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Coilgun (basics?)

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LAN_403

mogallin

Wed Dec 31 2008, 05:47PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Posts: 15

I would just like to ensure I've got a few things right regarding coilguns:

The higher capacitance, the longer discharge time
(Meaning too high capacitance would decelerate the projectile, unless the current is switched of mid-pulse)

The higher voltage, the stronger magnetic field
(Does the voltage effect the discharge time?)

Will there be a point at which the projectile wont accelerate towards the magnetic field due to some strange physical phenomenon?

And will any capacitor do, or just some?

rp181

Wed Dec 31 2008, 08:25PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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The higher capicitance does mean a longer pulse, if everything stays the same. Higher current means a bigger magnetic field. But high voltage allows more current to be drawn, due to ohms law. Higher voltage = shorter pulse with higher current.
A projectile will stop accelerating in a coil when the magnetic fields have a neutral affect, When magnetic forces act equally on the projectile. This happens in the middle of the coil (for longer coils, it will be a middle section). This is why you want the current to be switched off when the projectile reaches the middle, if it is still on and the projectile is past the middle, the fields will start pulling it backwards.

blackgrunge

Wed Dec 31 2008, 08:27PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Like rp181 said higher capacitance means longer discharge if everything is kept the same. Sometimes that can be good, sometimes bad. It all depends on the inductance of your coils. With that value you can calculate discharge time.

Use these to save you time and find out discharge relationships with inductance:

El_Roberto

Wed Dec 31 2008, 08:31PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Dont forget that the discharge time and current also depends on the size of your coil, try using barry's simular to play around with various values.

Edit: Got beaten to it!

mogallin

Wed Dec 31 2008, 10:04PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Thanks. Good to know these things for sure.

Another thing i would like to know just so that i dont go and buy stuff that dont works;
Will any capacitor with the right voltage and capacitans do, or are some capacitors intended for use with, say AC currents and therefore wont work?

blackgrunge

Wed Dec 31 2008, 11:14PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Its all about voltage. Anywhere around 300v and up will stand up to inductance pretty nicely but aim for 400 to 450v. Another factor is internal resistance but most of the time it won't really matter. The only time it does is when your trying to you a capacitor for a car audio system but those are only made for 24v. Read carefully the voltage ratings and if it says it has an operating voltage of 400VAC check its DC voltage rating because they might not be exactly the same.

rp181

Thu Jan 01 2009, 12:02AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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I realized i wrote a short article on this:

disregard the first paragraph, its wrong =p ( I think, someone wanna clarify this?)
Take a look at this page for good graphs showing voltage vs speed (look at the no enhancement graphs)

Also,
Do AC capacitors discharge AC?

Barry

Thu Jan 01 2009, 12:17AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

rp181 wrote ...

... Higher voltage = shorter pulse with higher current...

Wrong, higher voltage simply means higher current. The pulse width stays the same for all coilguns that use LC timing. The LC time constant does not depend on the capacitor's voltage. You can also verify it using my RLC simulator.

For a fixed inductance and capacitance, the discharge looks like a sinusoid. The frequency is fixed, and the only way to change it is to add some type of turn-off circuit or tinker with L and C values.

The LC timing is so important that I made a separate lookup table in Java to help you find arrive at your desired values: LC Time Simulation.

Cheers, Barry
pÉÇÉ¥ ÊŽÉ¯ oÊ‡ ÆƒuÄ±É¥snÉ¹ sÄ± poo×Ÿq ÇÉ¥Ê‡.

rp181

Thu Jan 01 2009, 12:58AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

Sorry i wasn't clear, i meant highering voltage while keeping energy the same, so lowering capacitance.

blackgrunge

Thu Jan 01 2009, 03:40AM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Posts: 55

@rp181

there's no such thing as an AC capacitor...
When a capacitor is rated or talks about being AC it means that its purpose is to be a filtering capacitor. So no an AC capacitor cannot discharge AC because it cannot be charged with AC current because the voltage is constantly changing. Unless you want to try and stop the voltage at its peaks

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