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Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi guys I am new to this topic and i want to start with some coilgun experiments.
I have 3 BIG 18mF with 350V capacitors. This may give around 3.3kJ.
This is what i know until now: To build a gun is verry complex if things should do an optimal job. It is also verry expensive I have played around with some simulators that google showed me.. (barrys simulators seems verry known here) I do not want to accelerate heavy projectiles but they should go really fast. For fast Projectiles the single coils must not be on for a long time. That gives me the problem that the capacitors do not unload completely before i have to turn off via IGBT, right? All that is switchable for be is lower than 1.2kA. So.. how to make a good design?
Would it be possible after a long optimazion period to reach >100m/s for 15g projectiles? So far i have read that a multistage gun is more efficent than a singlestage one. Given the problem of too much capacity, i would suggest to use every capacitor twice so building a 6stage gun. I have MUCH experience in programming and configuring CPLDs, so timing isses wil not be a problem. I just dont have much experience with high current and magnetic fields and other issues such as saturation in the projectile and so on.
O have also a LOT of questions about experiences of other people, because i am unfortanely not able to research completely on my own due to a low studens budget
But simply at first i would love to hear some ideas for a concept for a gun with these capacitors. This is just for the future. Of course i will start with one stage and collecting some experiences, but i really like to know to which aspects i have to pay ny attention on.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Hi, and welcome to the forum! I just wanted to toss in a quick response, and let you know that barry does keep an eye on what's happening in here. The bottom line is that timing is largely controlled by the LC time constant. So large capacitance can be balanced by small inductance to get the short timing needed. It generally works best to use a few turns of heavy wire in the coil: the resistance of thin wire is bad. You would have fun starting with one capacitor and a coil with a few more turns than needed; then see what happens to performance as you gradually remove turns from the coil.
Cheers, Barry Does your party() #include <margaritas.h>?
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi and thanks for your reply and for your JavaSkills I want to controll the timing with IGBTs, and not just pull the trigger an discharge the cap. Due to the expensiveness of the IGBTs i want to keep the currents below 1.2kA. I have run some simulations with LTSpice, just to look whats happening. With 0.5mH and a time of 2ms (is that kind of dimmension right?) and some ESRs (in cap and coil) i am getting 1.2kA. (interestingly that is not waht vour tool says.) This discharges my capacitor from 350V to 260V. So a second and maybe a third shot can be supplied from the cap. So theoretically a 9 stage gun should come up.
The positive thing of the half dischage is, that i do not care about switch-opening and do not use a antiparallel diode (of course there will be one for safety, but normally Id=0.) because the unused magnetic field is feed back to the cap and can be used to power the next coil. I have attached my simulation. (this is whar LTSpice says.)
If i am correct, the timing becomes shorter the faster the projectile is. I thought about this, and and want to compensate that with lower inducance every coil - is that a good way? Also it is possible to power neighbored coils at the same time and shut off the first one earlyer to have a more constant timing. (but nevertheless it becomes shorter) I would be pleased if my ideas would be commented, or maybe just someone may say that i am stupid Also in my head, there is the idea of the changing inductance then the Projectile approaches. Would it be correct to calculate the inductance of the Coil with the iron inside?
And there is another thing.. which IGBTs for 1.2kA?.. Soooo many questions ]draft3.zip[/file]
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
DerAlbi wrote ... I have run some simulations with LTSpice, just to look whats happening. With 0.5mH and a time of 2ms (is that kind of dimmension right?) and some ESRs (in cap and coil) i am getting 1.2kA. (interestingly that is not waht vour tool says.) ... The positive thing of the half dischage is, that i do not care about switch-opening and do not use a antiparallel diode (of course there will be one for safety, but normally Id=0.) because the unused magnetic field is feed back to the cap and can be used to power the next coil. I have attached my simulation. (this is whar LTSpice says.)
Did you notice that in your simulation, when the MOSFETs turn off with 950 amperes in L1, the voltage spikes to 50 kV? I think your SPICE model of MOSFET is not very accurate in the reverse-breakdown operating region. No matter how it works, the 300 joules stored in L1 will mostly be dissipated in the FETs. You can protect the FETs by connecting anode of D2 to the drains instead of the sources; then the 300 J will mostly heat R2, but the current in L1 will drop off relatively slowly.
Where do you have a problem with Barry's RLC sim result? It's consistent with simple formulas for very lightly damped RLC (since you have unrealistically low R of 1 milliohm): half-period is 11 ms, peak I is 1800 A (this assumes that the switch stays closed).
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
I have discovered this over the day.. it is already fixed. New File is attached. Today the university gave me a 6kV 1.2kA IGBT for testing Nice guys.
The new graph of the current in the coils looks bad. If i would shut down the current if the projectile is already in the middle of the coil, there will be an equal force that sucks it back due to the current through the diode that sinks so slowly. is that conclusion right?
In general.. what is better? -SingleStage - 3-5xMultistage with high currents (1.2kA) - more stages with moderate currents (~300A)
Anny suggestions to the design? ]draft3_1.zip[/file]
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Looking good so far.
To get the current pulses to stop faster:
1. Use a realistic value for the coil resistance.
2. Increase the resistance in series with the clamp diodes (D1, etc.) to perhaps 0.5 ohms ... as much resistance as you can add without exceeding the breakdown voltage rating of the FETs. Inductive kick voltage is proportional to the value of R5, and to the coil current at the moment of commutation.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
OK, so can you give me a quick hint what is realistic? Please for both: OnTime for a shot AND Resistance of the Coils and their inductance? (Please note that the coils in the simulation also have their own ESR ~0.1Ohm - the additional Resistor is just fake with 1mOhm )
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Thanks for the reminder: I had overlooked the 0.1 ohms hiding inside the L models of your sim. Not the first time LTSPICE fooled me with that. Since you have an explicit resistor in the schematic, how about setting the hidden resistor value to zero, to avoid confusion? Or at least, if the parasitic resistance is significant, make its value visible!
0.1 ohms -is- realistic. Try this tool: With a coil ID of 14 mm, I got to your 470 microhenries with about 0.16 ohms of AWG14, wire. The same volume wound with AWG10 would be 74 uH, 0.025 ohms, giving a much shorter pulse (of similar field strength) if you want a self-timed pulse using (for example) an SCR switch.
As you know, the circuit simulator is a useful tool. Let me know if you want an SCR model for LTSPICE. The IGBT lets you shorten the pulse & leave energy in the capacitor, but you need to manage the braking resistance and voltage. Do you have time to make a simple, single-stage, SCR-switched coilgun for practice?
>>have also a LOT of questions about experiences of other people, because i am unfortanely not able to research completely on my own due to a low studens budget YOu could learn a lot by reading all of Barry's websiste. Perhaps start with the "Mark 4" chapter.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Thanks for your reply.. and sorry for your confusion My idea so far is to use IGBTs. This is an extra topic about their calculation an overstressing. The university gave me juste on 6kV @ 1.2kA IGBT for testing. As you have said, i already have read a lot of barrys stuff. But still i have these questions mybe becaus i still have no idea about reality. This brings me to the point that currently i say: Hey let me just get some practice, measure the force on a projectile for your self and try some simulations via FEM. After this i will try an easy singlestage gun. For experimental timings, velocityn friction, eddy currents and so on. Then i want to create a FEM model for this too. But: currently i just try to get an overlook about all this. It is a really expensive hobby for me so i do not want to have so many iterations. To get an overlook e.g. i try to simulate on/of currents via IGBT with the circuit you have seen. But for this i already need some information like "what concept is efficent (singeltage, multisage highcurrent, multisage medium current)" what are normal Timings and "onTimes" for the coils. Also it is interesting to choose the material for the tube for the projectiles.. i have read about plastictubes but my head says that will melt on high velocities and maybe slotted coppertube hast too much eddycurrents between the slots.. so i NEED do ask for your experience And reading will never bring me the answer to that because i cant weight the problems that brings every solutions for now
This is my current plan: - The first thing i will do now is to try to find out what makes it possible to charge 3x 18mF caps to 350V in a good time (20sec) from a battery (12-24V). - Next thing will be: before connecting the charger: build a discahrger. - Then i will limit the voltage to 50V for safe experiments for a future singlestage gun. - Then: how to switch these currents - IGBTs - which ones, how much can they be overstressed due to the short time of stress. - Then i have to invent a kind of a security system, like overcurrent protection for each coil (and their IGBTs). - After this i need a mangagement for the timings. Maybe CPLD or FPGA with a µC that loads configurable delays into the logic-IC. - add some Stages an optimze ...
Is something wrong with this? Please add forgotten things and strikeout waht is not necessary. ..and maybe give me a hint how to cahrge caps efficent - this would be my starting point for the next days.
Thanks for your support so far, i hope this will continue
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3x 18mF @ 350V -> What to do?
I have played around with some simulators that google showed me.. (barrys simulators seems verry known here) I do not want to accelerate heavy projectiles but they should go really fast. For fast Projectiles the single coils must not be on for a long time. That gives me the problem that the capacitors do not unload completely before i have to turn off via IGBT, right? All that is switchable for be is lower than 1.2kA. So.. how to make a good design?
