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What would happen if a coilgun was built using AC current through the coil.

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Yanom

Thu Sept 06 2012, 01:31AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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I don't plan on building this, but here's a thought experiment: What if it was possible to build a coilgun where the current through the coil was A/C current? How would the rapidly reversing current through the coil affect the magnetic field and the projectile? Would it increase or decrease performance over the same amount of energy DC?

The reason I'm asking is because I played around with Barry's RLC sim, and found that at extremely high voltages and low capacitances, there's a "ringdown effect" when firing, as the current reverses multiple times in the coil before dieing away to nothing. This effect looks a lot like A/C current, but with gradually decreasing voltage as it dies away.

jnbrex

Thu Sept 06 2012, 04:55AM

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An AC current would create an oscillating magnetic field which would, if we assume the projectile to be conductive, induce eddy currents on the surface of the projectile and exert a repulsive force on the projectile. Thus, creating an accelerator with an AC current would most likely decrease efficiency, as energy would be wasted as heat on the surface of the projectile and the repulsive force of the changing magnetic field would cancel out some of the attractive force caused by ferromagnetism. Accelerators that have a non-ferromagnetic, conductive projectile (like copper) are actually fairly common, and work through the repulsive force on the projectile caused by the changing magnetic field.

Yanom

Thu Sept 06 2012, 02:23PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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jnbrex wrote ...

Accelerators that have a non-ferromagnetic, conductive projectile (like copper) are actually fairly common, and work through the repulsive force on the projectile caused by the changing magnetic field.

That's called a Thompson coilgun, isn't it? Is it possible to convert a normal coilgun to a Thompson by just replacing the projectile material? Would this type of coilgun work with an AC current?

Barry

Sat Sept 08 2012, 05:43AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
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Alternating current coilgun? Sure, why not, it was done quite handily in 1937 by EF Northrup and documented in considerable detail in his book "Zero to Eighty". See some extracts from "Zero to Eighty" republished online. These can be really quite efficient with proper design.

Enjoy, Barry

Yanom

Sat Sept 08 2012, 08:43PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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a "linear induction motor". interesting.

I tried winding about 2ft of wire into a coil around a straw, and putting the small coil about halfway into the coil of my big (DC) coilgun, which shoots iron slugs about 4m. I fired it and the little coil didn't move at all. Am I not setting this up right?

ConKbot of Doom

Sun Sept 16 2012, 12:31AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Did you short out the ends of the coil? If you didnt, then no current is going to flow in the wire, and you just have copper in a magnetic field, so nothign will flow. Feed one end of the wire though the center of the coil and twist/solder it with the other end and you should begin to at least see *something* happen.

Yanom

Sun Sept 16 2012, 01:36AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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ConKbot of Doom wrote ...

Did you short out the ends of the coil? If you didnt, then no current is going to flow in the wire, and you just have copper in a magnetic field, so nothign will flow. Feed one end of the wire though the center of the coil and twist/solder it with the other end and you should begin to at least see *something* happen.

Yes, the ends were shorted. I think the problem may have to do with "di/dt", the rate of rise of current through the coil. What's considered good di/dt? How about 100Amps/millisecond? I had a theoretical circuit design that would produce an AC waveform with that di/dt level each cycle.

BigBad

Fri Nov 23 2012, 06:16AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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The standard version of this is the jumping rings isn't it?

You can get levitation/repulsion with single phase, no problem.

Running at 50hz, there's a ferrite(?) rod down the middle I think; I think it makes it work better.

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