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"The degree of inductive coupling determines the rate at which the energy transfer occurs, the intensity of current, the tuning response, and among other things, the voltage gradients developed along the secondary coil and at the topload. If coupling is too tight, some points along the secondary will have a voltage gradient high enough to cause corona inception and ultimately spark breakout of the secondary winding. Typically, we observe this phenomenon as racing sparks traveling down the length of the secondary. When this occurs, the coupling is too tight and needs to be loosened up (position the coils further away from each other)." -JavaTC
This is certainly true for spark gap coils. The coupling directly determines the number of cycles in which the primary energy is transferred to the secondary. There may be 2 causes, why this leads to racing sparks: a) The arc loading the secondary does not grow fast enough to keep up with the energy transferred to the secondary. This might cause excessively high voltages along the secondary. b) The fast power rise in the sec excites higher order modes in the sec, which also leads to high voltage gradients along it.
DRSSTCs have a much slower primary rampup, though, so that the energy transfer speed to the sec is limited by primary rampup speed. Coupling probably still plays a role but not as much as in SGTCs.
If you still have problems with arcs originating from the strike rail ends, I can only emphasize Maurizio Valvos comments. Insert a snugly fitting ball bearing halfway into the rail end or screw in a bolt with a semi spherical head. Filing alone does not help as well. It is all about radii of curvature. You can also try lengthening your breakout rod. This will limit excessive voltage in the secondary somewhat.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Thanks! I actually did make a new breakout point from some scrap 6 gauge solid wire I used for the ground. Both ends were rounded off at first then I sharpened one end to very sharp point. I bought two small aluminum corona suppression spheres last week too so I have those for backup if needed. Hopefully not though my plan was to use them as strike targets. Which also reminds me, when I put a strike target close to the breakout it encourages the sparks to jump and also seems to reduce the parks that jump off of the strike rail.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Remember that your strike rail is ground. The same reason you ground your target, is the same reason the arcs are drawn to your strike rail. If there's enough energy, arcs will reach out to find ground. The reason the arcing to your strike rail decreased when you were arcing from the breakout point to your target is that the energy was being dissipated from the secondary. (Had somewhere to go!) That lowered the amount of charge available near the strike rail. Think of it like this, when the lights dim in your house its because they aren't recieving enough energy from the transformer outside. A big draw on the line somewhere makes your bulbs loose power. The bulbs are just like the portion of your coil near the strike rail.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
doctor electrons wrote ...
Remember that your strike rail is ground. The same reason you ground your target, is the same reason the arcs are drawn to your strike rail. If there's enough energy, arcs will reach out to find ground. The reason the arcing to your strike rail decreased when you were arcing from the breakout point to your target is that the energy was being dissipated from the secondary. (Had somewhere to go!) That lowered the amount of charge available near the strike rail. Think of it like this, when the lights dim in your house its because they aren't recieving enough energy from the transformer outside. A big draw on the line somewhere makes your bulbs loose power. The bulbs are just like the portion of your coil near the strike rail.
Thanks, and yes I understand that, but, thats not exactly what was happening. Its not like the Arc's are jumping off the strike target and going to the secondary like they were with the strike rail. The strike target is placed much farther away then the strike rail too as measured from the tip of the breakout point and the direction the breakout point is facing. I get what your saying though. I understand that the energy in the secondary is looking for the nearest object to transfer electrons with whether or not the object is grounded or made of metal etc... I've been able to isolate the problem down to poor coupling and poor house ground, which is why I constructed a dedicated RF ground outside using ground rods, also raised the secondary, and lowered the strike rail.
That lowered the amount of charge available near the strike rail
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Fun isn't it! For some reason a tesla coil can completely draw people into learning and being driven to totally understand electricity! That's what got me going! Yet another reason 4hv rocks.
Besides all that, your coil looks great! Nice work! There's always a few glitches to work out, no worries. With a dedicated ground and your adjustments to the pri/sec coupling you should see some nice arcs in no time!
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
doctor electrons wrote ...
Fun isn't it! For some reason a tesla coil can completely draw people into learning and being driven to totally understand electricity! That's what got me going! Yet another reason 4hv rocks.
Besides all that, your coil looks great! Nice work! There's always a few glitches to work out, no worries. With a dedicated ground and your adjustments to the pri/sec coupling you should see some nice arcs in no time!
Thanks you for your kind words. Indeed a mighty learning experience!
teravolt wrote ...
have you redone your primary is there changes fom your original pictures?
I'm not changing anything with the primary. The only thing different is I lowered the strike rail and raised the secondary.
Speaking of.. I got out and tried more tests tonight since I changed the coupling and I'm having more problems when I detune primary for optimal performance. I started by detuning about 17% (roughly 65 - 66 kHz) and figured I would go down from there (in frequency) as I try to find optimal performance. First thing that happened was the IGBT's on the negative DC supply side blew at about 50% Vin from the variac, 100Hz PRF 150uS PW.. I was stumped why this happened, so I swapped them out figured it was probably best to go back to natural resonance and then detune from there (probably was a bad idea actually). Well, at least when I did this, I can put out 2 to 2.5 ft streamer to air, only when I exceed about 60-70% of Vin from the variac is when I get racing sparks.
So I assumed the problem was now one of two things, or both: 1) need to raise the secondary a little more. 2)I need to find optimal tuning point.
First thing I did was detune again, this time I tried 8% (about 72 kHz) and the IGBT's on negative supply side exploded violently again.
Question: Do IGBT's normally blow up when the system is out of tune? (I expected to at least have better results as I detune the primary)
EDIT: I think it's time to wind a CT to scope primary current, that may or may not help to see where any problems might be (should have done this in the first place). I'll try to upload some scope shots next time around.
Do IGBT's normally blow up when the system is out of tune? (I expected to at least have better results as I detune the primary)
The energy transfer from the primary to the secondary is larger, if the coil is in tune. This will then reduce primary current. Imagine 2 scenarios:
a) The primary is tuned only a liitle bit below the secondary. This will ensure initially a good energy transfer to the secondary, but as the arc grows, the secondary frequency will shift below the primary freq und put the coil out of tune, limiting further spark growth.
b) The primary is tuned far below the secondary. Then energy transfer is low and sparks will be meek and not able to draw the secondary frequency into the vicinity of the primary freq. Primary current can ring up high in this case, since there is not much arc loading.
The sweet spot is somewhere inbetween. When I tune, I start off with a primary f slightly below the secondary f and then lower primary f until my OCD kicks in or my IGBTs explode.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Uspring wrote ...
Physics Junkie wrote:
Do IGBT's normally blow up when the system is out of tune? (I expected to at least have better results as I detune the primary)
The energy transfer from the primary to the secondary is larger, if the coil is in tune. This will then reduce primary current. Imagine 2 scenarios:
a) The primary is tuned only a liitle bit below the secondary. This will ensure initially a good energy transfer to the secondary, but as the arc grows, the secondary frequency will shift below the primary freq und put the coil out of tune, limiting further spark growth.
b) The primary is tuned far below the secondary. Then energy transfer is low and sparks will be meek and not able to draw the secondary frequency into the vicinity of the primary freq. Primary current can ring up high in this case, since there is not much arc loading.
The sweet spot is somewhere inbetween. When I tune, I start off with a primary f slightly below the secondary f and then lower primary f until my OCD kicks in or my IGBTs explode.
Okay, so in scenario A im just not tuned low enough, and scenario B it is tuned too low. So if my IGBTs are exploding at 8% detuned, and even 17% detuned, then both are too low? (I hope im understanding you correctly, but what im wondering now is: will the IGBTs explode in both scenario's? If so then that is making it much harder to tell whether I need to tune up or down or in-between 8%-17% )
You say you start by tuning just slightly lower. I assume by just a few kHz or so. Are you just guessing really and/or do you ever do that tuning method where you hang a piece of wire from the toroid?
You say you start by tuning just slightly lower. I assume by just a few kHz or so. Are you just guessing really and/or do you ever do that tuning method where you hang a piece of wire from the toroid?
Yes, it is a guess. The amount of detuning depends on the ratio of arc capacitance to toroid capacitance, so it is different for every coil. You might start with 2-3%, which is surely at the border of the range. Do you know, how big primary current gets? There also might be a problem with the electronics. And don't start using burst length beyond 200us. I dunno about tuning with a wire hanging from the toroid. Can you explain?
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Strike rail arcs to secondary. -coupling & tuning-
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