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Sir Fixalot

Wed Apr 28 2010, 04:17AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
Location:
Posts: 22

I was trying to figure out what the peak discharge would be on my cap bank using "Barry's RLC Simulator." My bank is going to be 40 350 v, 2900 uf, 177J caps all in parallel for a total bank storage of 7080 J. When I plugged these numbers in, along with my coil resistance (.58 Ohms) and Calculated inductance(1.056 mh), it said the peak discharge current would *only* be about 550 amps. Does this sound right? I figured with such a large bank I would be able to top a few KA no problem. Of course these numbers are rough estimates(esp. inductance) and by no means set in stone, but I figured even with numbers that weren't super close I would be hitting at least 1 KA. Like I said, these are just rough-Once I get some time I will actually wind my coil so I can get a better idea of inductance.

So if anyone has any ideas-like maybe if i miscalculated something, or maybe the program did, or maybe I just was expecting too much, then they would be greatly appreciated.

PS-What do you guys think, calculators aside-just a guesstimate-my cap bank will put out current-wise?

Any help appreciated.

Turkey9

Wed Apr 28 2010, 04:53AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

That's probably about right, you have a pretty large inductor there, along with a very large capacitance. The pulse itself will be about 70mS. Thats pretty long for a pulse discharge like this. SCR data sheets give surge current ratings with a 10mS pulse., and yours is 7 times longer.

Make your coil smaller and you'll get a lot more current through there. Also, you could do 20 strings of 2 caps in series and make the whole thing 700v. That'll give you more current and a faster pulse.

GhostNull

Wed Apr 28 2010, 01:05PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Your total capacitance is 116mF!

and at 350v your short circuit current would be well inexcess of 2000amps!

If you are thinking that the peak discharge current is high then it is normal for a capacitor bank of this size to exceed 1KA. Something this powerful would be better suited to making a railgun. There will most likely be problems with the mechanical stress on the coil and heat generated by the coil. With this you might want to consider a railgun or toning down a bit

cduma

Wed Apr 28 2010, 03:18PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

I think a railgun would be better. They often do better with low V and high F and coil guns do better with high V and low F. 70mS is a really long time and will actually slow your projectile down since it will restrain it.

klugesmith

Wed Apr 28 2010, 07:52PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sir Fixalot wrote ...

I was trying to figure out what the peak discharge would be on my cap bank using "Barry's RLC Simulator." My bank is going to be 40 350 v, 2900 uf, 177J caps all in parallel for a total bank storage of 7080 J. When I plugged these numbers in, along with my coil resistance (.58 Ohms) and Calculated inductance(1.056 mh), it said the peak discharge current would *only* be about 550 amps. Does this sound right?

PS-What do you guys think, calculators aside-just a guesstimate-my cap bank will put out current-wise?

For the last question: short circuit current would well exceed 200 kA (5000 A per capacitor) based on single-cap ESR under 70 mOhm. Of course for that current, the busbar and crowbar resistance together must be less than 0.001 ohms.

For the first question: your coil is not a short circuit across the capacitor, it's almost a whole ohm!
What would be its current if capacitance were infinite, and inductance were zero?
350 volts divided by 0.58 ohms = 603 amperes. So where is the surprise?

[edit] Your coil has too much resistance -and- inductance for that C and V. Try simulating a winding in the same volume, with wire about 10 AWG's thicker. We look forward to reviewing your next design.

Turkey9

Wed Apr 28 2010, 10:10PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

You could use that bank as a single supply for an H-Bridge multi stage coilgun. The thing won't discharge completely even for a few stages. I bet you could get really good efficiency with a single stage too. But your coil is still going to have to be much smaller (as in L and turns).

Sir Fixalot

Thu Apr 29 2010, 02:11AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
Location:
Posts: 22

Well, Im gonna use 12 awg wire for my coil, but pulse length was not something I had thought about. I am now thinking instead I will use some of the leftovers I ordered and maybe make a single stage 2-ish kj coilgun, and use my massive bank of 40 (7.08 KJ) for a railgun. Hmm, I guess I know less than I thought I did about this...Don't worry though bc I will take all the necessary precautions like a bleeder and voltage dividers and all that good stuff.

So I guess an SCR is completely outta the question, right?

ps-how much current do you think this wire can safely take?

pss-I live near an engineering college so I should be able to get my hands on some Ln2 to make the coil superconductive, but thats not for a while, at least not till i've got my bank all boxed/wired up.

klugesmith

Thu Apr 29 2010, 03:03AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sir Fixalot wrote ...
So I guess an SCR is completely outta the question, right?

For which bank, for which load? There have been several recent threads here, showing how to interpret and apply SCR datasheets, and discussing series C/ parallel C options. SCR's exist that can comfortably discharge your 7 kJ bank in 1 millisecond, if you opt for a mostly series configuration.

Sir Fixalot wrote ...
ps-how much current do you think this wire can safely take?

To heat from room temp. to 150 degrees C? Same as 10 strands of AWG14. About 1300 A for 5 seconds, 13 kA for 50 ms, or 130 kA for 500 us.

Sir Fixalot wrote ...
pss-get my hands on some Ln2 to make the coil superconductive

Superconductive coil made of what? You do have a lot to learn. On the bright side, buying parts without a design is a good way to narrow down your options of what to make.

Sir Fixalot

Sat May 01 2010, 02:57AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
Location:
Posts: 22

Fine, how 'bout one of these for a coilgun or would the pulse be too short?

Turkey9

Sat May 01 2010, 07:11AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Way too short. Your bank would be fine for a coilgun, you just need to change your coil. A good guidline is to keep your bank at around 400v and your inductance in the mid uH range. That will give you a baseline to then play with in calculators like Barry's.

When you get the right coil, you can use a large stud SCR or a small puck just fine. You'll also want to use a large projectile as it will match the discharge time of your coil better.

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