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LAN_403

Ragnarok

Fri Jul 11 2008, 09:49PM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

This idea might be a complete dud and I don't even know if it's original, but I thought I'd throw it out there anyway.

For best efficiency, a reluctance coilgun will generate magnetic fields of about 2 or 3 tesla to avoid wasting too much energy to exceed the magnetic saturation of the steel armature.
This generally needs a relatively low current and voltage.

The inevitable problem that goes with this is that the coil's inductance slows the rate of current rise, and it can take some time to reach it's peak, restricting the impulse that's given to the projectile. This slow pulse also restricts the upper limits of velocity, because the current can't rise or fall fast enough to make it feasible for it to work on a faster projectile.

Now, an idea that's come to mind is that two capacitor banks could be used for each coil. One high voltage, relatively low capacitance one, with the grunt to push coil currents up to appropriate levels as quickly as possible, and one lower voltage, higher capacitance bank to maintain the coil current after it has reached it's peak.

Obviously, switching and charging would be more complex, but in my opinion, the idea could help erode some of the limits that plague coilguns.

I'd like to try the concept, probably in a rifle style design, but I'd also like some feedback on it first.

EDIT: Damn it, I was sure I was making this thread in the coilgun section. Apologies - could a friendly mod move it please?

teslacoolguy

Sat Jul 12 2008, 12:47AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Shouldent this be in the Electromagnetic Projectile Accelerators section?

Wolfram

Sat Jul 12 2008, 02:18AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Moved.

Anders M.

Ragnarok

Sat Jul 12 2008, 10:35AM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

teslacoolguy wrote ...
Shouldent this be in the Electromagnetic Projectile Accelerators section?

Yeah, I spotted that right after I'd posted it.
I was so sure I was making the thread in the EPA section - I have no idea how I got it in high voltage by mistake.

Shaun

Sat Jul 12 2008, 04:33PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I'm pretty sure this is why people use multiple stages in larger coilguns, because you can't have any single coil be too powerful since it will just saturate the projectile.

People DO use higher voltage and lower capacitance banks to get the current to rise faster, but they use them on the later coils in a multi-stage gun, when the projectile is moving very fast already.

Ragnarok

Sat Jul 12 2008, 07:37PM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

That's not quite what I said.

The point was to have two banks feeding each individual coil, one high voltage, low capacitance to allow the benefit of the fast pulse rise, but also a lower voltage one to maintain the optimal flux density for longer to maximise the impulse on the projectile.

My question is whether it's worth me making a prototype of the idea, or is my poor electronics knowledge is feeding me incorrect logic?

OZZY

Sun Jul 13 2008, 06:00PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

As I understand it your idea is to use a dual voltage drive to create a pulse with shorter rise and fall times. This is a very good idea, a pulse with a low ratio of peak to average current should be more efficient, especially if the fall time is shorter. As I see it this could be achieved with a variable voltage drive as you suggest or by pulse width modulation. Building a working prototype may not be straight-forward but please give it a go.

Ozzy

Ragnarok

Sun Jul 13 2008, 09:12PM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

Yes, that was the general idea. Multiple voltage drive to try and shorten pulse rise and fall times.
It would obviously be more complex to implement such a thing, but it might be worth it.

I have some other ideas I'd like to try out, and I was planning on doing a multi-stage coil pistol to test them.

Ragnarok

Sat Oct 04 2008, 03:50AM

Registered Member #659 Joined: Fri Apr 20 2007, 09:14AM
Location: SW Hertfordshire, UK
Posts: 75

I've come back and bumped this because I really would like some sensible feed back on it.

I've been discussing it with some other people, and they like the idea, although they're not particularly knowledgeable on coilguns, so I need the advice of people like you.

Please, it would really help.

guitardude012

Sat Oct 04 2008, 04:55PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

This is an interesting idea. It is the opposite of a v-switch.

Saturation speed is still related to acceleration right? If you slow things down do you plan on having sufficient time to accelerate the projectile before the projectile center of mass aligns with the center of the coil and it has to be turned off?

I will be thinking more about this idea today. If I understand correctly you want to reach a target current as quickly as possible and maintain it? This is similar something I have been working on with my third coilgun. At high speeds I believe it is necessary to "warm the coil up" before the projectile gets there to make the most of the coil's time at high current. If you time a v-switch right you can actually get higher currents than without it. Triggering the v-switch just before di/dt crossed zero would be ideal. I imagine you could do the same thing with larger caps for the quenching caps, in multiple v-switch stages, and you could get something close to a wave around your target current.

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