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Registered Member #61593
Joined: Mon May 01 2017, 07:55PM
Location:
Posts: 14
I've seen in many places ideas that supercapacitors are not good for projectile launchers. Here is a small device built to demonstrate that it can be done:
The device is not optimized in any way, here are some design details: -source: 2000F, 2.7V -switch: 4 mosfets AUIRFS8409-7P -coil: 17 turns using two 2.5mm wires in parallel, coil internal diameter around 6mm -pulse: fixed around 10ms, I tested various pulse widths to find an optimal value -working current measured around 1600 A
LE: I do not understand why the video is not visible, here is a direct link:
[Edit: Fixed youtube link. Needed ID instead of URL]
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi and welcome to the forum i guess ^_^ Nice built - good to see some digital control.
I would be curious how you measured the 1.6kA.. this correspondents to 1.69mOhm which is indeed plausible.
Just a comment on:
I've seen in many places ideas that supercapacitors are not good for projectile launchers. Here is a small device built to demonstrate that it can be done: [..] The device is not optimized in any way..
See, the point is when you try to optimize it, the 2.7V is one of the first things you tackle. So all of your statement is still true (supercaps are not good; but it can be done; unoptimized). I mention this because got the feeling that you meant it in the way that because of your demonstration the general attitude towards supercaps for coilguns is wrong.. its not
The device is not optimized in any way -pulse: fixed around 10ms, I tested various pulse widths to find an optimal value
Cheater hahaha. I was about to ask you if you could do some PWM during the 10ms, then i noticed that this is maybe not a good idea. You do currently not have any diode in place that is keeping the inductive peak at bay. So you currently rely on the fact that your built has virtually no inductance (its more a wound resistor than not a wound coil) and the avalanche capability of your mosfets or slow switching. Just keep that in mind if you start to not optimize it further
Edit: i find it interesting since you have a small prototype where you can make quick adjustments: try to shoot aluminium and turn it into an induction launcher. for this you have to place a non magnetic slug behind / at the back-end of the coil... and i guess the pulse length can be way shorter. i would love to hear if this moves the projectile... your current may be high enough.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Yeah, interesting. I have a 2500F super cap here I might have a play with, it should be relatively easy to increase the number of stages using the same cap, as it gets nowhere near discharged. The only problem is the number of FETs needed to keep resistance down, and the long length of barrel. I was intending to use my super cap for a battery spot welder, but the current is about a 1/3 too low with only one cap.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I suspect that one way to significantly improve efficiency is to increase the load resistance, by increasing the number of turns, e.g. x10 this will require a physically larger coil, and a projectile of much increased mass. ... more like artillery than small-arms.
A reduced current for an extended period should reduce losses in the coil, switch, wiring and cell. The recoil is equal and opposite to the impulse given to the projectile, so for both your present setup and any other prototypes, support the coil against recoil forces for efficiency.
e.g. tightly grip the coil of your launcher when you fire it, ... the coil should not waste energy by moving.
P.S. just in case you have not been there, Barry's coilgun site
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi hen918, you might be on to something with a multistage design. The interesting part here is that the supercapacitor's energy density is 10x - 100x higher than aluminium caps (lets say 50x) according to wikipedia. This means you actually achieve a superior system by achieving just 0.4% efficiency which would give you the same energy density or weight-effectiveness as a 20% efficiency traditional coilgun*. Not sure if 0.4% is doable however.. the low voltage makes the system sooo bad, that this could be quite a hard goal to reach.
Lets just do some calculations: The video uses 2.7V * 1600A = 4.32kW * 10ms = 43.2J. (i assume no inductive component here, because the L/R-ratio in this build is negligible short) At 0.4% this means 0.1728J would be within the projectile which seems about right or even underestimated, so the efficiency is likely to be higher than 0.4%
The problem will arise when you try to scale up the system due to the problems you mention. Since a low voltage system will likely to be restisively dominated. The biggest flaw in the system above is the way-too-long coil.. it would be interesting whats possible actually.
*My above statement is tremendous false if you consider real world application btw.. in a traditional coilgun you use up all the energy in the caps while you store much more than needed in a supercap which wastes all the energy density in the end
Sulaiman:
I suspect that one way to significantly improve efficiency is to increase the load resistance, by increasing the number of turns, e.g. x10 this will require a physically larger coil, and a projectile of much increased mass. [..] A reduced current for an extended period should reduce losses in the coil, switch, wiring and cell.
Be careful with such proposals. First of all you mix up efficiency and overall power transfer. It is true that scaling up the thing will transfer more power, because simply said its just a bigger motor then. However this does not mean that its efficiency increases. Increasing the "load resistance" the way you describe it also wastes more energy while transferring more. It is however true that a change of the coil shape would increase the efficiency in this case, but only if you change it to a better shape ^_^.
Registered Member #61593
Joined: Mon May 01 2017, 07:55PM
Location:
Posts: 14
Well, since it started some interest, I will elaborate a bit
wrote ...
I would be curious how you measured the 1.6kA.. this correspondents to 1.69mOhm which is indeed plausible.
Simplified DC calculation, I calculated the wire resistance, if I remember correctly, the total length is around 70cm..ish, there are 2 wires, 2.5mm diameter, connected in parralel. The voltage across the coil, during the pulse, is around 2V, measured with oscilloscope. From the total of 2.7 volts, some is wasted on the mosfet switch and some on the capacitor esr.
wrote ...
I was about to ask you if you could do some PWM during the 10ms, then i noticed that this is maybe not a good idea. You do currently not have any diode in place that is keeping the inductive peak at bay.
No pwm, just an off-on-off pulse, 10 ms wide. It is generated by a silabs 8051 development board. I do not have peak suppressing diode, as this is not a big problem so far. Looking with the scope, at mosfet on-off, there is a spike up to 40 volts or so (breakdown voltage of mosfet) but so far it looks capable to withstand the avalanche energy. To be honest I did not make calculations for this. However, the coil has an inductance in 10-100nH range, without projectile. The switching is not slow at all. Each mosfet is driven by a 9A capable mosfet driver with 12 volts levels.
That being said, my ideas toward optimizations are:
-first problem is mechanical in nature. The projectile is actually a beheaded screw. The plastic tube used to prevent contact between the projectile and the coil is very rough on the inside. This, plus the screw thread, creates a lot of friction during initial acceleration. Smoother surfaces are required. -I would increase the projectile mass. More inertia keeps the projectile longer in the barrel, to accumulate energy. -I would increase the current peak, maybe towards 10kA and would increase coil length. Consequently, the required readjustments on the coil resistance would seriously increase the required wire diameter, by my calculations, above 5mm -there are some serious challenges when dealing with such low resistances. You just cannot connect two copper wires (in my case coil ends to bus bar) just by soldering them, as I did in the prototype. I am sure there is a serious loss in that place. Normally, they should be welded together, and this raises yet another issue, since for welding you have to use oxygen free copper and TIG welding procedure. -soldering switching transistors to a 20x3mm bar, as in the prototype, was a real pain. Heating such copper mass is not easy, and when heated, it oxidize rapidly, the solder deposition is not even and there are a lot of residues which are standing in the way of a good connection. -the coil should be inserted in some form of mechanical restriction. The intense current creates a mechanical shock in the coil windings and this propagates toward mosfets. Repeated mechanical shocks may damage them. -during firing, indeed, little energy is spent from the full capacitor charge. There is a lot remaining for successive firing. But putting 1600A in 2x2.5mm wires is a lot. After 10 consecutive triggers with 1-2s in between, serious heat builds up in the coil and you cannot keep the hand on it.
LE:
wrote ...
The device is not optimized in any way -pulse: fixed around 10ms, I tested various pulse widths to find an optimal value Cheater hahaha.
You caught me :) Maybe 'optimal value' is not the correct term. With other values it barely worked. I had to try some values because this being my first such project, I had absolutely no idea where to start from. First pulse I tried was around 10 microseconds. I only knew that it is probably a very bad idea to keep mosfets on for 1 second or so.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Looking with the scope, at mosfet on-off, there is a spike up to 40 volts or so (breakdown voltage of mosfet) but so far it looks capable to withstand the avalanche energy. To be honest I did not make calculations for this. However, the coil has an inductance in 10-100nH range, without projectile.
Seems legit. Well... 100nH/2*1.6kA^2 = 0.13J. So if you would do PWM the mosfets are dead within.... maybe 50 cycles.. good enough safety margin if you switch just once ^_^
Simplified DC calculation, I calculated the wire resistance
Awww dont! I mean yes, do! But, measure or higher confidence. Your soldering and contact resistances do matter at those levels. I recommend you use a mV-capable multimeter (if you have?) and use a 10A or 5A Power supply set it at high current and do a 4-wire measurement. Those experiments can tell you a lot in terms of where its correct to just do the math and where its important to be pessimistic. (its quite possible that doing the math is sufficient!)
-I would increase the current peak, maybe towards 10kA and would increase coil length.
A longer coil makes worse performance. The length of the coil should be as long as you projectile for your applications. Please go through Barrys website (theory and the prototypes) before you waste your time with bad ideas - i am just saying that because i am not sure if your setup withstands a lot of modifications . Besides that you indicated that you want to use thicker wire meaning less turns.. less turns will pull less.. so you dont get any benefit from that - in general a constant coil geometry will perform equal, no matter which wire you use.
Registered Member #61593
Joined: Mon May 01 2017, 07:55PM
Location:
Posts: 14
A new design would mean other capacitors and pretty much other everything (coil, mosfet, connections, etc). This one cannot give more than 1800A. There are some 3400F/2.85V, 4-5 of them could go there. They have 2kA per piece max I. But this is just theory, currently I do not have time to put it to practice. However, I will study the theory, thanks for the link.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Ok, i just looked up your capacitor and i see where your 1.8kA-limitation comes from. However this does not really apply to you since the 1.8kA is rated for a 1 second on-time. This thing can go higher for shorter time, no worries. Your transistors are rated for 800A for 10ms, so 4 in parallel should give you quite some current headroom. However as i watch your video closely your mosfets are driven individually... which might be a bad idea. Due to driver mismatch the current sharing can be bad during switchting... not sure how much that matters ins your case, but on the other hand, paralleling all gates directly cant really hurt.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
directly paralleling fast transistors can cause parasitic oscillation at surprisingly high frequencies, better to have individual gate drive resistors (with diodes, capacitors etc.) and sometimes ferrite beads on the gate leads too, I have had this happen.
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Supercapacitor coilgun
hahaha.
Those experiments can tell you a lot in terms of where its correct to just do the math and where its important to be pessimistic. (its quite possible that doing the math is sufficient!)