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Single Coil Alternating Current Gauss Gun

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Woodie

Thu Jan 08 2009, 10:05AM

Registered Member #1916 Joined: Thu Jan 08 2009, 09:50AM
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Posts: 3

I didn't see this when I did a search for it, so sorry if this has been brought up before.

Ok, first we know that AC current is a sine wave. There is a brief period of no current when the wave is at 0. We know that coils make electricity. So instead of using many batteries of coils couldn't you use a single coil over the entire barrel?

My thinking is this. As long a there is a current through the coil there is a magnetic field. When the wave hits 0 the projectile can move past the part of the coil which was pulling it forward until it is closer to the next part where the charge begins to get strong enough to begin pulling it again. Rinse and repeat with the coil stretched to compensate for the coil going faster while the wave is not.

Couldn't you with this method make a fairly powerful coilgun from a single coil and power source? If so this would eliminate the need for all those switches, gates and large expensive capacitors.

Backyard Skunkworks

Thu Jan 08 2009, 10:41AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Because a traditional coilgun uses magnetic attraction, the force on the projectile will be more like |sin()|. So this method wont really do much good...
Also, where would you get something that can do several tens of kilowatts peak power AC?

The most likely outcome of this setup is the projectile either sitting in the coil or if the power is shut off fast enough, being ejected with little force.

Dr. Slack

Thu Jan 08 2009, 12:23PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

What you're describing is a linear, single phase, synchronous, reluctance motor.

This is on of these things that in theory could work, but in practice has a lot going against it. Energising the whole coil, with only one part of it doing any useful work is rather inefficient. The profile of the windings limits you to only one accelearation profile, which makes it very non-flexible. Multi-coil guns solve both of these problems by triggering several coils in sequence.

Prof Eric Laithwaite had fun with a 3 phase linear motor, his "magnetic river", but that worked best as an induction motor with a non-ferrous armature, which it levitated into the bargain. And he did have a multi-100kVA supply to play with. Wouldn't have worked nearly so well with iron, or absent the other 2 phases.

Barry

Thu Jan 08 2009, 02:52PM

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

Almost. The AC current is sine wave, but at any instant in time the whole coil experiences the same current. So you don't get a magnetic field to form a sine wave in space, only in time.

Also note that solenoid coils have uniform magnetic field inside the coil. Once the projectile is fully enclosed by the coil, there is no longer any physical forces upon it, so it's merely coasting whether the coil is energized or not.

You can accomplish what Woodie describes, all you need is a series of coils end-to-end which are energized with three-phase AC. Each coil has the appropriate phase connection to pull the projectile further down the tube. Now you have a linear induction motor.

This seems to be reasonably efficient.

There is little power wasted while idling since energy is stored inductively and released on the next part of the AC cycle.

EF Northrup, who held many patents for induction heating, built some small models in the 1920s, and wrote a fanciful story about using this coilgun to launch to the moon in "Zero to Eighty". I saw some pictures of them dropping wrenches and small tools into the opening and having them fly away. Or am I thinking of Prof Laithwaite's coilgun? I forget.

Cheers, Barry
Have a Happy Electromagnetic New Year!

big5824

Thu Jan 08 2009, 05:39PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

It will be hard to find an AC power source that can supply enough current, a mains line certainly wont cut it. I think your best option would be something like a compulsator....I guess they could supply the huge current needed as a sine wave.

Woodie

Thu Jan 08 2009, 10:36PM

Registered Member #1916 Joined: Thu Jan 08 2009, 09:50AM
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Posts: 3

@Dr. Slack: Couldn't you have a repeating coil gun using this model? As long as the power is a steady sine wave and you feed it ammo at the correct intervals wouldn't this then make it much more efficient? Most household AC is 60Hz which even at .5 or .25 that would still be considerable would it not? Also, why does it need multiple acceleration profiles?

@Barry: On the train of thought I obtained from my lack of sleep and reading Dr. Slack's reply, would you be able to use a switch or something to restart the current to create a repeater? I will read up on linear conduction motors in the mean time.

Marauder709

Fri Jan 09 2009, 02:22AM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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Posts: 22

Also note that solenoid coils have uniform magnetic field inside the coil. Once the projectile is fully enclosed by the coil, there is no longer any physical forces upon it, so it's merely coasting whether the coil is energized or not.

I don't think so Barry. An armature is in equilibrium when it is in the exact center of the magnetic feild. In the case of a solenoid, in the very center of the coil. The force exerted on the projectile becomes less and less (by an inverse square law i believe) the further it goes into the solenoid. If the coil's length is equal to the armatures length then I would agree with you. However, in the case of a solenoid whose length is quite long compared to the length of the armature, the feild will continue to accelerate the armature until it is in the center of the feild.

rp181

Fri Jan 09 2009, 02:56AM

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

I agree with barry, There is a central region, not a central point. I just added it to the wiki today too =p

The magnetic field from the end coils wont reach all the way to the other side, when you reach the central region, the net force is zero.

Dr. Slack

Fri Jan 09 2009, 08:05AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Barry wrote:

So you don't get a magnetic field to form a sine wave in space, only in time.

Exactly, that's the problem with a single phase synchronous machine. It's only when you interact with a projectile that's already moving, *in the right phase*, that you get a net force. It's similar to the problem that old synchronous clocks had, they'd sometimes start backwards.

Woodie wrote:

... Also, why does it need multiple acceleration profiles?

It's not so much that you need multiple profiles because you need more than one, it's because you need to have it varaible to change it until it works, and then change it every time you change anything else, like the projectile mass, starting position, coil voltage, friction of the armature on the barrel, it's just not practical.

With a rotary synchronous machine, once it starts in the right direction, it pulls into the right phase, and it runs as a motor with the previous pulses ensuring that the armature is in the right place for the next pulses. With a linear machine, it's all got to happen correctly from the start, one pass through the barrel, everything's got to be right. It's why single stage guns are easy. Two stage guns can be made to work with fixed timing, but they're easier triggered. At least you can control the relative timing of the two coils, to set it up for your projectile, a degree of freedom you don't have with the fixed coil position, fixed frequency setup that is the subject of the thread. 3 stages and more really need armature-triggered timing. You just can't get a sufficiently repeatable position versus time for fixed timing for all the coils to contribute the desired forward force.

Woodie

Fri Jan 09 2009, 10:34AM

Registered Member #1916 Joined: Thu Jan 08 2009, 09:50AM
Location:
Posts: 3

Ok, so the 3 phase AC linear motor is the best way to go about this.

@Dr. Slack: What if you used specially made bullets which are already in motion in the direction you want through tradition gunpowder and used the coils in assist to achieve higher velocities? Also, I don't think that would be much of a problem since you would try to find something that works and stick with it. I'm not thinking along the ideas of endless experimentation but of finding something easy to use and cheap enough that other people would want to use it, then if the idea works you could sell it and afford much more research then just soliciting people for donations for research (For example Northrup Grumman is trying to market their Firestrike battle laser system and if they can sell it in the future they will be able to do the R&D to sell smaller, cheaper, more efficient and yet more powerful lasers which could have commercial uses like literally carving rock to safely mine without needing explosives or megawatt power generators like the engines found inside the Abrams tanks)

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