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Ragnarok

Sat Sept 08 2007, 11:00PM

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

Well, having mostly finished my latest air cannon project, I've decided I should pick up the threads of my old coilgun plans. As I'm going to university next week (studying engineering), I could do with something a bit smaller, quieter, and rather less destructive than my average air cannon to satisfy my ballistic urges.

I've gone and done lots of research, and I now understand all the basic principles (and some of the more complex ones) of the thing, but still I have a few questions.

For starters, it's just these ones, but I guess there will be more:

- Is there any accepted maximum voltage for a reluctance coilgun? (Safety issues aside) For example, would a 1000V (or maybe slightly more) cap bank be reasonable and practical, or just plain excessive with practical problems? I recognise that higher currents will induce higher eddy currents in the projectile and extra coil heating, but do these disadvantages outweigh the extra attractive force that would arise?

- Other than weight, size, extra components or more winding, would extending into multiple coils (above
about 3) have any major disadvantages?

- If eddy currents can be overcome, what are the other problems with external iron?

- Do you think a Half-bridge design would be too complex/large for a fairly small coil pistol/rifle? I did some primitive research into half bridges, but I'm no good at judging the size of these things.

EDIT: Hmm, I might have expected a couple of responses. Oh well, I'll add some more of my newbie-ish questions.

- Searching through the HV wiki, I found this statement about photoflash capacitors:
"However experimentation has showed that they are not suitable for series connection to increase their voltage rating."
I tried both a google search, and search of the archives to try and find out more than this, but the results I got from the searches didn't relate to this particular issue. Could anyone point me towards a topic/webpage that explains why they aren't suitable? I had heard different before, but I never know really.

- Are these IGBT's suitable for a coilgun or is there a problem with them? The internal schematic shows a diode across two of the terminals, which from my limited knowledge, I think is an absolute killer for trying to use them (I think problems with back EMF are likely), but I'm really not sure.

If they can't be used, it's a pity because they seemed a quite reasonable price. If anyone knows of a good source for a moderate quantity of some suitable and reasonably prices 1200V igbts for the UK, then I'd love to know about it.

Well, if my questions can be answered, I'm looking at a slightly more modest project than my older rifle plan, just building a (fairly) small 3-stage coil pistol to start with.

Barry

Mon Sept 10 2007, 02:41AM

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

Whew! That's a mouthful to start with. Let me just reply to a few points off the top of my head, and hopefully others will drop in to contribute.

Ragnarok wrote ...

Is there any accepted maximum voltage for a reluctance coilgun? (Safety issues aside) For example, would a 1000V (or maybe slightly more) cap bank be reasonable and practical, or just plain excessive with practical problems? I recognise that higher currents will induce higher eddy currents in the projectile and extra coil heating, but do these disadvantages outweigh the extra attractive force that would arise?

The maximum voltage you might choose is limited by practical and financial reasons. Remember, the field strength of a coil is proportional to current (not voltage) so it's okay to use whatever voltage to get the current you want. The advantage of higher voltage is that it results in steeper ramp-up of current for a given inductance. That is, for a given amount of stored energy, you can lean toward high voltage and lower capacitance, or toward lower voltage and higher capacitance.

In theory there is not limit to the voltage. In practice, it gets more difficult and expensive to find parts with very high voltage ratings. IGBT ratings are often up to 1200v or so. Much higher than that and you'll be looking at triggered spark gap commutation. But the practical matters of keeping the sparks where you want them can be difficult.

Ragnarok wrote ...

Other than weight, size, extra components or more winding, would extending into multiple coils (above about 3) have any major disadvantages?

Cost and complexity and the time to debug/tune all those stages are the main disadvantages of more stages (as you noted).

One big advantage of more stages is that efficiency is expected to increase as the entry velocity is greater.

One aspect to be aware of (I'm not sure it's a disadvantage or not) is the effect on velocity. The theory says that later stages will not affect velocity as much as earlier stages. They add kinetic energy, but the effect on velocity is not the same. Let's take a fabricated example ... let's suppose that capacitors for every stage holds 100J of potential energy. Further, suppose we can transfer 1J into kinetic energy in every stage (1% efficiency) into a 1g projectile. Note KE = 1/2 m v^2. Solving for velocity, v = sqrt(2KE/m)

Stage 1: adding 1J raises the speed from 0 to 44 m/s

Stage 2: adding 1J raises the speed to 63 m/s

Stage 3: adding 1J raises the speed to 77 m/s

The speed difference is smaller at each stage, although the kinetic energy goes up as desired

In the real world this may not matter; the efficiency will be different in each stage (hopefully better!)

Ragnarok wrote ...

If eddy currents can be overcome, what are the other problems with external iron?

External iron can saturate at high levels of flux, whereas air-core coils do not. In my opinion, this means we should design the magnetic path to be dominated by the reluctance of the projectile, rather than by the reluctance of the external iron. For example, if the external iron has twice the cross-section of the projectile, then adding more external iron will have no effect.

Another thing to note is the external iron will raise the inductance. So you'll generally end up with fewer turns in the coil, when you're done tuning.

Okay, enough to start with.

Anyone else?

Cheers, Barry
I used to have a handle on life but it broke off

Ragnarok

Mon Sept 10 2007, 01:37PM

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

Thanks for the help.

I wasn't planning on exceeding 1200V by any means, so I won't have to worry about finding a different triggering method. My (probably primitive) logic on more voltage but the same energy was that for a higher voltage, a higher current and flux level will arise, therefore potentially allowing more joules per stage, although it might eat at the efficency levels.

I'm not really worried about the extra complexity of multiple stages. Actually, it'll be nice to work on something where unlike my air cannons there is real fine tuning involved. With my pneumatics, there's a clean rule like : "More barrel + More pressure + More air flow = More power". Simple, but not subtle.
Playing with the Coil Gun simulator, I spotted the increasing efficencies with increasing injection velocity, but also noticed the fact that the energy from each stage is always slightly less than the last, so obviously there is a law of diminishing returns, even if the efficency is increasing.

I suppose the important question about external iron is "Should I try it?". I'm not short of spare offcuts of steel pipe and fairly large washers, so adding it wouldn't be too hard.

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