Author Topic: DIY induction guns? (warning:long)  (Read 7905 times)

Offline davekni

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Re: DIY induction guns? (warning:long)
« Reply #40 on: April 01, 2024, 01:54:46 AM »
Quote
I actually bought some of the (IKY75N120CS6 1200V 300A peak) IGBTs you linked a while back as they do seem very promising for my small applications. The peak current is lower than the thyristors by a lot so I'll have less margin for error, but on the other hand they are cheap and small and I can use as many as are required.
Less peak current, but much faster allowed current rise rate without burning out.  (Unless you were using pulse-discharge specific thyristor types.)
For IGBT experimenting, I'd suggest soldering a TVS diode between gate and emitter leads, something in the 18V-33V range.  That will drastically reduce chances of damaging IGBT when handling and use.  Excess Vge due to ESD etc will punch through gate oxide.  Per Mads, typically requires about 80Vge to destroy an IGBT.  Humans easily get charged above 2kV walking around.
BTW, that would be one advantage of thyristors.  They are much less prone to ESD damage.
IKY75N120CS6  should be good.  Includes internal anti-parallel diode.  It can likely handle even more than 300A for short single pulses, probably to 500A.  Drive Vge to 18V or 20V (or even 24V) for enhanced high-current pulse capability.

Quote
I'm watching LTspice tutorials now and hopefully that will be illustrative.
Yes, simulation will help answer many questions.  There are on-line user groups for LTSpice with lots of advice too.  One suggestion for your specific needs:  Use a built-in FET model instead of finding an IGBT model.  For macroscopic behavior, FETs behave close enough to IGBTs and they simulate much faster.  LTSpice built-in FET models do not model Vds limits, which is convenient for your use.  For a high current switch, use a low on resistance part even though it is rated for low voltage.  For example, IRFH5250 will simulate fine at 1000A and at least 5000V per my quick test.
David Knierim

Offline Benbmw

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Re: DIY induction guns? (warning:long)
« Reply #41 on: April 01, 2024, 11:58:01 PM »
I have had similar thoughts to you on this topic (see me post, https://highvoltageforum.net/index.php?topic=1755.0) and have access to ANSYS Maxwell where I have set up some 2D axisymmetric simulations. My interest is primarily for "polyphase" launchers with higher aspect ratio projectiles which I think take advantage of the LIL concept the best (though you do eventually run into issues with projectile length if you are targeting very short acceleration distances). Unfortunately my free time to tinker is quite limited but I would be happy to collaborate with you in discussion and provide simulation capabilities to your efforts!

While Dave is correct that thyristors generally suffer from low dI/dt capabilities unless specifically designed for pulsed applications (see Solidtron datasheet attached, 100kA/us !) It is still possible that something like a VS-80TPS16L-M3 with 500A/us di/dt could still work for you. Assuming target velocities in the 100's of m/s and a pole pitch ~10cm, that gives half sine pulses in the 10kHz range which gives you plenty of room to reach the peak current capability of on of these devices before having acute dI/dt limitations. With that said, depending on your drive configuration, IGBTs may still be advantageous. As alluded to in my post, I am partial to a "resonant drive" topology that allows you to use higher energy density, lower voltage, polarized caps (ie electrolytics) as you main energy storage which is then switched, ideally with an H bridge, to your series drive coil and resonant cap which you can allow to ring up to 2,3,4x the main drive voltage. Though his adds complexity and it may not be ideal given your constraints of size and weight.

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #42 on: April 02, 2024, 03:23:46 AM »
Quote
I actually bought some of the (IKY75N120CS6 1200V 300A peak) IGBTs you linked a while back as they do seem very promising for my small applications. The peak current is lower than the thyristors by a lot so I'll have less margin for error, but on the other hand they are cheap and small and I can use as many as are required.
Less peak current, but much faster allowed current rise rate without burning out.  (Unless you were using pulse-discharge specific thyristor types.)
For IGBT experimenting, I'd suggest soldering a TVS diode between gate and emitter leads, something in the 18V-33V range.  That will drastically reduce chances of damaging IGBT when handling and use.  Excess Vge due to ESD etc will punch through gate oxide.  Per Mads, typically requires about 80Vge to destroy an IGBT.  Humans easily get charged above 2kV walking around.
BTW, that would be one advantage of thyristors.  They are much less prone to ESD damage.
IKY75N120CS6  should be good.  Includes internal anti-parallel diode.  It can likely handle even more than 300A for short single pulses, probably to 500A.  Drive Vge to 18V or 20V (or even 24V) for enhanced high-current pulse capability.

Quote
I'm watching LTspice tutorials now and hopefully that will be illustrative.
Yes, simulation will help answer many questions.  There are on-line user groups for LTSpice with lots of advice too.  One suggestion for your specific needs:  Use a built-in FET model instead of finding an IGBT model.  For macroscopic behavior, FETs behave close enough to IGBTs and they simulate much faster.  LTSpice built-in FET models do not model Vds limits, which is convenient for your use.  For a high current switch, use a low on resistance part even though it is rated for low voltage.  For example, IRFH5250 will simulate fine at 1000A and at least 5000V per my quick test.

Thanks for the tip on the diodes and the LTspice simulation. I ordered some 28v TVS diodes for some cheap insurance on the IGBTs. I never would have though of using the fet model so that's helpful. I'm going to try to start up some simulations this week; I was going to try tonight but I'm too tired to do anything after working out and a full day at work haha.

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #43 on: April 02, 2024, 03:38:01 AM »
I have had similar thoughts to you on this topic (see me post, https://highvoltageforum.net/index.php?topic=1755.0) and have access to ANSYS Maxwell where I have set up some 2D axisymmetric simulations. My interest is primarily for "polyphase" launchers with higher aspect ratio projectiles which I think take advantage of the LIL concept the best (though you do eventually run into issues with projectile length if you are targeting very short acceleration distances). Unfortunately my free time to tinker is quite limited but I would be happy to collaborate with you in discussion and provide simulation capabilities to your efforts!

While Dave is correct that thyristors generally suffer from low dI/dt capabilities unless specifically designed for pulsed applications (see Solidtron datasheet attached, 100kA/us !) It is still possible that something like a VS-80TPS16L-M3 with 500A/us di/dt could still work for you. Assuming target velocities in the 100's of m/s and a pole pitch ~10cm, that gives half sine pulses in the 10kHz range which gives you plenty of room to reach the peak current capability of on of these devices before having acute dI/dt limitations. With that said, depending on your drive configuration, IGBTs may still be advantageous. As alluded to in my post, I am partial to a "resonant drive" topology that allows you to use higher energy density, lower voltage, polarized caps (ie electrolytics) as you main energy storage which is then switched, ideally with an H bridge, to your series drive coil and resonant cap which you can allow to ring up to 2,3,4x the main drive voltage. Though his adds complexity and it may not be ideal given your constraints of size and weight.

Thanks for the generous offer to share information and simulations, I need all the help I can get. I did see your thread but to be totally honest I don't fully understand the concept of the resonant inverter drive, it's a little over my head right now I think.

If I understand what you're saying, the electrolytic capacitor provides the initial energy pulse into the coil which then is fed back to an AC capacitor via the ringdown?

Being able to use lower voltages and electolytic caps would be helpful. I actually got my film capacitors from digikey today and I have to admit I didn't look that closely at the dimensions lol, I saw that they were the blue box style and assumed they were fairly small but I was quite surprised how big they are after unpacking them! I think maybe the only way capacitors like these would compete with electolytic is by using far higher voltages since the capacitance increases with the square of the voltage. These are 45uf and are around the same volume and weight of  the lower voltage 1100uf electrolytic capacitors I have.

I'm not ruling out using the polyphase linear motor style drive yet, it's just that I'm also curious what the pusher design is capable of.

With regards to the aspect ratio of the projectile I'm wondering about adding 3d printed nose and tail pieces to them to improve their aerodynamics but otherwise keep the metal part on the short side (for the pusher). I'm not really sure if metallic mass is advantageous for the inductive coil gun in the same way that is is for a reluctance coil gun.

The thyristor you linked is interesting, I wonder what one costs? I got a quote on a different thyristor (one of the ones dave linked, the smaller of them) and after explaining what I was going to use it for received a quote:

[edit: see this; The cost on these would be $829.23 each.    Lead time is 24-26 weeks.     Please let me know your information so we can send you a formal quote.    As mentioned earlier, this will require a UK export license.]

I don't have a UK export license, I'm not waiting 6 months, and I don't really want to spend $829 times however many I need lol.

Dave mentioned they were $100 on ebay or whatever it was, but after trying to buy one, the seller messaged me saying there was a mistake with the listing and refunded my money.

This week I'll post the specifications for my barrels and projectiles in case you or anyone is curious, as well as what electronics I have for the first prototypes. I'm not necessarily trying to build the best possible article but more of a proof of concept that's also not terrible, hopefully.
« Last Edit: April 02, 2024, 03:52:30 AM by Michelle_ »

Offline OmGigaTron

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Re: DIY induction guns? (warning:long)
« Reply #44 on: April 03, 2024, 11:50:44 PM »
Thanks for the tip on the diodes and the LTspice simulation. I ordered some 28v TVS diodes for some cheap insurance on the IGBTs. I never would have though of using the fet model so that's helpful. I'm going to try to start up some simulations this week; I was going to try tonight but I'm too tired to do anything after working out and a full day at work haha.

I just finished reading through this thread.
What a fantastic build idea so far.
Coil Guns / Induction guns are awesome.
Pushing the magnetic field to do the WORK for us, has always been an interest of mine.

In my research on IGBTs, they look great for super-fast switching for this type of application.

I have been doing a bunch of comparisons on IGBTs Vs Mosfet Vs Sic Mosfet in my Plasma Toroid design, looking for the literal fastest possible Vt/On Vt/Off switching times.
I can pull up a few of those that I have found that are reasonably priced, to see if they can be applied to your design here, if that is helpful.

I plan on going through and re-reading this again, after another cup of coffee this evening.
There are plenty of great papers that you have provided and lots of fantastic tidbits of information provided by this discussion.

Keep up the build log and discussion.

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #45 on: April 04, 2024, 05:17:50 AM »
Thanks for the tip on the diodes and the LTspice simulation. I ordered some 28v TVS diodes for some cheap insurance on the IGBTs. I never would have though of using the fet model so that's helpful. I'm going to try to start up some simulations this week; I was going to try tonight but I'm too tired to do anything after working out and a full day at work haha.

I just finished reading through this thread.
What a fantastic build idea so far.
Coil Guns / Induction guns are awesome.
Pushing the magnetic field to do the WORK for us, has always been an interest of mine.

In my research on IGBTs, they look great for super-fast switching for this type of application.

I have been doing a bunch of comparisons on IGBTs Vs Mosfet Vs Sic Mosfet in my Plasma Toroid design, looking for the literal fastest possible Vt/On Vt/Off switching times.
I can pull up a few of those that I have found that are reasonably priced, to see if they can be applied to your design here, if that is helpful.

I plan on going through and re-reading this again, after another cup of coffee this evening.
There are plenty of great papers that you have provided and lots of fantastic tidbits of information provided by this discussion.

Keep up the build log and discussion.

Thanks for the kind words.

Feel free to link the switches you've found, if they can handle a lot of current they could be useful to me.

I'm going to start posting more information about what I'm actually doing in the following posts, versus throwing out ideas and asking questions.

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #46 on: April 04, 2024, 05:44:42 AM »
OK so basically I have two goals right now:

1.) Build a single stage induction launcher
2.) Build a single stage reluctance launcher* may provide less info on this generally

Based on how these go, I will decide how to move forward; increasing number of stages, going back to the drawing board, etc...

One objective I have is starting with a small projectile, which Dave says will be difficult and I believe him, however nobody else has made a small induction launcher or made any reasonable attempt at optimizing the projectile air gap or thin-ness (critical factors). I put a lot of thought (aka mcmaster-fu) into these first barrel and projectile combinations because I'm entirely convinced the ratios especially the air gap is CRITICAL to efficiency, and a lot of people are taking flyers using loose fitting projectiles in PVC pipes.

Here are some of the materials I have available for the first prototypes:

4x 630VDC/310VAC 45uF film capacitors
4x 480VDC 1100uF electrolytic capcitors
8x IKY75N120CS6XKSA1 IGBT 1200v 150A
x Wire ranging from 10-30awg
3x 660v 1800A thyristors
x Misc electrical components for circuits, switches, optical sensing
2x Arduino

I have a first iteration of a design document (which covers the projectile and barrel) that I based off of analyzing one of the coil guns discussed earlier in this thread. I like to design things this way, just start listing out parameters and fill them in as I go. Typically I use a spreadsheet and also include notes, status, relevant links, part numbers, vendors, etc... However I'm just going to blast this thing as plain text right now in the thread. This could be helpful for someone maybe or it can just be something I update periodically for informational purposes.

Now that I have projectiles, switching devices, and capacitors I can and will begin simulating and designing coils, then seeing if things are feasible as far as currents, frequencies, etc...

This document doesn't yet include all possible stages but it is a template that can be expanded upon:

ITEM      IND proto 1
      
      
Projectile Material      3003 Aluminum
Projectile Conductivity      
Projectile OD Designed mm      4.78
Projectile OD Used mm      
Projectile Wall Thickness Designed mm      0.5588
Projectile Wall Thickness used mm      
Projectile Length Designed mm      12.7
Projectile Length Used mm      
Projectile Mass Designed g      0.257
Projectile Mass Used g      
Projectile Payload Mass g      
Total Projectile Mass g      
Projectile Mass Density kg/m3      2730
Projectile Melting point ©      660.1
      
Air Gap Designed mm      0.605
Air Gap Used mm      
Projectile as % of coil ID dsn      0.799
Projectile as % of coil ID act      
      
Critical Slip      
Critical Slip Velocity m/s      
      
Pole Pitch mm      
      
Barrel Material      CF
Barrel ID mm      4.978
Barrel OD mm      5.99
Total Length cm      
Total Number of Sections      
Section 1 Length cm      
Section 2 Length cm      
      
Total Number of Coils      
Number of Coils in Section 1      
Number of Coils in Section 2      
Coil OD mm      
Coil ID Designed mm      
Coil ID Actual mm      
Coil Length mm      
Number of Turns in Coil      
Coil Winding Type      
Cross Section of Copper cm2      
Cross Section of Copper in      
Coil Wire Shape      Round
Insulation Type      Enamel
Coil Winding Continuous y/n      
Coil 1 Inductance H      
Coil 2 inductance H      
      
Section 1 Frequency Designed Hz      
Section 1 Frequency Actual Hz      
Section 1 Initial Voltage Designed V      
Section 1 Initial Used Voltage V      
Section 1 Peak Current A      
Section 2 Frequency Designed Hz      
Section 2 Frequency Actual Hz      
Section 2 Initial Designed Voltage V      
Section 2 Initial Used Voltage      
Section 2 Peak Current A      
      
Section 1 Phases      
Section 1 Capacitor per Phase Design uF      
Section 1 Capacitor per Phase Actual uF      
Section 2 Phases      
Section 2 Capacitor per Phase Design uF      
Section 2 Capacitor per Phase Actual uF      
      
Section 1 Total Energy Actual Max      
Section 2 Total Energy Actual Max      
Total Gun Actual J      
Designed Gun J      
Maximum Projectile Speed Designed m/s      
Maximum Projectile Speed Observed m/s      
Projectile KE Designed J      
Projectile KE Observed J      
Gun Efficiency Designed      
Gun Efficiency Observed       
Gun Efficiency Simulated (as built)      
      
Stage 1 Max Velocity m/s      
Stage 1 Max KE       
Stage 1 Efficiency       
      
Ignition Method      IGBT
Timing Method      Arduino




« Last Edit: April 04, 2024, 05:53:24 AM by Michelle_ »

Offline davekni

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Re: DIY induction guns? (warning:long)
« Reply #47 on: April 04, 2024, 06:19:43 AM »
Quote
Feel free to link the switches you've found, if they can handle a lot of current they could be useful to me.
Ran into another 1200V IGBT, this one rated for higher pulse current, 560A:
IKY140N120CH7
As with others, you can likely use it somewhat above 560A for shorter than 1ms pulses and higher Vge (ie. 20V or 24V instead of normal 15V).
David Knierim

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #48 on: April 04, 2024, 03:43:38 PM »
Quote
Feel free to link the switches you've found, if they can handle a lot of current they could be useful to me.
Ran into another 1200V IGBT, this one rated for higher pulse current, 560A:
IKY140N120CH7
As with others, you can likely use it somewhat above 560A for shorter than 1ms pulses and higher Vge (ie. 20V or 24V instead of normal 15V).

Thanks that's a gem, big amps and low price! I'll put some in my cart for if/when I blow up the ones I have.

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #49 on: April 05, 2024, 07:00:17 AM »
This is probably a dumb question but I'm wondering what factors would prevent one from using coils with less and less resistance so less capacitance is needed to generate the same current and timing? Especially for a reluctance coilgun. Would it not make sense to design the coil to be the least resistance possible so as to need less capacitor mass?
« Last Edit: April 05, 2024, 07:04:20 AM by Michelle_ »

Offline Benbmw

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Re: DIY induction guns? (warning:long)
« Reply #50 on: April 05, 2024, 07:33:08 PM »
what factors would prevent one from using coils with less and less resistance so less capacitance is needed to generate the same current and timing? Especially for a reluctance coilgun. Would it not make sense to design the coil to be the least resistance possible so as to need less capacitor mass?

In theory less resistance is always better, all else equal. However it is rarely possible to keep all else equal. In this case depending on your method of lowering resistance, it will have inherent effects on the number of turns and inductance.

First off, the peak current will almost certainly be limited by the coil inductance rather than resistance, scaling with sqrt(C/L). Assuming we are talking about changing winding configurations rather than materials, (and maintaining the barrel diameter) you could lower resistance in one of two ways: maintaining the coil cross section and increasing wire size, or maintain the wire size and use less turns (shrinking the coil cross section).

In the first case, it turns out that for a fixed coil cross section to the first order changing the wire size gives you the same amp turns, which is what matters. Double the wire cross section to cut the resistance by a factor of 4 and you also halve the number of turns which is a 4x reduction in inductance leading to double the peak current. Your losses (again to the first order) will stay the same since RI^2=(R/4)(2I)^2

In the second case, if you were to say cut the number of turns in half, your resistance would only halve and your inductance would be ~4x lower (assuming your coil cross section is approximately square, for single layer coils your inductance would only drop by a factor of 2) . This means a doubled peak current again and therefore the same amp turns, but now your losses are doubled! Not great, which is fundamentally why small launchers are hard to make efficient (think how increasing coil/barrel diameter has the same relationship). Note that efficiency does not quite scale with launcher diameter, since for multilayer coils there are second order effects (skin and proximity) that change the resistance and inductance scaling. However, it does still stand that you gain some efficiency going larger.

So, generally you will want to pick a coil cross section that gives you the field distribution you want (another topic, but nominally close to square is desirable), then pick your wire gauge to give you the inductance you want. This will be driven by component availability on one end, namely what is the maximum voltage your components can handle, and launcher parameters on the other, namely what resonant frequency do you want.

If you fix the resonant frequency then you basically have a fixed LC product. To maximize force you want to maximize current. Given our progression of constraints up to this point, you can double the current by either doubling voltage, or doubling capacitance and halving inductance. You can basically pick your poison within reason. Though if you choose to decrease inductance (lower voltage higher capacitance) and therefore increase wire size, you will stray from the linearity assumed above as the skin depth becomes less than your wire radius and you are no longer efficiently using your wire cross section. Litz wire would be an (expensive) way to overcome this. At the other end, with high voltage comes expensive semiconductors and thick insulation (reducing coupling and coil packing density). There is usually a reasonable middle ground that fits the semiconductors, capacitors, etc. that you have available.

Hope this is helpful, also since this was a bit hand wavy and not tied to exact math, please double check my reasoning!
« Last Edit: April 05, 2024, 07:53:22 PM by Benbmw »

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #51 on: April 05, 2024, 09:37:56 PM »
what factors would prevent one from using coils with less and less resistance so less capacitance is needed to generate the same current and timing? Especially for a reluctance coilgun. Would it not make sense to design the coil to be the least resistance possible so as to need less capacitor mass?

In theory less resistance is always better, all else equal. However it is rarely possible to keep all else equal. In this case depending on your method of lowering resistance, it will have inherent effects on the number of turns and inductance.

First off, the peak current will almost certainly be limited by the coil inductance rather than resistance, scaling with sqrt(C/L). Assuming we are talking about changing winding configurations rather than materials, (and maintaining the barrel diameter) you could lower resistance in one of two ways: maintaining the coil cross section and increasing wire size, or maintain the wire size and use less turns (shrinking the coil cross section).

In the first case, it turns out that for a fixed coil cross section to the first order changing the wire size gives you the same amp turns, which is what matters. Double the wire cross section to cut the resistance by a factor of 4 and you also halve the number of turns which is a 4x reduction in inductance leading to double the peak current. Your losses (again to the first order) will stay the same since RI^2=(R/4)(2I)^2

In the second case, if you were to say cut the number of turns in half, your resistance would only halve and your inductance would be ~4x lower (assuming your coil cross section is approximately square, for single layer coils your inductance would only drop by a factor of 2) . This means a doubled peak current again and therefore the same amp turns, but now your losses are doubled! Not great, which is fundamentally why small launchers are hard to make efficient (think how increasing coil/barrel diameter has the same relationship). Note that efficiency does not quite scale with launcher diameter, since for multilayer coils there are second order effects (skin and proximity) that change the resistance and inductance scaling. However, it does still stand that you gain some efficiency going larger.

So, generally you will want to pick a coil cross section that gives you the field distribution you want (another topic, but nominally close to square is desirable), then pick your wire gauge to give you the inductance you want. This will be driven by component availability on one end, namely what is the maximum voltage your components can handle, and launcher parameters on the other, namely what resonant frequency do you want.

If you fix the resonant frequency then you basically have a fixed LC product. To maximize force you want to maximize current. Given our progression of constraints up to this point, you can double the current by either doubling voltage, or doubling capacitance and halving inductance. You can basically pick your poison within reason. Though if you choose to decrease inductance (lower voltage higher capacitance) and therefore increase wire size, you will stray from the linearity assumed above as the skin depth becomes less than your wire radius and you are no longer efficiently using your wire cross section. Litz wire would be an (expensive) way to overcome this. At the other end, with high voltage comes expensive semiconductors and thick insulation (reducing coupling and coil packing density). There is usually a reasonable middle ground that fits the semiconductors, capacitors, etc. that you have available.

Hope this is helpful, also since this was a bit hand wavy and not tied to exact math, please double check my reasoning!

Thanks for the really helpful reply. I'm starting to make simulations and design coils so I'm trying to make sense of all the possible variables. Who knew building a coilgun would be so complicated lol.

Offline Benbmw

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Re: DIY induction guns? (warning:long)
« Reply #52 on: April 05, 2024, 10:48:17 PM »
I'm starting to make simulations and design coils so I'm trying to make sense of all the possible variables. Who knew building a coilgun would be so complicated lol.

Indeed, it becomes much more complex to optimize as you get into the weeds. One random thing (which you may already be well aware of!) that really helped in my thinking about these launchers was when I realized: when it comes to imparting force to the projectile, the only two quantities we care about are the azimuthal currents in the projectile and the radial B field produced by the coils. Where these interact we get a JxB force in the axial direction (helping or harming depending on the sign).

So, for example, longer coils might do well to induce large currents in the projectile with their axial field, but only at the ends of the coils where the field is radial do we produce force. This is why in an induction pusher, if the projectile is sticking out of both ends of the coil we dont get any net force. The axial field induces currents all in the same direction, but the radial magnetic field at one end of the coil is pointed inwards and at the other end it is pointed outwards resulting in equal and opposite forces on the projectile (it is being stretched axially by the radial fields and squished radially by the axial fields).

My intuition used to be what you often hear, "the changing coil current creates a changing magnetic field which induces a current in the projectile, which in turn creates its own magnetic field that 'opposes' the field that created it" giving me the intuition that the "north pole pushes the north pole", when in reality magnetic fields do not interact, they simply sum. The currents of the coil and projectile are what interact with the resulting net magnetic field.
« Last Edit: April 05, 2024, 10:50:53 PM by Benbmw »

Offline Michelle_

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Re: DIY induction guns? (warning:long)
« Reply #53 on: April 05, 2024, 11:13:35 PM »
I'm starting to make simulations and design coils so I'm trying to make sense of all the possible variables. Who knew building a coilgun would be so complicated lol.

Indeed, it becomes much more complex to optimize as you get into the weeds. One random thing (which you may already be well aware of!) that really helped in my thinking about these launchers was when I realized: when it comes to imparting force to the projectile, the only two quantities we care about are the azimuthal currents in the projectile and the radial B field produced by the coils. Where these interact we get a JxB force in the axial direction (helping or harming depending on the sign).

So, for example, longer coils might do well to induce large currents in the projectile with their axial field, but only at the ends of the coils where the field is radial do we produce force. This is why in an induction pusher, if the projectile is sticking out of both ends of the coil we dont get any net force. The axial field induces currents all in the same direction, but the radial magnetic field at one end of the coil is pointed inwards and at the other end it is pointed outwards resulting in equal and opposite forces on the projectile (it is being stretched axially by the radial fields and squished radially by the axial fields).

My intuition used to be what you often hear, "the changing coil current creates a changing magnetic field which induces a current in the projectile, which in turn creates its own magnetic field that 'opposes' the field that created it" giving me the intuition that the "north pole pushes the north pole", when in reality magnetic fields do not interact, they simply sum. The currents of the coil and projectile are what interact with the resulting net magnetic field.

Thanks that's a good explanation that makes it easier to understand. Feel free to drop any tidbits like this that come to mind, there's a lot to wrap your head around with these things and it's easier for me to understand concepts than a bunch of math formulas.

Offline Benbmw

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Re: DIY induction guns? (warning:long)
« Reply #54 on: April 18, 2024, 06:17:15 AM »
I had the chance to fire up my ANSYS simulation and thought you would enjoy some examples of what it can do. Attached is an animation and plot of a nominal induction launcher design with the following parameters. Note that this is a time harmonic solution (not transient) with 10 identical coils and an armature that is longer than the "barrel". A case which would be great for efficiency, since all of the coils are running at the same frequency and all doing work on the armature. So a bit of a simplified launcher.

Armature diameter: 25mm
Armature wall thickness: 3mm
Armature material: Aluminum
Armature weight: 200g

Coil number: 10
Coil cross section: 20mm x 15mm
Coil wire gauge: 10 AWG
Coil turns: 45
Coil drive current: 1kA

With this model I can sweep all sorts of variables like armature-coil gap spacing (one I knew you were keen on), drive phase number, drive frequency, etc. etc.

The animation shows the product of the azimuthal current density J and the radial magnetic field density B which has units N/m^3 (if you do a volume integral you get total force on the armature). The plot shows the total armature force vs drive frequency for gap spacing of 1, 2, and 3mm. As you can see, a smaller gap is better, but the gains aren't astronomical, more on the order of 10 or 15%, at least for this design.

I would be happy to plug in your numbers and run a similar simulation sweeping whatever variables you are interested in. Just let me know!

High Voltage Forum

Re: DIY induction guns? (warning:long)
« Reply #54 on: April 18, 2024, 06:17:15 AM »

 


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