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Registered Member #16021
Joined: Fri May 03 2013, 08:16PM
Location: Konstanz, Germany
Posts: 5
Hi,
my name is David, I'm from Germany and a "coiler newbie"
My wife and me want to build a moderately sized DRSSTC and before starting on the project, I would like to hear some opinions on our planned design. Some background: My wife has a PhD in physics, I am currently writing my PhD thesis in solar cell physics and we consider ourselves moderately fit with electronics. She just started her new job, being responsible for designing and running the experiments shown in the physics lectures at our university. We thought that an audio-modulated DRSSTC would be kind of a nice first project. I have built a lot of small electronics projects in the last years, however none of us ever built a tesla coil.
I spent some time reading websites and the forum to come up with a set of components to get us started, hopefully without too many problems. The coil layout was designed with JAVATC (metric units) to be around 100kHz. But we still have some questions:
Is there anything in our design which is a Bad Idea (tm) and probably wouldn't work?
Regarding the IGBTs: There seem to exist some with and without bypass diodes, does it matter? I chose the ones with diodes, as the proven CM300 bricks also have diodes. However, the IXYS seem to be faster and a lot cheaper (at least here in Germany).
Do foil capacitors like those used for the MMC work as snubbers? Do I need snubbers?
What is the best primary layout to avoid strikes, flat or cylindrical? Are there design rules where to put those strike rails?
How do we prevent frying other expensive lab equipment with HF incoupling on the power mains or ground lines? Is it sufficient to use one of those LC mains filter modules?
Should we go for three-phase power or is single-phase better?
-----------------------------------------
----------- Secondary Outputs: -----------------------------------------
----------- 106.56 kHz = Secondary Resonant Frequency 90 deg° = Angle of Secondary 80 cm = Length of Winding 25 cm = Turns Per Unit 0 mm = Space Between Turns (edge to edge) 785.4 m = Length of Wire 6.4:1 = H/D Aspect Ratio 108.9788 Ohms = DC Resistance 47292 Ohms = Reactance at Resonance 0.877 kg = Weight of Wire 70.634 mH = Les-Effective Series Inductance 74.677 mH = Lee-Equivalent Energy Inductance 73.271 mH = Ldc-Low Frequency Inductance 31.582 pF = Ces-Effective Shunt Capacitance 29.872 pF = Cee-Equivalent Energy Capacitance 47.306 pF = Cdc-Low Frequency Capacitance 0.2151 mm = Skin Depth 24.758 pF = Topload Effective Capacitance 187.1522 Ohms = Effective AC Resistance 253 = Q
-----------------------------------------------
----- Primary Outputs: -----------------------------------------
----------- 106.31 kHz = Primary Resonant Frequency 0.23 % high = Percent Detuned 0 deg° = Angle of Primary 445.64 cm = Length of Wire 3.99 mOhms = DC Resistance 0.87 cm = Average spacing between turns (edge to edge) 3.48 cm = Proximity between coils 3.6 cm = Recommended minimum proximity between coils 10.359 µH = Ldc-Low Frequency Inductance 0.19907 µF = Cap size needed with Primary L (reference) 0.848 µH = Lead Length Inductance 119.59 µH = Lm-Mutual Inductance 0.137 k = Coupling Coefficient 0.132 k = Recommended Coupling Coefficient 7.3 = Number of half cycles for energy transfer at K 33.93 µs = Time for total energy transfer (ideal quench time)
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
To tackle your list:
Igbts: freewheeling diodes are necessary in a bridge design. As the devices turn off, the diodes must conduct and act as clamps to the positive and negative supply rails because current in the primary has to ring down.
Metalized film capacitors such as the Cornell Dubilier caps we use for our mmc's are built to be snubber capacitors, we use them for resonant capacitors simply because nothing else can withstand the current we pump through them(sort of- other types can, but are impractical).
A spiral coil is less subject to *flashovers*, but costs you coupling. Both types of coils can easily receive a strike, so a strike rail is necessary.
The RF common mode: I have no idea... You could build a filter, but I've never heard of someone doing that before.
Three phase is 'better' because you have more voltage peaks through the rectifier, but you won't be needing the raw power from three phase unless you're building some 10kw monster(which based on your ~fres, you're not)
Everything else checks out fine by me, but I can give a few tips:
Look at the snubber built for igbt modules on ctrsurplus ebay store, they'll lower the stray inductance on the way to the snubber, so you'll have less ringing. You could also go for a higher TVS rating; your bricks are rated to 650v, let the snubber eat chunks out of the transients, give some phase lead to soften switching, and save the TVS diodes for when you really need them- just before you blow a brick!
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Hello and welcome! Bens tips are good. Go with the flat pancake primary. It is a bit easier to construct and gives you tighter coupling. You will much rather have an arc to the strike rail than raving sparks and flashovers. Typically the strike rail sits about an inch or so above the outer most turn of the primary. Sometimes you can completely avoid strike rail arcs if you make a long breakout point. I use breakout points that are 3-4ft long and never get arcs to strike rail. Using a long breakout point will slightly lower your resonant frequency but it can easily be simulated in javatc or using an oscilloscope and function generator when you find the freq of your secondary. I also get a little bit of corona on the long breakout points but find it to be negligable in regards to have a negative effect to the spark length. And to repeat what Ben said, no need for 3phase on a coil that size. 240V rectified will do good.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Hi David, Welcome to 4hV.
I would recommend a pancake primary as well. They are much easier to make then solenoid coils, although they take up more space.
For IGBTs, i would recommend using either CM300s or CM600s. They are very robust, and probably less expensive than the IXYS components and pretty much the defacto standard for DRSSTCs.
If you need additional help, just contact me offlist and i would be glad to help you with any additional questions you may have.
Registered Member #16021
Joined: Fri May 03 2013, 08:16PM
Location: Konstanz, Germany
Posts: 5
Thanks for all the tips. I will change the TVS to use 4x 1.5KE110A in series for each IGBT, which should have a clamp voltage of around 650V.
Regarding the breakout point: Most larger coils I've seen use breakout to the side and the smaller ones often use a vertical breakout point. Is there a reason for this and can a large coil be used with a breakout to the top?
@EVR: Unfortunately I haven't found a source here in Germany which sells CM300 bricks at a reasonable price. The IXYS are 25 EUR a piece from Mouser, so around 100 EUR for a full bridge. A CM300 full bridge would be more than 300 EUR (we cannot buy from ebay for the university). Also, the IXXN IGBTs seem to be a little faster than the CM300 or CM600.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
As far as I know the breakout point is typically located on the side for larger coils because the spark is going to want to strike the ground or nearby object. So having it directed outwards with a slight tilt up will help to have the spark strike something other than the primary.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
David Kiliani wrote ...
Thanks for all the tips. I will change the TVS to use 4x 1.5KE110A in series for each IGBT, which should have a clamp voltage of around 650V.
Regarding the breakout point: Most larger coils I've seen use breakout to the side and the smaller ones often use a vertical breakout point. Is there a reason for this and can a large coil be used with a breakout to the top?
@EVR: Unfortunately I haven't found a source here in Germany which sells CM300 bricks at a reasonable price. The IXYS are 25 EUR a piece from Mouser, so around 100 EUR for a full bridge. A CM300 full bridge would be more than 300 EUR (we cannot buy from ebay for the university). Also, the IXXN IGBTs seem to be a little faster than the CM300 or CM600.
A note on the TVS. If you have the luxury of designing an IGBT bridge with very low inductance and can ensure soft-switching (i.e. a driver with phase lead adj) , you won't need to have the TVS snubbers.
Registered Member #16021
Joined: Fri May 03 2013, 08:16PM
Location: Konstanz, Germany
Posts: 5
So, after half a year I'm reviving this thread with some progress and images. I have completed the MMC and full bridge assemblies. The primary, secondary and toroid are also finished, but I haven't taken images of them yet. Unfortunately I misunderstood the "major diameter" term in javaTC and made the toroid a little too large. So currently the secondary is at 75kHz instead of 100kHz, I hope I can adjust for that with the primary...
One thing about which I'm not sure is the placement of the current feedback coil in a full bridge setup. Does it matter if I place it at the primary coil side or the MMC side of the full bridge?
Registered Member #40550
Joined: Sun Nov 17 2013, 09:22PM
Location: Germany
Posts: 1
Nice pictures! I like the construction of your mmc very much. One thing I'm curious about are those Wima MKP10 caps, because they are rated 1600V DC but only 400V AC. I thought that the maximum spikes of 400V AC are only about about 560V high, so why does the voltage rating differ that much?
Might be a stupid question but i hope someone will answer.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Looks very nice! Wish I could do work like that!
Current feedback is fine either side of the bridge, as long as the phasing is correct.
AC rating of these capacitors is determined by internal heating, and probably other factors I'm not sure about (corona maybe?). I'd like a good answer too!
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First DRSSTC design questions
