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Coil Size

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KillerRabbit

Wed May 30 2007, 02:04PM

Registered Member #803 Joined: Fri May 25 2007, 01:58AM
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Posts: 5

I was wondering, how do you calculate the correct coil size for your capacitor? I tried going to the Magnetic Gun Club's site like an old post suggested, but it seems to be down.

Dave Marshall

Wed May 30 2007, 05:38PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

There is no 'correct' coil size. Actual coil dimensions can vary wildly from a small diameter, long coil to very large diameter, short, high current coil.

Really you have to look at a variety of things to come up with a good starting point. Voltage of the capacitors is a major factor. If you've got a very high capacity, low voltage capacitor, you'll want to keep resistance and reactance to a minimum, to maximize peak current and keep the discharge as short as possible.

With a smaller capacity, higher voltage capacitor, you may want to use somewhat smaller diameter wire, and more of it to give you maximum flux density and extend the discharge somewhat so it can achieve max coupling.

All this is for a reluctance based system obviously. For an induction based coilgun, its pretty much maximum current, maximum surface area, minimum discharge time, all the time.

Material availability will play a big roll in determining your coil dimensions, as will your intended goal. You should have some idea of what you hope to accomplish with the system. For max efficiency, careful design of the coil will ensure max coupling and minimum 'suckback'. For all out speed, you may just want to go for max rise time and try and keep peak current as high as possible.

I know I've rambled a bit here, but heres an example. I've done two major coilgun projects, and numerous smaller ones. The two major ones were on opposite ends of the spectrum. The first was for all out velocity. 4,300 joules of stored energy, 1200V 1,000uF capacitor banks, 6 stages, and a .25" 15g steel projectile. Discharge currents approached 5KA, and max velocity was around 100m/s consistently. The coils were short, exactly as long as the projectile at 2". They were 13AWG wire, and between 4 and 6 layers of wire. It was attrociously inefficient, but made a hell of a show.

The new system is a mains powered (yes, straight from the wall), IGBT switched system that peaks at 220v, 75A. Because my peak current is limited not by my switch, but by the fact that I don't want to blow a breaker, I can easily calculate the ideal impedance of my coil, right around 3 Ohms. Because this is such a large resistance compared to most other coilguns, I can afford to use smaller diameter wire, and lots of turns for maximum flux density and coupling. Because of the small amount of energy imparted by each stage, there will be 25-30 stages when its finished. Also because of the lower power involved, switching is far more efficient. The fact that I'm using a very large and heavy projectile, around .5" diameter and between 50 and 75g, velocity is fairly low, making precision timing easier. This should hopefully lend itself to an efficiency greater than 10%, with a kinetic energy similar to the first system at 150j. Velocities will be dramatically lower though.

This should give you a bit of an idea where to start atleast. There are a multitude of other factors that have to be considered as well, but the above are probably the most important to start with.

Dave

KillerRabbit

Wed May 30 2007, 07:37PM

Registered Member #803 Joined: Fri May 25 2007, 01:58AM
Location:
Posts: 5

So, wha you're saying is that a high voltage, low capacity cap will use a smaller, less resistant coil, while a high capacity, low voltage one will work better with a larger one? Could you explain how high is high capacity? I'm a bit new to all this.

uzzors2k

Thu May 31 2007, 07:29PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

It all depends on what speed the projectile is moving at, and getting the pulse to die away in the time it takes to center in the coil. Pretty tricky to do without trial and error in other words. What you should do is design your coil to limit the current pulse to what your switch can handle. Say you have a 500A SCR and a a 1000ÂµF 200V capacitor. Your coil could then have 0.1 ohm of resistance and 110ÂµH of inductance. That would limit the current at 485A max. Then you design your coil for 0.1 ohms of resistance and 110ÂµH of inductance. For newbie coil gunning just design parts of the coil gun to suit the rest of the coil gun.

Check here to see what your peak current and pulse length will be.

Dave Marshall

Thu May 31 2007, 07:58PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Actually you've got that backwards. Any RC circuit has whats known as a time constant. The time constant is defined as T=RC (where t is time in seconds, R is resistance in ohms, and C is capacitance in farads). A capacitor discharge is whats known as an inverse curve. Each time constant is the amount of time it takes the capacitor to discharge 50% of the energy stored in the capacitor.

Heres a site with a good example

Essentially a higher voltage, lower value capacitor will discharge much more quickly into a given coil than a lower voltage, higher value cap, and it will achieve a higher peak current. In some cases the capacitor might discharge so quickly that the projectile doesn't have time to reach the optimum point in the coil for full effect. To counter this you'd use a more resistive coil to slow the discharge slightly. Other times the discharge lasts too long, causing 'suckback', basically the magnetic field holds the projectile back, slowing it down.

The ideal coil is one that will allow the capacitor to fully discharge in exactly the same amount of time as it takes the projectile to reach the center of the coil. Once the center of the projectile passes the center of the coil, the forces reverse, and the projectile is being pulled back into the coil.

You should definitely read up on the dynamics of coilguns. Barry's Site is a great place to start.

I'd highly recommend the following experiment to help you see how different variables change the performance of your coilgun. Build a couple different coils, maybe one with largish wire, say 14AWG, and another with thinner wire, maybe 20AWG. Wind maybe 12 or 14 layers of wire on the coil. Then with the first coil, try positioning the projectile different distances from the start of the coil. Start it out just outside the coil on a couple shots, then even with the start of the coil, then just inside the coil. Cut off a layer and try it again. Eventually one of the combinations will likely produce a large jump in the projectile's velocity. This will indicate a more efficient setup, when you cut the next layer off, if you've reached the peak efficiency for that setup, it will begin to drop in velocity when you remove another layer after that.

Try it with both types of coils and see where the differences are. Through trial and error you can gain an understanding of the interaction between the different variables in the system. Once you have a set coil size that works well, you can calculate interesting values like inductance, impedance, and peak current.

You haven't said what your capacitor values are yet, but I'd recommend some sort of heavy mechanical switch to start with, like a light switch. SCRs take some skill and experience to use, and household light switches are both cheap and fairly tough.

Dave

uzzors2k

Thu May 31 2007, 08:49PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Oh yeah, the RC time constant...

I was mostly thinking of the efficiency and therefor projectile speed though, which is difficult to calculate ahead of time, and determines how long the pulse should be.

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