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currentkills91

Wed Apr 20 2011, 07:42AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Hey again,

I've began building a small handgun coilgun while waiting for parts to arrive for other projects. The gun is going to be a single stage design, hopefully small enough to fit in a pocket. The coil I have made is 35 mm longs, and 6 layers of 18 gauge wire on a 5 mm brass tube. As i was working with some capacitors I have laying around, i realized i don't really have what i need.

My question is, is what do you guys think is better. Should i go with a low voltage, around 200 volts, or a higher voltage, around 400 volts? If I play around with different values of capacitance is it possible to get similar results with a lower voltage as compared to a higher voltage. I'd experiment myself but i don't have any capacitors available to use laying around.

Thanks in advance!

(if your interested theres a video on my youtube channel of the coil being fired when it had 8 layers - telsadude19)

Saz43

Thu Apr 21 2011, 04:37PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

wrote ...

Should i go with a low voltage, around 200 volts, or a higher voltage, around 400 volts?

It's hard to say. I've seen huge successes and huge failures with coilguns operating in a wide range between 12 and 1000 volts. It really depends on your ability to match your coil to your capacitor bank. I would reccomend using the simulators by Barry to tweak your coil and capacitor bank to get the most amp-turns possible in a brief period of time (2ms<t<8ms). I'm sure that there is a good soultuion for the parts you have, but it may come down to trial and error untill you get optimal results.

A coilgun designer's two best friends:

Barry

Fri Apr 22 2011, 02:42PM

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

currentkills91 wrote ...

Should i go with a low voltage, around 200 volts, or a higher voltage, around 400 volts? If I play around with different values of capacitance is it possible to get similar results with a lower voltage as compared to a higher voltage?

Either one can work equally well. The reason is the magnetic field comes from current, not voltage; the field strength is a simple product of current times number of turns. The projectile itself has no way to know what voltage caused the current.

Suppose you have a small box of capacitors, and are trying to figure out whether to connect them in series or parallel. The bottom line is, given the same amount of stored energy, a properly tuned coilgun can have the same performance in either arrangement.

Just be aware that lower voltage designs (assuming that same box of capacitors and stored energy) will necessarily have a coil with fewer turns and higher current peak. That is, when you put caps in parallel to operate at lower voltage, they have higher equivalent capacitance, so to keep the same LC time constant you choose lower inductance. Also to get the same amp-turns of magnetic flux, with fewer turns there is higher current. The point here is that lower voltage designs require thicker wire and lower-resistance connections to achieve the same results.

I believe the choice of voltage is a matter of practical and economic decisions. Figure out what power supply you have or how much it costs. Choose an SCR or switch that can handle the load at the current and voltage you expect. See if you can get magnet wire in the size you need. See what you can afford and and can get in the time you want, and your choice of voltage will magically make itself known.

For myself, I like to work under 50 volts for safety reasons. But the economic sweet spot generally includes everything from 200 to 400 volts. My bigger coilguns are at 310 volts because I can make a cheap Q&D power supply from a voltage-doubling transformer and diode bridge. These transformers are commonly sold for international travelers moving between 110v and 220v regions. My voltage is therefore (110 vac) * (2) * (1.414) = 310 vdc. To get an adjustable voltage all I have to do is connect a variac, then it can reach about 370vdc.

currentkills91 wrote ...

The coil I have made is 35 mm longs, and 6 layers of 18 gauge wire on a 5 mm brass tube.

If you already have the coil built, run the inductor simulator to find its inductance. Or just measure it. Then use the RLC simulator to see what the timing will be for various capacitance.

What timing will you need? You'll have to work that out for yourself. I have some ideas about it spread across a few of my web pages starting with estimating kinetic energy. Damn, I should turn that into another simulator, eh.

Cheers, Barry
Centrifuges: They're what separates the men from the boys.

currentkills91

Fri Apr 22 2011, 09:22PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Thank you both so very much. I'll just tinker around for a while with the sims and stuff and see what happens. ill post new stuff as do it (:

Xray

Sun Apr 24 2011, 04:27PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Barry wrote ...

If you already have the coil built, run the inductor simulator to find its inductance. Or just measure it. Then use the RLC simulator to see what the timing will be for various capacitance.

Cheers, Barry

Should you measure the inductance with the projectile in the core? I ask this because it seems to me that the inductance will be quite different with the projectile in the core versus it NOT in the core.

Thanks,
Xray

Barry

Sun Apr 24 2011, 11:12PM

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

Xray wrote ...

Should you measure the inductance with the projectile in the core? I ask this because it seems to me that the inductance will be quite different with the projectile in the core versus it NOT in the core.

Measure it without the projectile, this is the most important number.

Actually, for any over-achievers out there, measure it both with and without the projectile. Divide the one number by the other, that is, divide the larger by the smaller inductance to get a number greater than one.

This is the "saliency ration" and is a useful figure of merit used by conventional motor designers. If you can tweak your design to increase the saliency ratio then you will improve your coilgun. Cool!

Note 1: "Inductance" is weakly defined in coilguns. Once the magnetic field at high current exceeds the linear region of the B-H curve, then its difficult to make accurate predictions of how the system will operate. A further complication is that the motion of the now-magnetized projectile will also affect the coil's voltage.

So we generally use the "no projectile" or minimum inductance for a quick estimate of the LC time constant. Then just build the damn thing and adjust things for best performance. Otherwise you too may suffer the debilitating medical condition "paralysis by analysis".

Note 2: Most engineering schools will teach V = L di/dt, and EE students are accustomed to using this everywhere. But it doesn't apply directly to coilguns because "L" is changing with respect to time. So the equation to use is: V = d(Li)/dt and from this you can then see how it's usually simplified to the more common equation.

Cheers, Barry
Actually, all diodes are zener diodes if you apply sufficient voltage.

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