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Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
wrote ...
Primary voltage, i.e. the voltage the on primary cap is about Iprimary * Zprimary. So a higher Z will make a higher voltage rating of the cap necessary. When the secondary gets out of tune, e.g. by a ground arc, primary current is limited only by the loss resistance in the primary tank and can shoot up severely.
Generally secondary voltage is roughly proportional to Iprimary*k*f*sqrt(L1*L2) so it is not a bad idea to go for higher L1. Streamer loadiing will reduce the voltage, though.
Hi, thanks for your reply..
Yes, low capacitance and high inductance, however, I think It will require more cycles on the bridge compared to the High Z, and that means slow ramping of current in the bridge means longer ON time... is that also good for obtaining 3x secondary arcs??
Anyway, why do we need to have lesser cycles on the primary (faster transfer of energy) if you say, its also good to have it in slow mode? do you have any comments about the voltage sec vs spark length output? Thanks..
Anyway, why do we need to have lesser cycles on the primary (faster transfer of energy) if you say, its also good to have it in slow mode? do you have any comments about the voltage sec vs spark length output? Thanks..
Aside from voltage, I believe, that the spark length depends on the duration of the burst and on the resonant frequency. A very short burst won't give the streamer enough time to grow and heat up. A higher frequency will put more energy into the streamer for a given voltage, so that the voltage requirement for a given spark length will be lower. Or putting it the other way around, for a certain voltage the sparks will be longer. This doesn't mean, that the power requirement is lower for high frequencies. It just means lower voltage in the arc but higher current. Freaus formula relates the power to the spark length. I don't know, if it also works for DRSSTCs but the magic of a well designed coil seems to be, that the coil will get as much energy as possible out of the bus and deliver it to the secondary. Since the load to the secondary due to the sparks varies greatly from breakout to longest spark, this is far from trivial. I failed up to now myself doing that right.
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
wrote ...
Aside from voltage, I believe, that the spark length depends on the duration of the burst and on the resonant frequency. A very short burst won't give the streamer enough time to grow and heat up. A higher frequency will put more energy into the streamer for a given voltage, so that the voltage requirement for a given spark length will be lower. Or putting it the other way around, for a certain voltage the sparks will be longer. This doesn't mean, that the power requirement is lower for high frequencies. It just means lower voltage in the arc but higher current.
I am sorry, I tried to understand the situations you have imposed on this paragraph, but it gets me a little confusion.. =( Specially this..
"A higher frequency will put more energy into the streamer for a given voltage, so that the voltage requirement for a given spark length will be lower. Or putting it the other way around, for a certain voltage the sparks will be longer."
What do you mean by this? please elaborate it clearly =( thanks...
A streamer will grow, when the top of the coil has enough voltage to push a sufficient amount of charge to the streamer tip. How much voltage is needed, depends on the conductivity of the streamer, i.e. if the streamer is hotter it will have a higher conductivity and then less voltage is required.
You can think of a streamer as being a resistor connected to your coil followed by a capacitor, which is then connected to ground. Streamers are modelled like this in e.g. scantesla. A higher frequency of the coil will reduce the reactance of the capacitor. This leads to a larger current in the streamer, which makes it hotter.
This is somewhat speculative. I was led to this by Steve Wards observation of long sparks with quite low voltages. He rran at 325 kHz. I don't think higher frequencies will help lowering the burden on your IGBTs. Only if you want to break the records for getting long sparks out of low voltages
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Oh wow that was a blunder on my part big cap = low impedance Doh! Well that's what happens when I try and answer tech questions late at night I mix the simple stuff up. Anyway that high fres really can only help you out if you are using a QCW though because of it's mode of operation and the way it grows sparks. IN a regular DR you want a nice low frequency so that you can have low switching losses in the IGBTs because they are very slow indeed!
Trying also to help clear up the question about frequency. If you go back to your days of high school physics you will remember that a higher frequency has more energy because it has more cycles in a given amount of time. This being said you can trade voltage for frequency and still have the same average power input to the spark. However this is at the cost of raising switching losses in the switch. So in the end it's a trade off and frankly I only see it being worth while with the QCW.
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Thanks Uspring and Eric,
@ Uspring. I THink I cannot make my fres a little higher because of my consideration about the IGBT speed characteristics, yes you are probably right about heating the air for higher conductivity of streamers. But I may not would want to make a Quasi Con Wave DR .. =) I just need to make a DR medium first =)..Uhmm.. I think I just want to have long sparks do not care if its in the range of 70Kv or 100Kvs.. since I have already made a low fres sec and prim and that would make it more difficult.. Thanks.
@ Goodchild. yes I think my IGBTs would not want to run higher switching speeds and would make the losses on its switching severely. I guess I have to run my 150A 300A Icm IGBTs (fullbridge) to see what will be the output of my dr if it will produce a decent 6' arc =).. my setup will be .225uf , 17.162 uH = 8.73 ohms (impedance), full bridge switching, 315v input, 120 us on time, 7.5 turns of my 1/4" 10" diameter with 1/4" spacing .. coupled to sec by 1.3K only..(will be slowly coupled higher once seen the output) I think I will run IGBTs 2x the ICM rating ,, is that good or over rated>???
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
zzz_julian_zzz wrote ...
Thanks Uspring and Eric,
@ Uspring. I THink I cannot make my fres a little higher because of my consideration about the IGBT speed characteristics, yes you are probably right about heating the air for higher conductivity of streamers. But I may not would want to make a Quasi Con Wave DR .. =) I just need to make a DR medium first =)..Uhmm.. I think I just want to have long sparks do not care if its in the range of 70Kv or 100Kvs.. since I have already made a low fres sec and prim and that would make it more difficult.. Thanks.
@ Goodchild. yes I think my IGBTs would not want to run higher switching speeds and would make the losses on its switching severely. I guess I have to run my 150A 300A Icm IGBTs (fullbridge) to see what will be the output of my dr if it will produce a decent 6' arc =).. my setup will be .225uf , 17.162 uH = 8.73 ohms (impedance), full bridge switching, 315v input, 120 us on time, 7.5 turns of my 1/4" 10" diameter with 1/4" spacing .. coupled to sec by 1.3K only..(will be slowly coupled higher once seen the output) I think I will run IGBTs 2x the ICM rating ,, is that good or over rated>???
Most people run CM300 no more than 1.5KApk as a max upper limit so you should be just peachy
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
"Most people run CM300 no more than 1.5KApk as a max upper limit so you should be just peachy"
-peachy? haha! The problem is that mine is only CM150(but in Full bridge) haha! ,. but i will peak the pri current to as much as 2.5x of the ICm rating.. which is 750Amperes. I hope this will not explode my IGBTs, another thing, on the spice,, the primary voltage exceeds 5kV, but my caps are only 4 kV rated only (limited caps) =/ so I am worried about this, I can limit the prim Voltage if I decrease the Impedance (putting larger cap and reducing turns, but this would require a much Bigger IGBTs) so If you were in my case , which would you prefer?? =/ thanks Idol =D
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
zzz_julian_zzz wrote ...
"Most people run CM300 no more than 1.5KApk as a max upper limit so you should be just peachy"
-peachy? haha! The problem is that mine is only CM150(but in Full bridge) haha! ,. but i will peak the pri current to as much as 2.5x of the ICm rating.. which is 750Amperes. I hope this will not explode my IGBTs, another thing, on the spice,, the primary voltage exceeds 5kV, but my caps are only 4 kV rated only (limited caps) =/ so I am worried about this, I can limit the prim Voltage if I decrease the Impedance (putting larger cap and reducing turns, but this would require a much Bigger IGBTs) so If you were in my case , which would you prefer?? =/ thanks Idol =D
I saw 300A and and thought CM300, regardless The CM150's should take that just fine. But I wouldn't go much higher than that.
And yes I would make your tank impedance much higher, Not just for a lower tank voltage put also the fact that you will have less primary current for a longer period of time. This will deliver the same average energy but thermally it's much better because the peak current doesn't have to be nearly as high. Another thing to check is the RMS AC voltage, In LTSPICE hold down control and ether right or left click (can't remember which) the name of the cap bank voltage waveform, this should give you the average and RMS voltage across that bank. Make sure you meat that in the AC voltage rating as well
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
@ ERIC, It reads my Average as 186.41v and RMS @ 2.12 KV,. (but as you have said, this is for AC) My caps are 4KV DC rated but did not meet the 6kv voltage flowing on the tank wavefrom , looks like I have to add some of other caps that is not the same as the CDE, =/.. I have noticed that whenever I increased the On time above 210us, In the primary Tank current waveform, The last cycles are almost the same of their previous ones, (i mean it does not increase the prim current visually/dramatically) looks like the current does not ring up higher and already on its peak side, but I can still have more ON time.. Is that still okay? After 210 us, it hits the prim current @ only 600 Amps =D (pretty low!) hehe.. I hope that this will produce the sparks I expected hehehe!,. Thanks!
EDIT: Since my caps are only 7 pcs.. of .15uf/2kvdc CDE, I am going to find another different capacitor with the same capacitance and voltage rating but different in peak current handling to mix with it..(i dont have the part numbers but I think its not poly propelene caps)
I do already built a 187.5nf / 8KVDC primary MMC capacitor from my previous coil, but my concern is the primary impedance maybe insanely high of about 11.809 ohms... =( The primary current as per spice simulation, has 600 Amps on about 210 us. but the voltage are in 7kv range so the problem in V were solved, but again, the impedance... I am afraid that It will affect the output I expected.. =/
the bottom line:
if I go for 9 ohms impedance, Prim turns is only 9, the voltage across tank exceeds my capacitor rating( i can change it to .225uf /6kv, but it will be mixed by another type of cap which I am afraid hindered the output of the coil)
If I go for 11.8 ohms impedance, the capacitor will be changed to 0.187/8kv and prim turns into 10.. (1 type of capacitor will be used, no mixing, but lower in current peak compared to CDE type)
Both setup will have current peak about 650 Amperes for 210 us. What will be the best set up that will work for this case? =/ thanks!
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LTSPICE Top load Voltage output
Well that's what happens when I try and answer tech questions late at night I mix the simple stuff up. Anyway that high fres really can only help you out if you are using a QCW though because of it's mode of operation and the way it grows sparks. IN a regular DR you want a nice low frequency so that you can have low switching losses in the IGBTs because they are very slow indeed!