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30,000v Ceramic Capacitors for Railgun

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JohnL

Sat Jun 25 2011, 01:32PM

Registered Member #3842 Joined: Thu Apr 21 2011, 12:19PM
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Posts: 10

Hi everyone, I've been mucking around with low voltage coilguns recently and also tried a neodymium augumented railgun. While looking on Ebay I came across these:
http://www.ebay.com.au/itm/10x-30KV-10nF-0-01uF-103-High-Voltage-Ceramic-Capacitor-/290565294644?pt=AU_B_I_Electrical_Test_Equipment&hash=item43a70a8a34
I found the specs a bit hard to believe, but I don't know how big these things are. With 30,000v and 10 nf they store 4.5J each according to E = CV^2/2. My idea was that with 30,000v, it would be possible to get the massive current pulses that a railgun needs to produce a strong enough magnetic field to actually be slightly efficient.

I have no idea what the equivalent series resistance of these capacitors is, I couldn't find any datasheets or info. The ESR could be far too high for this to work. There's also the big problems of charging to 30,000v, switching, and not getting electrocuted : )

Any ideas/suggestions?

Forty

Sat Jun 25 2011, 05:33PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

you'd need about a thousand of them.
the series resistance only matters when.. they're in series. for that kind of money you could do way better.
you could even make your own:

,_3.5_nF

Pinky's Brain

Sat Jun 25 2011, 09:46PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Railguns accelerate projectiles in ms range ... in that range electrolytics are unbeatable.

JohnL

Sun Jun 26 2011, 05:00AM

Registered Member #3842 Joined: Thu Apr 21 2011, 12:19PM
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Posts: 10

ESR still could affect the maximum current pulse. For example if one of these capacitors has 10 ohms esr the maximum current would be 3000A. I've noticed in electrolytics that as capacitance increases esr decreases. As voltage increases esr seems to increase. If ceremic capacitors are the same then these would probably have very high esr.

On powerlabs railgun research: http://www.powerlabs.org/railgun2.htm
Under rails and rail enclosure design there's a graph that shows the field strength between the rails compared to current. a field strength of 1T requires over 30,000A. does anyone have any knowledge of the ESR of ceramic capacitors?

Forty

Sun Jun 26 2011, 07:29AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I wouldn't believe much of anything off of powerlabs. ESR literally means Equivalent Series Resistance. if no capacitors are in series, then the ESR doesn't really apply. If anything, putting a bunch of caps in parallel would reduce this equivalent resistance, because resistors in parallel decrease overall resistance. 30kA for one tesla? that sounds terribly inefficient.
whatever your argument may be, railguns are inefficient no matter what. if you wanted to buy 1000+ of those ceramic capacitors to make only a 4.5kJ railgun, then your money could be spent much better elsewhere. big pulse discharge capacitors could give you much more energy per dollar and huge peak currents, but still wouldn't work great for a railgun. A railgun takes an object from rest (or at an initial velocity insignificant to the final velocity) and accelerates it up to speed over time. a massive, short current pulse is either going to:
-accelerate the projectile briefly, which will then slow down due to friction with the rest of the rails (best case)
-fuse the projectile in place between the rails despite any initial velocity
-explosively turn the projectile and adjacent rails into plasma (likely case)
A railgun has to accelerate it's projectile for the entire length of it's rails to be effective at all, and at 30kV, you're just not going to have a long enough pulse.
for the kind of money you're talking about spending on those ceramic caps, you could make an awesome railgun with electrolytics. A huge, short pulse like the one you're describing, would be better suited for an induction launcher.

JohnL

Tue Jun 28 2011, 11:22AM

Registered Member #3842 Joined: Thu Apr 21 2011, 12:19PM
Location:
Posts: 10

A huge, short pulse like the one you're describing, would be better suited for an induction launcher.

That's a really good idea, I did some roundabout calculations on qucs and it looks like these sort of caps are perfect for that. It seems like they would cause the current in the coil to oscillate at a frequency in the 10KHz-100KHz range, I think thats about right for an induction launcher.

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