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Tips to making an efficient coilgun coil without a chronograph

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jpoopdog

Wed Jun 26 2013, 11:30AM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

Hi,

I just went to toyworld whilst handing out resumes everywhichwhere at all the stores i liked. Now benifiting greatly from the toysRus across the road being burnt down, they were having one of their many sales and i got some aluminium hobby tubing, some 6mm stuff, super thin, very cheap, and it occured to be right before i bought it, i should actually get to work on my coilgun once again, i have a mini inverter, i have all the parts to properly make a boost converter if i dont want to use the inverter, i have heaps of wire and now ive got a perfect shaft, also i have a lego NXT and a buttload of lego technic i recently sorted from my regular lego ( a milk crate sized box of technic lego) from which to make a coilwinder, plus i have a 3d printer to fabricate any random parts i want, so whats stopping me?

Now, a while ago i go into a chat with someone who actually knows a heap about coilguns, and i found out that most coils are less than 1% efficient! which is quite bad.
to get to the point, i want to build another coil, my last ones were an estimated 0.001% efficient or some ridiculously low number, and i wound those by hand and spent quite a bit of effort on those, but i didnt know what i was doing, and thus the consequences.

I am going to use some of this 6mm aluminium tubing to build two coils, one hand wound perfectly with superglue applied over each layer so each layer is perfect, the other with a coilwinder i shall build which should roughly do the same thing, wind perfectly, but with no resins or glues or anything.

Now, given i have a 6mm shaft, ill be using roughly 3-5mm munitions, either steel ball bearings or neodymium ball bearings of the same size (if it works better), ill be using 4x 450v 470uF capacitors in a 450v 1880uF 190j bank, or 10x 2200uF 220J, or 10x 330uF 334J
How should i go about winding my coil? i got lots of different guage wire here already, essentially im asking anyone how i should make my coil to attain maximum output efficiency for 450V at either roughly 2000uF or 3000uF 200 and 300J respectively, so i get the best possible effeciency possible?

Im after, best guage, coil length(number of windings per layer) and number of layers, as well as any tips or tricks to wind the coil as efficient as possible.
With the 3 capacitor bank sources ive located 3 lots of capacitors i can get for under $30, but im not sure between the 190J 4pc bank and 220J bank, whether its best to have few capacitors, or more, and if more, then could i go higher, to 330J. Heat isnt much of an issue for me, also i do plan on playing around with the SCR to switch off after the bulk of the load has been dumped, or strategically switching the capacitor off to both conserve charge, and to increase power output to the amunition, by preventing any drag back, where the amunition is pulled back by the electromagnetic feild if the capacitor isnt totally empty by the time the amunition has gone past the center.

Im asking for opinions based on logic and possibly personal experience, im aiming at making a decent portable single stage coilgun which i can shoot through cans at long distances.
Also, what would be a reasonable efficiency to attain? essentially im after a little more than an air rifle which ive heard was roughly 5 joules, but the more the better. Is the asumption that i can get that kind of power out of a 200-300J bank realistic?
Because i dont have a chronograph, i dont know how to measure efficiency, however im sure i can come up with a system of sorts to measure wether im getting stronger or weaker shots, but the question is what do i do with this information? would i have the coil permanently set and just be playing around with the best amount of energy to dump through the coil or how far or close to be placing munitions near the coil? what would i be doing?

Any information or tips anyone could give be as to selecting the best bank (im dead set on 450v), and the ideal wire guages and coil size, would be greatly appreciated.

Wizzup

Wed Jun 26 2013, 01:04PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

I am not an expert on coilguns but I know some things.

Firstly a plastic barrel would be much better compared to the aluminum barrel to avoid eddy currents. Eddy currents eat a part of the energy that would otherwise be used to accelerate the projectile. At your energy levels there is practically no risk of crushing the barrel on firing so it is highly recommended.

Next thing is the projectile. I would recommend using much longer projectile than a bearing, because a bearing has very little mass that the magnetic field can grab compared to an iron rod. If the projectiles diameter is 5mm then a good length for the projectile would be something like 3cm, more or less, its your choice. You don't want to make the projectile too long because the magnetic field around the coil gets weaker by the square of distance, so if the other end of the projectile is 10cm away it is basically dead weight. Also you want to make the projectile just slightly smaller than your barrel's ID, because of the same reason as above you want to make the distance between the projectile and the coil as small as you can.

Next to the capacitor bank. I would take the 3300uF 450V bank as it has the most energy. I dont know where you live but if you live in europe then this would also be a good choice:

I used 3 capacitors with identical stats for my own coilgun in parallel for 14100uF at 450V so the more the better.

For the wire gauge, you should use as thick wire as you can as long as you can wind it without leaving large gaps. Generally the larger the wire gauge, the higher the inductance is compared to the coil resistance. I used 1.5mm diameter magnet wire myself for my coilgun as it was the largest I could find in my local electric store but I would have used even thicker wire if they had had it. I recommend winding the coil by hand when using 1mm or thicker wire because you have to use some force to get it wound tightly.

For the number of layers it really depends on the bank capacitance and wire diameter and some other things, I can not tell you the exact number but it would probably be between 4 and 8, again it depends greatly on lots of things. The coil length should be generally the same as your projectile length to get the best linkage between the coils magnetic field and the projectile.

An SCR is a good choice for a coilgun but it doesn't protect the capacitors from negative voltage alone. You should place an anti-parallel diode across the coil so that it is in the opposite direction compared to the capacitor bank, so that when you fire the gun, the current goes through the coil normally and when the coil reverses the voltage the diode activates and the bad current will go through the diode and not store in the capacitors. The diode doesn't need to be massive, I used 5x UF5408 diodes in parallel for my coilgun and they handled it well.

I recommend using this simulator to get a rough idea of how many layers to use and so on:

I designed my own coilguns coil using that simulator and I achieved something like 3-3.5% effiency using a 10mm diameter 4.5cm long projectile. Here is a video of it if you want to check it out:

Not a very good video but it shows the main parts of it.

I hope I helped you and cleared some things.

jpoopdog

Tue Sept 17 2013, 02:03PM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

oh hey, the coilgun simulator file is missing mind reposting?

Wizzup

Wed Sept 18 2013, 11:56AM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

Yeah it was probably deleted because not many people downloaded it probably, here it is again:

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob