Re: Best Coil for a given capacitor size
Quantum Singularity, Sat Apr 01 2006, 03:34AM

Well I cannot tell you all the details myself, but as indicated on another thread here titled "need assistance on my first coilgun" there is a link to the magnetic gun club which has all sorts of info and simulators.

Sounds like you have the right idea about testing different number of layers for your coil... sometimes these things take alot of trial-and-error design. However if you wound only 8 layers and thats giving you the best results then chances are you might want to wind more layers and test with more than 8. The only way to know for sure is if you can see the projectile energy rise to a point and then fall again. That peak is obviously were the setup will work best at. If you tried all your windings and its onyl gone up and not back down yet then you probably havent reached that peak. I am no expert myself but these are the few things I have learned. Coil length is determined by projectile length (should be about twice as long), wire diamter is set by cap voltage and ESR values (cant be much more help here, I would be asking those who have used similarly sized caps succesfully) to small wire wil either melt or be to high resistance and too large wire will not create as strong of magnetic field density, the number of layers will set the pulse length and is which is tuned with the cap value and how heavy the projectile is. I cant really give you the forumlas but alot of them are on that link. Good luck and hopefully someone with more reluctance-based coilguns can give you some more advice - that is the type of gun your building right?

Re: Best Coil for a given capacitor size
Reaching, Sat Apr 01 2006, 01:56PM

yep, reluctance based.
i tried the calculator on the website and wound a new coil with the calculated windings,layers etc. it performs very well, but how can i calculate the number of turns,layer for the second and third stage? is there a rule of thumb to do this, without braincracking myself with alot of formulas? all stages have the same capacitance., cause the projetil accelerates in the first stage and reaches the second with a relatively high velocity, i only have to wind half the winings and on the third stage half the windings again or so?yeah, dont think its so easy .

Re: Best Coil for a given capacitor size
Desmogod, Sat Apr 01 2006, 02:09PM

Maybe try calculating the "ideal" using the sims available, then experiment by changing the amount of turns, layers etc.
Keep going in the direction that gives increasing velocity, then stop when it tapers off.
I'm sure it would be specific to your setup though, environmental conditions etc, much like TC's.

Re: Best Coil for a given capacitor size
Quantum Singularity, Sat Apr 01 2006, 06:43PM

I forgot to consider the fact that each coil will be different and only addressed one... Sounds like you already know whats going on just want a calculator for it. Not sure if theres one out there that will take into consideration inititial velocity (as the only difference that is a given is that the projectile is an initial velocity upon entering the coil) . But if you already built a test coil you should have eveything you need. For example this is what I would do. Build a test coil with enough layers to be sure youve reached the optimal amount for 1 stage. This 'test coil' should have each layer wound so you can test each layer (with a connector for each layer). Once you have determined the best 1st stage coil then build one with those specs. For the second stage then re-use your test coil. Since your using the same caps and same projectile the only thing that really has to change is the number of layers/turns. Already have a test coil so start at a layer less and keep going smaller until you have found the peak velocity. You need to have the electronics done before all the coils because testing it this way means you have to have your trigger/timing circuit and everything so that both stages are firing. Once you found the right number of layers then build a coil to that spec and replace your test coil with it. Same procedure for the third stage, throw you test coil on it and start with the same or one layer less turns and keep going down until you have the max velocity for all 3 stages firing. You could even keep on going with more stages using this procedure. As I have never built a reluctance based cg before I am not sure if its critical if you tune for tha actual # of turns in your coil or just the number of layers (like for example maybe 5 layers isn quite enough but 6 layers is to much). That would take more work if you needed to be that close.

Re: Best Coil for a given capacitor size
Yohan, Sun Apr 09 2006, 04:28PM

Reaching,

Are you familiar with differential equations? The solution of a differential equation is another equation...

I've tried to stress to several people on the forum that, for CG's, the best approach is to use a very large C and trigger with an IGBT so you can avoid the decaying pulse that you have no control over with the SCR.

The equations and resulting waveforms representing the pulse are different between the two triggering schemes. Therefore, the approach to deciding what coils, tpi and gauge to use can vary drastically.

I'm not going to explain both approaches so post what trigger you have chosen and we'll go from there.

Re: Best Coil for a given capacitor size
Sonny, Sun Apr 09 2006, 07:31PM

I agree with Yohan. Go for the IGBT/FET way, that would give you the far best efficiency though i don't really like IGBTs because it's hard to paralell them together. I tried this with 2 IGBTs rated at 1200V 180A peak each with a coil at 3.4 ohms and 300V capacitor charge. After 4-5 shots one of the IGBTs blew up. After that i went for the FET-way and now i'm playing with a 2.3 ohms testing-coil (at the same voltage, 300V) with 2 FETs in paralell rated at 300V and 140A peak each with no problems at all.

Have been shooting 200+ shots and so far no problems at all :)

Re: Best Coil for a given capacitor size
Reaching, Sun Apr 09 2006, 08:05PM

mhh, no, i got my scrs very cheap and i have a bunch of them. they are ratet for 10kA at 1,2kV, i can short circuit them with a 6kJ cap bank .. its impossible to destroy them. using fets and igbts will end up in burned semis

Re: Best Coil for a given capacitor size
Sonny, Sun Apr 09 2006, 09:31PM

Yes, sure SCRs are alot more easy to build with. The only "hard" part in an SCR based coilgun is to make the coil right. Your capacitors holds 273 joules charged at 450V. I don't really know the efficiency on an SCR based coilgun but it should be maybe ~5% so that would give you ~13 joules of kinetic energy. If you know the weight on your projectile shouldn't one be able to calculate the maximum speed of the projectile then? And if you know that speed you can get an "on-time" for the coil.

Then you can calculate how big current you will need to draw the capacitor to ~0V. Dunno if this really works but it should give you a clue to start with.

Re: Best Coil for a given capacitor size
rupidust, Mon Apr 10 2006, 03:23AM


Those are some rather high ohms inductors you made. Are you using thin <= 22 awg wire or more than 12 layers and long coils? At 300v and 2.3 ohms the currents are about 100 amps. I expect no component failures at this current range.

Now as for the best coil for a given capacitor size, that remains to be found by the builder. If you found the best coil for X Farads, this would no longer be best if you decrease or increase charge voltage. Change projectile mass, length, or metallic propertiers, then best coil will have to be remade to accomodate the change. Same is true whether you implement a pulsed naturally commutated system or a forced communtated constant current source, the coil may need to be changed depending on the capacitor voltage at time of discharge.


Pulse width:
The best coil for a given capacitor size is the one where when the capacitor is discharged and accelerates a load, the pulse width is approximately equal to what you wanted it to be and the magnetic force exerted on the load is approximately equal to what you wanted it to be. A coil might have the correct pulse length as desired but yet not exert sufficient force on the load due to a low inductance value, 1 layer and 5 windings for example. As for pulse width, say the force on the projectile exerted is satisfied, but the pulse width is longer than desired, so long that the projectile is slowed down or oscillates. Proper force alone without proper pulse width would then be meaningless.

Pulse Shape:
The best coil for a given capacitor size is the one where when the capacitor is discharged and accelerates a load, the pulse shape is approximately equal to what you wanted it to be. For a forced commutation system (on/off) controlling a constance current source (excess capacitance) the waveform should be as horizontal as possible or as designed. If the waveform significantly drops as it approaches the end of the pulse width, then the coil needs to be altered or the capacitor size (Farads) need to be increased, which negates the whole "Given capacitor size" idea.

Load constraint:
The best coil for a given capacitor size is the one where when the capacitor is discharged and accelerates a constant load, the dynamic pulse width and pulse shape requirements are satisfied. A coil may actually produce the correct pulse width and shape for an unloaded (dry fire) discharge. However, the coil's inductance does dynamically change when in close proximaty to a metallic object, thereby effecting pulse width and shape, and consequently demoting the coil from the rank of best. Another example of load constraint is to have the best coil for a 5 gram slug during experimentation but not have the best coil for a finally decided same 5 gram mass slug but with a different diameter, length, or metallic property.

Since the coil alone is not the only component effecting the end result, a better question is "What is the best arrangement for a given Joules and Velocity using a fixed capacitance?"