currently using. These should work great as your bus capacitors. Two caps seem a bit on the low side though for a 8kW coil, I'd use no less than 4 of them. Suppose your coil is drawing 8kW at about 300bps, that's 27J per bang. I usually size my bus caps no smaller than 20x bang energy. At 550V on the bus, that's at least 3500uF required.
Finally for your MMC, I'm not sure how much primary current you are planning to run the coil at, but I wouldn't go more than about 1.2kA (given your primary impedance of 6.8R) to keep within the AC ratings of the capacitor. Again for long term reliability, I'd go for even more capacitors for a higher voltage rating. But otherwise it should work fine.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
loneoceans wrote ...
I agree with Goodchild, especially if you're building a coil for a customer. The caps you are using will probably work fine for a while, but I wouldn't place my bets on it for any sort of long-term reliability or even any sort of high duty cycle runs.
A quick search on ebay turned up a few good capacitors for reasonable prices. I know many folks have had great success with Rifa capacitors which are affordable if you are on a budget:
These are 3400uF 420VDC rated. Look at the size of their terminals compared to the ones you are currently using. These should work great as your bus capacitors. Two caps seem a bit on the low side though for a 8kW coil, I'd use no less than 4 of them. Suppose your coil is drawing 8kW at about 300bps, that's 27J per bang. I usually size my bus caps no smaller than 20x bang energy. At 550V on the bus, that's at least 3500uF required.
Finally for your MMC, I'm not sure how much primary current you are planning to run the coil at, but I wouldn't go more than about 1.2kA (given your primary impedance of 6.8R) to keep within the AC ratings of the capacitor. Again for long term reliability, I'd go for even more capacitors for a higher voltage rating. But otherwise it should work fine.
I searched a few containers in the university and found 2 RIFA PEH169 3300uF 350V capacitors which I can use, so I feel lucky today. Yes the limit will be 1,2kA. I have a total of 100 snubbers, so a I can go probably a little higher in voltage. Like 17 caps in series for and 6 strings for a total of 705nF @ 17kV. But these are used caps withouth guarantee, so I assume a few will fail on the test I'm going to make with them before use.
Today I tested the dual conception with a small spark gap driver around 350W (NST 7kV) and it worked. Unfortunately I made no video, but I'll make one next month, as the university will be closed from next week. I made the driver transformer with helical primary and used a PA sheet between the coils. The primary wasn't tuned perfectly as I was needed to test the coil fast, but it made 35cm sparks without a problem. Small surface sparks appeared near the primary coil, but it was expected as the whole surface of the PA sheet was very smooth. The coupling was very high, probably near 0,5.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
A few pictures. You can see some snubber caps are broken. Not sure if the the customs did that or it was like that before the shipping. But the seller was nice he sent a total of 110 caps instead of 100.
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ÊŽ/2 DRSSTC
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