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Electromagnetic Cannon

A place to ask general spud cannon related questions.
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Electromagnetic Cannon

Unread postAuthor: Reactor4 » Sat Oct 03, 2009 9:43 pm

Okay, i have just started building an electromagnetic cannon. the idea is that the emf generated from the windings around the barrel with send a spud with a steel rod thru it out the end. I was wondering if anyone has suggestions for what to use as a power source, possibly a rectified source? transformer maybe? any design ideas accepted too! Thanks![/b]
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Unread postAuthor: tghhs » Sat Oct 03, 2009 9:57 pm

Sweet dude, more deatiles though. Are you planning to make a coil gun or a rail gun? Will you use a injector to get it up to speed before putting the final push on it?

As for power source...

http://www.spudfiles.com/forums/sparker ... 13030.html

with this sort of power bank

http://www.spudfiles.com/forums/28-8kj- ... 13449.html
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Unread postAuthor: Reactor4 » Sat Oct 03, 2009 9:59 pm

well my plan is to use a coil, but im kinda goin back and forth between that and the rail. and as for the injector, possibly, or maybe some sort of capacitor, like in a capacitor start motor.
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Unread postAuthor: inonickname » Sat Oct 03, 2009 10:11 pm

You don't need too much voltage for EM weaponry, but a lot of current. Unless you have a very large capacitor bank ignore railguns completely as the lorentz force drops off rapidly at lower energy. A coil (gauss) gun or electrothermal/electrochemical gun would be a better idea.

If your a whizz with electronics you could work on a multistage coil gun.

Injectors are only of use in a gun in which the current is passed directly through the projectile; i.e. a railgun. It helps prevent welding and erosion (which some people will deny) and removes the need for a massive solid state which can handle the current.

It would be good to understand the concept first.

You will only need a small transformer/rectifier to bring the voltage up to around 300-400 volts and charge your capacitor bank. Pulse capacitors are best, though most kinds will work.

You will need a capacitor bank. Just pumping mains directly through doesn't work.

as for the injector, possibly, or maybe some sort of capacitor, like in a capacitor start motor.

Why did you think a capacitor would have any use as an injector?

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Unread postAuthor: Reactor4 » Sat Oct 03, 2009 10:22 pm

yea i didnt mean use the capacitor as an injector, but thanks for the advice. My dad is an electrical engineer so he could help me with a multistage coilgun, but im not sure if I want to go that far. And yea i get the whole voltage vs current thing its just been a while since physics :D (I have some electrical experience but mostly from motors i've made.)
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Unread postAuthor: Technician1002 » Sat Oct 03, 2009 11:52 pm

I don't recommend using an iron core. The coil would attract the iron core. An back EMF launcher requires inducing a large as possible current in the core for the core to create an opposing magnetic field. Using a poor conductor such as iron is not a good choice. A copper ring works much better.

There is a Make article on them that uses the high energy DC pulse that others have mentioned. This is not your AC transformer floating ring launcher. Be advised, dangerous voltages and currents are present.

http://blog.makezine.com/archive/2006/06/induction_launchers.html
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Unread postAuthor: i-will » Sun Oct 04, 2009 1:34 am

when i did it i used a capacitor bank made from a lot of disposable cameras. i can't really think of this project being full or semi auto because of the charge time. you have to balance voltage, charge time, size and weight of the system, etc. also if you want to do it right you'll need math. youtube has a lot of great examples.

have fun.
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Re: Electromagnetic Cannon

Unread postAuthor: Ragnarok » Sun Oct 04, 2009 4:32 am

Reactor4 wrote:Okay, i have just started building an electromagnetic cannon. the idea is that the emf generated from the windings around the barrel with send a spud with a steel rod thru it out the end.

My recommendation is actually understanding electromagnetics.

A coilgun works because the projectile provides a good path for the magnetic flux. The better it will complete the magnetic path, the stronger the force on it.

So, anything where the rod diameter is so small compared to the coil diameter will be very inefficient. We're talking fractions of a percent here. In fact, many coilguns with reasonably decent projectile/coil matches already have efficiencies of under 1% - so you'll be getting even worse.

Not a recommended idea. If you're doing a coilgun, the projectile must be (almost entirely) steel, and as close to the coil diameter as feasible, and even then, you can't expect efficiencies of more than a few percent (although I'm personally working on one that might achieve 10% efficiency)

Also, I'm somewhat confused as to why you think a railgun would work in this case.

I think what you've done is looked at the idea of magnetics and thought "Hey, neat" - and then proceeded to try and implement it without actually learning any of the details.
These things are a LOT more complicated to set up than any "pressure based" cannon like this site normally deals with, and coilguns in particular are very fussy about their projectiles if you want any performance out of them.

@i-will: The horrible charge times from a disposable camera set up are because of the charger only having a 1 watt output. Bolt together a decent charger, and you can get up to about 100 watts - although you need a sizeable battery pack to supply that. A mains driven transformer can easily manage 1 kW charge rates or more.
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Unread postAuthor: joepage2008x2 » Sun Oct 04, 2009 5:36 am

My opinion, dont bother - too much trouble and costs too much.

Ive built a lot of coilguns and they have awful efficiency, i myself have only managed 1.6% with a single stage.
Heres the problems, capacitor banks are very bulky, so if your looking for a hand-held one then it will be very weak.
You also need to use SCR`s as switches as the current is way too high for other switches and they need to switch fast, they are very expensive - so multistage will be very expensive.
You need between 300v and 1000v for good efficiencies, very expensive capacitors.
Also it takes just 16J to kill a person, mine was at 500J with 1.6% efficiency and acheived a muzzle energy of 8J, the projectile wont kill you but the capacitors can easily.
And like what ragnarok said a disposable camera only gives out 1W, so if you go for 500J it would take 8min 20sec to charge. A good way i found to charge a coilgun fast and make it handheld was an inverter.
Inverters in the uk step 12v up to 230v and with a voltage doubler you get 460V, say is a 100w inverter and your capacitor is 500j then it only takes 5secs.

Plus you need more maths, when the coilgun turns on the projectile moves into the coil, the coil turns off when the projectile is in the centre of the coil and therefore continues moving, if the coil does not turn off then the projectile will just stay in the coil. This is the reason people use capacitors because when the projectie gets to the centre of the coil the power in the capacitor has fully drained, plus capacitors have high current capabilities. The problem is trying to get the right coil specs and the right capacitence specs to acheive the high efficiencies. Coilguns need a lot of tweaking.

A railgun will just total abliterate the potato before it comes out of the barrel and they cost a lot.
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Unread postAuthor: rp181 » Sun Oct 04, 2009 9:02 am

You also need to use SCR`s as switches as the current is way too high for other switches and they need to switch fast, they are very expensive - so multistage will be very expensive.
You need between 300v and 1000v for good efficiencies, very expensive capacitors.
Also it takes just 16J to kill a person, mine was at 500J with 1.6% efficiency and achieved a muzzle energy of 8J, the projectile wont kill you but the capacitors can easily.


Most of this is wrong. First off, This is his first coilgun, for those low energies, just about anything to switch will work, though semiconductors must be used for a multistage. You never told us your energy level. Railgun's need much more energy, and if the energy is too low, will not even shoot. You do not need 300v to 1000v for good efficiency, it is more about matching the coil to the power source. Take a look at these:
http://www.coilgun.info/mark2/rlcsim.htm
http://www.coilgun.info/mark2/inductorsim.htm
His whole website is great for coilguns, look through it. It does take 16 joules to kill a person, but a 500J bank will not kill you, unless you use a absurdly high voltage, and stab the capacitor to either side of your heart. The hand to hand skin resistance is around 10kohms. Assuming 1000 volts, that is 100ma of current through your body.
The skin affect will further reduce the risk of death, Don't get me wrong, there is a small possibility it could kill you, and if it does not, it will still hurt. My worst shock has been from a camera flash capacitor, as one of the terminals pierced my skin, and I have been shocked by a bank around 600J.
Also going with ragnarok, small projectiles could also saturate, reducing the efficiency. The flux path determines the effeciency to a great extent, that is why coils with a iron powder shell will work better.

I have several pages:
coilgun switching:
http://www.rp181.110mb.com/index.php?p=1_12
coilgun charging:
http://www.rp181.110mb.com/index.php?p=1_10
coilgun concept:
http://www.rp181.110mb.com/index.php?p=1_11 (this is grossly simplified)

And for railgun:
http://www.rp181.110mb.com/index.php?p=1_28

Note, everyone, including me, is giving much more information than required for a first coilgun, it is still useful though. I say grab a flash camera, and do this:
http://www.instructables.com/id/Disposa ... a-coilgun/[/i]
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Unread postAuthor: Reactor4 » Sun Oct 04, 2009 11:20 am

Thanks for the info rp 181. I was looking at how maglevs work and thought about using the concept of switching polarities respectivley to the position of the projectile. I would imagine this would give a somewhat higher efficiency performance. Thanks everyone for the enormous amount of info, I think i will start of with a little prototype (using the flash camera) and then after success with that, move up to a bigger size. I actually just thougt of something too.

I could put the coil on the inside of the barrel, and have an iron ring slide down the oustide, an electromagnetic inductor based thing, like they have in science museums.
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Unread postAuthor: rp181 » Sun Oct 04, 2009 11:32 am

Im not quite sure what you mean about switching polarities, but from what I understand, that would not work. The projectile is not magnetically oriented, which means the projectile will react the same to both polarities in the coil. Take a look at disc launchers, rather than the iron ring you said:
http://fastmhz.com/?p=33
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Unread postAuthor: joepage2008x2 » Sun Oct 04, 2009 2:10 pm

My coilgun was 500J at 500V, im sorry but that is absolute death with 50mA through your body.
with electricity you need a certain voltage for it to get through the skin effectively, you can survive 10V at 1MJ because of the voltage, i survived a 100J shock from 230v mains, but when you get to the higher voltages theres less chance. Im just looking out for people, people dont know how dangerous electricity is. Also a camera flash capacitor is around 300V and about 5J and I bet that hurt a lot, hook 100 in parallel and youll realise how dangerous they are.

I meant SCR`s should be used because i was talking about high energies. I used microswitches for my first ones, anything above 200J and they would weld shut. SCR`s also have a lot better life.

And yes any voltage can be used but the lower the voltage the higher the current that passes (the higher the amps that passes i mean) and therfore the switches dont last long and a lot o energy is lost, its a lot harder to make it efficient.
Voltages of 300-1000v are used because SCR`s are in better use at these voltages and their size is not wasted on small voltages.
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Unread postAuthor: ramses » Sun Oct 04, 2009 3:01 pm

rp181 wrote:The skin effect will further reduce the risk of death,


IIRC, the skin depth of a human, even at tesla frequencies is 60 inches.

So unless you're really fat*, it won't help you
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*[size=.]you know who you are. you have at least a 500 inch waist. [/size]
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Unread postAuthor: rp181 » Sun Oct 04, 2009 3:07 pm

That is not true. Lower voltage means LESS current. Think about ohms law, current is equal to voltage divided by resistance. That means high voltage means higher current in a faster pulse. Energy is lost when the voltage is too low, the resistance has a greater impact. This is why I said the coil needs to be matched to the power source, low voltages will work fine, if the coil is compensated. Look at this:
http://www.coilgun.eclipse.co.uk/images ... ristor.gif
This is a surface plot of a SCR triggered coilgun. Notice the lower voltage of 50v did better than the 80v shot. That is because it was matched with the coil and the projectile. Higher voltage means nothing.
This guy has got the highest efficiency I am aware of
(amateur), with 8%, and that shot was with a iron powder shell (enhanced flux as ragnarok said)
http://www.coilgun.eclipse.co.uk/images ... _33000.gif
Again, the best result was at 30v, not anywhere near your statement.

Note, higher efficiency does not mean higher velocity, the capacitance was kept constant.

I have been shocked many times (not so much recently), from 5kJ 400v banks, to 1600v pulse capacitors, to mains. The mains (this was not my fault, I was in a constructing house and there were exposed wires :evil:) Was the worst, as my whole body clenched, and the time for me to move was much longer. My upper body stayed tense for almost 2 hours, all shaky and whatnot.

Anyway, my point is it is more than a simple current figure to kill you, theres skin affect, salinity, contact points, and some others.

EDIT: From what I read, the skin effect is more caused by the higher resistance of the skin, rather than frequency. I read this a while back, so feel free to shoot me down :)
EDIT EDIT: I looked it up, can't find anything, so disregard everything about skin effect :o
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