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Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117
Good day!
I am currently designing a coilgun that I am planning to build next summer. I have spent the last days reading up on the subject, doing simulations, and trying to get a grip of what design would be most effective. But I am a little blown back at how difficult it is to derive the perfect design, due to all the different factors involved, like coil turns, wire width, projectile weight and size, inductance, pulse width and length, etc. So I thought I should finally make a user here (I have lurked quite a bit already), and ask for some guidance. I am getting the feeling that when it comes to coilguns, experience beats calculations.
The coilgun will be powered by my 3,6kJ cap bank that I built last summer. It works like a treat! You can see two pictures of it below.
I have for now thought of using a 4 or 3-stage design, splitting the large cap bank into smaller ones. The projectile is a 200-300 gram iron rod, 2-3 cm diameter. The cap bank will be split into 3 or 4 smaller banks, one for each stage. I have literally no idea of how many turns my coils should have, how long they should be (except they need to be the same length as my projectile), and how thick of a wire I should use. I'll use a PVC tube for a barrel.
So, yeah, I'd very much like to hear your input on this. Please let me know how I can improve this design, or how you would have built it! The only thing that's set is the capacitor bank size, everything else can still be changed. Oh, and I've already worked out a good way to get the timing of the coils right, so at least that's out of the way!
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Sounds like a good project, and I think you're off to a good start. The multistaging is a great idea, but only if you plan on taking the time to tune each stage. Many people choose to make a multisage gun but don't put any time into tuning it, so they still end up with 2% eff or worse, which defeats the purpose and extra work.
Add one stage at a time, calculate its efficiency seperate from the other stages (simple math) and tune it until it's giving good performance. Depending on how you switch your coil, the first stage should get at least 2%, and the following stages should get 3% to 10%. Therefore, it would make sense to make the first stage less energetic than the following ones to get the low efficiency "pre-acceleration" or "injection" out of the way then hit it with the real power in the higer efficiency 2nd, 3rd, and 4th stages.
Balance that with the fact that, as the projectile speeds up, well timed coils will have shorter and shoret pulses so you will be able to add less and less energy efficiently. My best reccomendation is to use simulations and math to create a spreadsheet that predicts the timing and kinetic energy added for each coil before you go and buy or build anything. I'm more than happy to provide examples of the math if you ask, but judging by the pictures you posted I think you know what to do.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
If you have the funds or are lucky enough with your scavenging, I would try and build a multistage coilgun using IGBT H-bridges. I would make a small kicker coil to start the projectile going (something that doesn't need to be tuned perfect but only uses enough energy to start the projectile going down the barrel) and then have the remaining 3-4 stages switched by the IGBTs.
Here's the trick: Don't split up your bank. Since the IGBTs can be turned off (unlike SCRs) you don't have to worry about the first stage completely draining your bank. This will also cut down on the need for tuning (it will still need it, but not by physically changing the coils). I would make each coil pretty large so that the voltage on the bank won't drop much per stage. You can use an optical sensor before each coil tied directly to your IGBT for triggering. When the signal goes high, turn on the IGBT. When it goes low, turn off the IGBT.
Your main concern with this design is finding an IGBT that can withstand the current. To get an idea what current to expect, I would first start by picking the length of your projectile. Then you know your coil length too (1.5x projectile length). Start calculating inductances - either using Barry's calculator or the raw equations - for each layer added to the coil. Then using Barry's RLC sim you can find the peak current for each inductance. Keep adding layers until you find that the circuit is overdamped. The peak current for that coil is what I would use to find the right IGBT.
I know that this system is pretty complicated but you should be able to get the best efficiency out of it. Plus, that's what I would do!
In general though, I would just say to use thick wire (14 gauge and greater) and definitely make it multi-stage.
Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117
First of all, thank you both for your detailed comments. Much appreciated!
Saz43 wrote ...
Sounds like a good project, and I think you're off to a good start. The multistaging is a great idea, but only if you plan on taking the time to tune each stage. Many people choose to make a multisage gun but don't put any time into tuning it, so they still end up with 2% eff or worse, which defeats the purpose and extra work.
Add one stage at a time, calculate its efficiency seperate from the other stages (simple math) and tune it until it's giving good performance. Depending on how you switch your coil, the first stage should get at least 2%, and the following stages should get 3% to 10%. Therefore, it would make sense to make the first stage less energetic than the following ones to get the low efficiency "pre-acceleration" or "injection" out of the way then hit it with the real power in the higer efficiency 2nd, 3rd, and 4th stages.
Interesting. I had thought of having more power in the first stage, and less in the next. But I now see that this approach is wrong! Calculating efficiency is a walk in the park, I study physics.
Saz43 wrote ...
Balance that with the fact that, as the projectile speeds up, well timed coils will have shorter and shoret pulses so you will be able to add less and less energy efficiently. My best reccomendation is to use simulations and math to create a spreadsheet that predicts the timing and kinetic energy added for each coil before you go and buy or build anything. I'm more than happy to provide examples of the math if you ask, but judging by the pictures you posted I think you know what to do.
Actually, I would very much like if you could give me an example of how to calculate the kinetic energy added per stage. I will definitively plan everything very well in advance, but what's giving me the biggest headache in this project, is deciding exactly how many turns each stage should have. More turns increases resistance, which increases pulse length, which decreases magnetic flux. But at the same time, more turns adds magnetic flux. I am confused.
Turkey9 wrote ...
If you have the funds or are lucky enough with your scavenging, I would try and build a multistage coilgun using IGBT H-bridges. I would make a small kicker coil to start the projectile going (something that doesn't need to be tuned perfect but only uses enough energy to start the projectile going down the barrel) and then have the remaining 3-4 stages switched by the IGBTs.
So you think I should have the kicker coil powered separately from my cap bank? I can do that. I have some big caps lying around I could use to power it.
Turkey9 wrote ...
Here's the trick: Don't split up your bank. Since the IGBTs can be turned off (unlike SCRs) you don't have to worry about the first stage completely draining your bank. This will also cut down on the need for tuning (it will still need it, but not by physically changing the coils). I would make each coil pretty large so that the voltage on the bank won't drop much per stage. You can use an optical sensor before each coil tied directly to your IGBT for triggering. When the signal goes high, turn on the IGBT. When it goes low, turn off the IGBT.
Interesting approach. I searched for IGBTs in the electronics store I use, they didn't seem too expensive. But will this not give a lot of power to the first coils, and less to the others? I thought (after reading Saz43s comment) that the last coils needed the most power. I have already designed a circuit to switch an SCR on when a beam of light is broken by the projectile, shouldn't be too difficult to alter it to work with an IGBT.
Turkey9 wrote ...
Your main concern with this design is finding an IGBT that can withstand the current. To get an idea what current to expect, I would first start by picking the length of your projectile. Then you know your coil length too (1.5x projectile length). Start calculating inductances - either using Barry's calculator or the raw equations - for each layer added to the coil. Then using Barry's RLC sim you can find the peak current for each inductance. Keep adding layers until you find that the circuit is overdamped. The peak current for that coil is what I would use to find the right IGBT.
I know that this system is pretty complicated but you should be able to get the best efficiency out of it. Plus, that's what I would do!
In general though, I would just say to use thick wire (14 gauge and greater) and definitely make it multi-stage.
Hmm... Yes, a little more complicated than what I had in mind, but I can make it work. Multistage is a no-brainer! But should I not use thinner wire for the first stages, and thicker for the last ones, in order to increase current per stage, thus increasing magnetic flux? I was under the impression that the magnetic flux should increase, and pulse lenght decrese, per stage.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
To clarify, you want a small amount of energy in the first stage because it will be the least efficient. After that, you want to have considerably more energy in the 2nd stage, but from there you will use less and less energy in subsequent stages because as the projectile goes faster, there will be less and less time for your coil to act on it as it flies through faster and faster.
I'll do an example of the math and post it when I get home.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Pinkamena wrote ...
But should I not use thinner wire for the first stages, and thicker for the last ones, in order to increase current per stage, thus increasing magnetic flux? I was under the impression that the magnetic flux should increase, and pulse lenght decrese, per stage.
Are you thinking thinner wire with same number of turns will decrease the magnetic flux because of it's higher resistance than thicker wire?
When working with coilguns, the resistance from the wire doesn't have a noticeable effect on the peak current when compared to it's inductance. What the resistance of the coil does do (along with the ESR of the capacitors) is damp the waveform. What you'll want is to vary the number of turns with the same thickness of wire. More turns, higher inductance, less current, longer pulse.
The kicker coil from a different supply will take care of the efficiency concerns Saz was talking about. It will be terribly inefficient but that won't matter as it will be only a small percentage of the total energy. To take care of the issue of most of the energy being used in the first stage, you need to make your coils very large. The high inductance should help make sure that only the proper amount of energy is used in each coil. This however is where there is some uncertainty: how many turns? This is where you might have to guess or experiment. If the coil is too large, the current won't build up very high (and the magnetic field) by the time the projectile has reached the center and the coil is switched off. If the coil is too small, most of the energy of your bank will be used in the first stage, or worse, all of it.
For the IGBT, I doubt the ones you found in your store would work. They will need to handle at least 1000A @ your bank voltage.
Now that I think about it, the IGBT approach will work better the faster the projectile is traveling initially. Ideally, what you want is for the voltage on your coil to look like the very first part of the normal RLC waveform where the voltage is decaying the slowest. The faster your projectile is moving, the shorter time the IGBT is on and the less voltage decay occurs. Anyway... you can play around with a system like that and figure out the best configuration. There aren't vary many good studies of this type of coilgun on this site.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
As promised. The question on how to design a coil comes up all the time, so I figured I'd make something that explains it all at once. Maybe if I have more time in the future I'll make it in a nicer format.
Note that there are several assumptions and guesses required here, this is not an exact process, but it will result in a much better coil than you would get without it.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Nice Saz. I have a couple questions...
You say that efficiency is hurt by high peak current... Why is that? Is it because with a high peak current the system is under damped and much of the energy will be wasted in the protection diode? Or is it that with a high peak current and a critically damped system the pulse length would be too long? Maybe change that to a ratio of the pulse length to the peak current...
Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117
Turkey9 wrote ...
Are you thinking thinner wire with same number of turns will decrease the magnetic flux because of it's higher resistance than thicker wire?
When working with coilguns, the resistance from the wire doesn't have a noticeable effect on the peak current when compared to it's inductance. What the resistance of the coil does do (along with the ESR of the capacitors) is damp the waveform. What you'll want is to vary the number of turns with the same thickness of wire. More turns, higher inductance, less current, longer pulse.
Thanks for clearing that up.
Turkey9 wrote ...
For the IGBT, I doubt the ones you found in your store would work. They will need to handle at least 1000A @ your bank voltage.
I tried out Barrys RLC simulator, and I found that the coils never really got more than 350A. Also, can I not put some IGBTs in parallel to make them able to handle more current? I might have missed the point of an IGBT H-bridge, but it seems logical to me to do that.
Saz43 wrote ...
As promised. The question on how to design a coil comes up all the time, so I figured I'd make something that explains it all at once. Maybe if I have more time in the future I'll make it in a nicer format.
Note that there are several assumptions and guesses required here, this is not an exact process, but it will result in a much better coil than you would get without it.
Thank you very much, this will undoubtfully be valuable to me. I have a few questions though. 1. What turkey just asked. 2. What is a v-switch, I see you put it under efficiency pros. Also, what do you mean with Flux augmentation? 3. You say that the difference in t(rlc) and t should be less than 0.01 ms. Is this correct, or did you mean to write second instead of millisecond? I feel that would make more sense. 4. You say that a small/light projectile is bad for efficiency. How can I decide how big the projectile should be? Is it simply "Bigger is better"?
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Turkey9 wrote ...
Nice Saz. I have a couple questions...
You say that efficiency is hurt by high peak current... Why is that? Is it because with a high peak current the system is under damped and much of the energy will be wasted in the protection diode? Or is it that with a high peak current and a critically damped system the pulse length would be too long? Maybe change that to a ratio of the pulse length to the peak current...
High peak current or just high current in general hurts efficiency because it tends to saturate the projectile. After the magnetic field is strong enough to saturate the projectile, the projectile will respond poorly to any further increase in field strength, you're familiar with the phenomenon but for anyone who isn't, link. You can get around this to some extent with a projectile made of material with a higher saturation point, but most coilguns use steel nails or screws, which already have a pretty high H_sat.
Pulse length mismatch and poor commutation (current wasted in diode) are also on that list but mostly are separate issues.
Pinkamena wrote ...
Thank you very much, this will undoubtfully be valuable to me. I have a few questions though. 1. What turkey just asked. 2. What is a v-switch, I see you put it under efficiency pros. Also, what do you mean with Flux augmentation? 3. You say that the difference in t(rlc) and t should be less than 0.01 ms. Is this correct, or did you mean to write second instead of millisecond? I feel that would make more sense. 4. You say that a small/light projectile is bad for efficiency. How can I decide how big the projectile should be? Is it simply "Bigger is better"?
1. See above 2. V-switch: Flux augmentation: 3. I did mean 0.01 ms, coilgun current pulses are measured in ms, so the agreement should be within a fraction of a ms if you calculations are going to be relatively precise. 4. Heavier is better for efficiency, but bad for speed performance since heavier projectiles go more slowly. The best way is to do some research here, find other coilguns like yours that were successful, and look at the size of the projectile they used. The math I showed you will also help you predict how a given projectile will work. If it's way to light or heavy, you'll have a tough time getting t and t_RLC to converge for a realistic coil, bank & eff value.
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Some advices for my first coilgun
