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Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Jase, I'm pretty sure that if you have Zsurge of 8 ohms and ESR rating of 10M ohms, your primary Q should be .8u ohms (or micro ohms), I think, my conversion might be off. How did you come to 800? .8u ohms is so low it's negligible like Goodchild said.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
zzz_julian_zzz wrote ...
@ Eric Goodchild, What is the good Q for secondary?=) Thanks in advance!
Q is the loss of the system. Do you mean Z? The Z should be ideally 20-50k. My coil is a bit over that, buts it's just a guideline just don't have a value that's way off or bad things will happen
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
My reference was to the Z serge of the primary circuit. This will dictated ring up time in the primary. Typically between 5 and 20 ohms is selected. Having tried both high and low impedance tank setups I prefer the higher impedance systems at 10+ ohms.
This is mainly due to the fact that you experience less heating in the primary copper and for MIDI operation it's superior due to the fact that you have more cycles to work with when scaling PW for higher frequency notes. The result is a more fluid change in spark length rather than adding or losing 4 or 5 feet at a time when only one or 2 cycles are added or taken away.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
You can also drive low impedance system long and hard but it gets more expensive in form of physically huge tank capacitor and high power cooling for switching transistors and primary coil.
If you ask me it is worth of all that. Output is very bright and loud. For example my 90kHz MiniDR has 3 ohm primary and i run it at 100-120µs 350bps Primary current rises easily over 2kA but who the heck cares :) IGBTs can take absolutely insane abuse when cooled and driven properly. Interesting thing is that once sparks grow up to a certain level, the primary current actually stops rising and sort of levels out untill the end of the burst. I guess this happens because the spark load appears as constant current sink for the inverter.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, as the burst progresses, a balance point is reached where the power draw from the streamers equals the power output of the inverter. You can adjust the balance point over quite a wide range by detuning the primary. It is very sensitive, 1 turn will change it by almost a factor of 2.
I like to tune so the balance point is a little above the current I want to run the inverter at, and then use my pulse skipping current limiter to control it.
I've run single bursts over 10ms at 450A like this, using an extra bank of DC bus caps for energy storage. The bang energy must be nearly 1kJ. I'm hoping to bring some of this mayhem to my next coil.
As a Tesla coil "musician" I also agree with Goodchild's comments. My "loaded Q of 10" guideline results in quite a high impedance primary. I also promoted the "secondary z0 of 20-50k" rule as this gives a loaded Q of about 10 for the secondary too, according to Terry Fritz's streamer model.
Interesting thought: the 220k part of Terry's model is constant. So an infinite sized Tesla coil should maybe have a z0 of 22k?
If you ask me it is worth of all that. Output is very bright and loud. For example my 90kHz MiniDR has 3 ohm primary and i run it at 100-120µs 350bps Primary current rises easily over 2kA but who the heck cares :) IGBTs can take absolutely insane abuse when cooled and driven properly. Interesting thing is that once sparks grow up to a certain level, the primary current actually stops rising and sort of levels out untill the end of the burst. I guess this happens because the spark load appears as constant current sink for the inverter.
Did you measure the resistive loss in your primary tank? If that is just 0.1 ohms, you'll burn 200V of bus voltage at 2kA. Primary current might flatten out because of this.
Registered Member #2566
Joined: Wed Dec 23 2009, 05:52PM
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Posts: 147
Steve Conner wrote ...
I also promoted the "secondary z0 of 20-50k" rule as this gives a loaded Q of about 10 for the secondary too, according to Terry Fritz's streamer model.
Interesting thought: the 220k part of Terry's model is constant. So an infinite sized Tesla coil should maybe have a z0 of 22k?
I don't think anything fixed about secondary Z and size of coil can be claimed based on the performance (at least as concerns SGTCs).This is a big Wysock's coil: Secondary size is 97''x24" and it has (only) 342 turns. Surge impedance of this monster is only about 15k.Reportedly,the coil could produce 25 feet long sparks with 30 kVA input. I guess this means as long as you keep secondary loses low enough,the coil will perform fine.
Nope i have not measured that. Proximity losses in primary is quite a problem though :)
Yes and also the skin effect. It can all be lumped into a primary loss resistance. I think it is an interesting parameter especially for low primary Z coils. The measurement is easy: Remove the breakout point or better all of the secondary, if it is not too much of a hassle, lower the bus voltage, so that you don't get a breakout and increase burst length until the primary current has flattened. That will get you an estimate of the primary loss resistance and what fraction of the input power is wasted in the primary tank.
Steve Conner wrote:
I also promoted the "secondary z0 of 20-50k" rule as this gives a loaded Q of about 10 for the secondary too, according to Terry Fritz's streamer model.
I think the Terry Fritz model may work for shorter burst like Kizmos or with SGTCs. But it is probably much off for 10ms bursts.
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All around questions for DRSSTC
Primary current rises easily over 2kA but who the heck cares :) IGBTs can take absolutely insane abuse when cooled and driven properly. Interesting thing is that once sparks grow up to a certain level, the primary current actually stops rising and sort of levels out untill the end of the burst. I guess this happens because the spark load appears as constant current sink for the inverter. 
