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Steve Conner

Mon Oct 22 2007, 10:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The "problem" was rupidust constantly hating on noobs and we "solved" it by booting him off the forum.

Anyway, we agree that supercapacitors can't cut it, but how about ultracapacitors?

(If they bring out something even better, will they call them hypercapacitors?)

Eric

Mon Oct 22 2007, 05:20PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
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Posts: 116

'ultracapacitors' can't cut it either. Short circuit discharge time constant is ~1/2 sec. A tad too long. If Eestor's super capacitors are ever produced, they could be the next big thing for EM launchers. Super duper wicked neat capacitors.

Bjørn

Tue Oct 23 2007, 12:36AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The short circuit discharge time does not say much alone. If the current is high enough a capacitor is usable no matter how long it takes to discharge. It would just mean you have more shots before you need to charge again.

Eric

Tue Oct 23 2007, 01:25AM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
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I think it says quite a bit, it gives you an idea of the limit for how fast the thing can discharge. The launcher will need to get all of its energy in less than ~1 ms. so a 500ms discharge time constant is reeeally slow. Practically speaking.

Especially since the optimal power transfer will occur with a much longer time constant, say 1000ms. How many designs would be able to operate on 0.1% of the capacitor energy in a shot? You'd need a *load of caps...

Dave Marshall

Tue Oct 23 2007, 03:12AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

In most cases, you'd be correct Eric, but I think you've got a bit to narrow a scope on the issue here.

Those 650F caps store a grand total of 1,755 coulombs.Your time constant estimate is fairly close. At the listed ESR, short circuit time constant would be closer to 3.7 seconds. You're looking at an average current of just over 474 Amps. At 2.7v the stated ESR of 1.15 milliohms would allow for a peak short circuit current of about 2,300 Amps. More than sufficient for a coilgun, assuming your coil had zero resistance

Granted this is a most impractical example, but it still demonstrates that without the specifics of a system, generalizing and saying they are unsuitable for all coilguns is unwise.

Now, for your average mechanically switched, or SCR switched coilgun, they are indeed quite unsuitable unless you've got an absurdly long barrel. However, if you had enough of them to stack in series, I think they'd make a most attractive power supply for an IGBT or FET switched, multistage or rapid fire coilgun operating on relatively low power. The very steady current they'd provide would allow for highly efficient operation.

In fact, if it weren't for the expense, I would be very interested in them for my current coilgun design. Unfortunately at that low voltage, atleast 20 would be needed to facilitate reasonable current across the resistance of the coil itself, and car batteries are a more economical (though certainly less practical from a handling perspective) alternative, requiring only 10 for similar peak current and voltage, without the annoying voltage drop over time. They're also comparable in price per unit.

Dave

OZZY

Tue Oct 23 2007, 07:55AM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Dave has hit the nail on the head. For a transistor switched coilgun you can consider the cap bank as a power source that can be switched on and off much like a battery. It should be possible to add more caps in series or parallel to get the desired specs for the bank.

However in an SCR switched coilgun the cap bank forms part of the RLC circuit and its properties affect the length and shape of the current pulse. The maximum power transfer will occur when the impedance of the load is equal to the impedance of the cap bank, i`m not 100% sure on that bit I hope someone can help me out? But the efficiency of the power supply will only be about 50%. To get good efficiency for a coilgun the pulse length should be at least 10 times the short circuit time constant.

The P270 version designed for higher power applications has ESR of 0.8 mOhm, this gives a time constant of 0.52s hence a minimum pulse length of 5.2s for good efficiency in an SCR circuit. The short circuit current is 3500A, max power will be at 1750A with terminal voltage of 1.35V So max power is 2362W at 50% efficiency. This is low power by coilgun standards!

This capacitor stores 2369J, an electrolytic cap the same size would store about 100J

Eric

Tue Oct 23 2007, 10:04PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

Yeah, I suppose you could use it as a battery. But it doesn't seem to be much practical use for actually providing coil/rail drive current if you can only get maybe 2 joules out of one during a shot. Unless you have a few hundred of them, seems pointless.

So has anyone heard more about the Eestor capacitors being developed? They sound really cool, essentially a giant high k chip capacitor. Very high energy density and very low series resistance potentially. Troubling thing is they filed the patent in 2001 and I don't know of any working models yet. It is intended for use an electric car 'battery' but it seems it could have MANY uses. If it worked/existed...

Coyote Wilde

Wed Oct 24 2007, 05:24AM

Registered Member #175 Joined: Tue Feb 14 2006, 09:32PM
Location: Sudbury, ON
Posts: 111

FeelGoodCars, who were funding most of EEStore's work, didn't strike me as foolish people or dupes, but one never can tell with these things. Same with Altair Nanotechnology, who has lovely new LiIon "nanosafe" batteries and no 3rd party verification... either one could be the Next Big Thing, or both could be snakeoil.
Anyway, there is news from EEStore, latest I found is here:

I wonder if ultra-capacitor companies sponsor students? Maxwell, I understand, used to do that...
Off topic, but am I the only one who finds the use of super-, ultra-, and hyper- prefixes to just about everything these days rather annoying?

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