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Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Hi all! I have a question, and I wasn't able to find an exact answer for it. How much affects the secondary quality factor the spark lenght? I wonder if it's not linear. Actually I'm going to make a new secondary for my 2MOT coil, with a small spacing between turns (0,4mm wire 0,1mm spacing) for a tighter coupling, and probably make an oil, or wax filled secondary. So there is 2 ways, and I'm not sure which one I need to take. One is, keep the wire diameter constant, and add spacing to it, then the quality factor won't drop, but the coil lenght will be bigger (and a little more expensive :) ) Second is to reduce the wire diameter by the spacing distance to keep a better coupling, and make a shorter coil, but the quality factor will drop. Which one would you prefer?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Due to corona, arcs and sparks loading the secondary, the un-loaded 'Q' of the secondary coil does not need to be optimised, if you make a CW coil then high secondary 'Q' to reduce heating IS required.
I believe that the most efficient winding is closely spaced turns I get about 2.5 x dc resistance increase due to 'proximity effect', plus skin effect. If you used the 'classic' space=diameter winding you would significantly reduce 'proximity effect' and 'skin effect' increase in resistance due to frequency but the dc resistance would be 4x higher (1/2 diameter=1/4 area) and it's easier to wind! Looking at it another way, space-winding a given wire on a given former would reduce number of turns by a factor of two thus inductance by a factor of four, increasing frequency by 2x, which increases 'skin effect' x 1.414 not a problem in theory as the half length wire resistance etc. all cancel each other out but the nett effect is operating at twice the frequency causing increased primary losses.
Maybe you can give more details? I am at least curious but I don't know how to exactly help you. There is this:
Also, keep in mind that for bigger coils you want a ratio of 4.5:1 to 5:1 - 4" diameter or less 4.0:1 to 4.5:1 - 5" to 6" diameter range (This is the recommendation given by many coilers, as they have had good experiences with this setup and the math concurs) and so on. Because of the skin effect you don't want too small wire for larger coils.. I usually see successful results with 26 AWG. but 22-28 is good for bigger ones and I've used 30 AWG on small tesla coils...
that being said to answer your question: L = 1.7*sqrt(P) L - spark length P - power OK, so the greater the power the greater the distance (obviously) I am still a novice in Tesla coils, and considering you gave me no idea to the input power, or the total capacitance on the secondary.... well I would just recommend you play with the program I put above, it will give you the best setup you can get with what you are working with... Does that make sense? Even your coupling of primary and secondary is an important factor to consider... ONLY making your secondary longer doesn't necessarily make sparks longer. To put it bluntly It's all about balancing equations for the best results. I recommend "The Ultimate Tesla Coil Design and Construction Guide", he butchers the math a few times and it totally confused me for a while, but reading other dissertations along with the book it explains thoroughly why/how each component changes the system as a whole. Comes with a spreadsheet that each time you change a value it shows the efficiency of the spark length.
I'm sorry I can't help you more. Anyone feel free to correct any mistakes as well, I'm also eagerly learning how to share my knowledge appropriately.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Or you can wind using more wires in parallel. As for the secondary Q-factor, the unloaded Q really has no meaning in a working Tesla coil. I think the loaded Q doesn't matter much as well, anyway there is probably no way to predict it accurately. Just design your coil from the "power and energy" point of view.
Just to add to this Think of frequency (and the L and C's influence on Q) as really just an aspect to the design challenge. Still success at higher frequencies but hell it's harder.. haha. He had to design this Tesla coil with a quartz crystal to make sure he had accuracy because it's SO FAST, think about how easy it could be to not be in tune and your mosfets feel 1000A inside them turn into heat! HAHA! Crazy.
High Q from what I read though is cruicial just because it helps with the quarter-wave frequency last. When you strike a high quality bell it will ring for a long time... The higher the Q for an inductor is also like that. If you had low Q it would only oscillate like 2 times total... Which is acceptable for the primary side because you want to push ALL the energy from the primary after the first rise in oscillations of power. Because the first wave is the strongest... hopefully the spark gap quenches than so that all that energy is stored in the secondary.
But changing the windings won't produce a noticeable change because the amount of capacitance vs inductance is already crazy L >> C
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Sulaiman wrote ...
Due to corona, arcs and sparks loading the secondary, the un-loaded 'Q' of the secondary coil does not need to be optimised, if you make a CW coil then high secondary 'Q' to reduce heating IS required.
I believe that the most efficient winding is closely spaced turns I get about 2.5 x dc resistance increase due to 'proximity effect', plus skin effect. If you used the 'classic' space=diameter winding you would significantly reduce 'proximity effect' and 'skin effect' increase in resistance due to frequency but the dc resistance would be 4x higher (1/2 diameter=1/4 area) and it's easier to wind! Looking at it another way, space-winding a given wire on a given former would reduce number of turns by a factor of two thus inductance by a factor of four, increasing frequency by 2x, which increases 'skin effect' x 1.414 not a problem in theory as the half length wire resistance etc. all cancel each other out but the nett effect is operating at twice the frequency causing increased primary losses.
Thanks. Yes, I meant the skin and proximity effects is already presented in the unloaded Q factor. I want to get rid of the secondary surface sparks, to make a tighter coupling, and to make a very reliable system. I simply don't trust the magnet wire's coating, I made a lot of secondary coils with the conventional winding after winding method, but once before an event my bigger, 4 mot coil's secondary with a diameter of 300mm exploded in one point. I never saw anything like this. So it will be a fool proof secondary, and this will be my first one with spacing, this is why I wasn't sure about the Q factor (and a few more things, like the decreasing self capacitance of the coil, and the RF heating of the oil/wax effect between the turns), but I think I'll give a try, and try to keep the Q factor high for the first test. The current version gives a maximum of 105 cm sparks with 2 small mots, 100nF cap, 110mm secondary, and some big toploads az 133kHz. Link With a little tighter coupling it was able to do 120cm, but the surface sparks appeared immediately, so with this secondary this is the maximum lenght. I'm going to make this one, and make a report about the development.
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Quality factor vs. spark lenght
