Cheaper induction CG?
GimpyJoe, Tue Mar 14 2006, 08:04PMI'm really impressed with the speeds people are getting with these induction CG's and I'm hoping to put one together myself since my local hamfest is coming up in a week and I can get some parts. The trouble is that large pulse caps are expensive and hard to find. My question is: would it be possible to use one pulse capacitor firing multiple times and charge the pulse cap with those cheap surplus energy storage caps? The storage cap would charge the pulse cap through an inductor. Does this sound like it would work?
Re: Cheaper induction CG?
pulslaser, Tue Mar 14 2006, 11:00PM
Hi,
the caps I used for my last coilgun aren't real pulscaps although they work very well in this application. I think caps which are used in the powersupply of high power shortwave amplifiers should work as well, maybe you will find on this hamfest.
I'm think your idea is a bit difficulte to realize, you have to recharge the puls caps in a short time, some hundert mikroseconds, unless you make the distance between the coils very long, and I' think the aktive spark gap will not quench in this short time to allow recharge. So you have to use quenched spark gaps or hydrogen thyratrons for the switches
But on the other hand it would be an interesting experiment to try
pulslaser, Tue Mar 14 2006, 11:00PM
Hi,
the caps I used for my last coilgun aren't real pulscaps although they work very well in this application. I think caps which are used in the powersupply of high power shortwave amplifiers should work as well, maybe you will find on this hamfest.
I'm think your idea is a bit difficulte to realize, you have to recharge the puls caps in a short time, some hundert mikroseconds, unless you make the distance between the coils very long, and I' think the aktive spark gap will not quench in this short time to allow recharge. So you have to use quenched spark gaps or hydrogen thyratrons for the switches
But on the other hand it would be an interesting experiment to try
Re: Cheaper induction CG?
Quantum Singularity, Wed Mar 15 2006, 05:52AM
Hamfests are a great source for stuff. Maybe I am spoiled though because I have only gone to the Dayton Hamfest (in Ohio) and if I am not mistaken its the biggest one in the U.S. HV pulse caps are still pretty rare though, but my first year there I got 2 HV caps that were 10kV 4uF for $10. They guy was selling them cheap because they have PCB's in them and there is not a whole lot of demand for these type of caps. I am not too worried about pcb's so long as they dont leak and I take certain precautions. It was a great way to start of the hobby cheap. I scored several used NST's for free from local neon sign shop (asked for donation for college project) and some HV diodes for $2 each mailorder. I wrapped coils out of regular household wire (14awg-8awg) and used a simple spark gap out of 2 bolts. For under $20 I was flinging disks and copper rings in the air
Quantum Singularity, Wed Mar 15 2006, 05:52AM
Hamfests are a great source for stuff. Maybe I am spoiled though because I have only gone to the Dayton Hamfest (in Ohio) and if I am not mistaken its the biggest one in the U.S. HV pulse caps are still pretty rare though, but my first year there I got 2 HV caps that were 10kV 4uF for $10. They guy was selling them cheap because they have PCB's in them and there is not a whole lot of demand for these type of caps. I am not too worried about pcb's so long as they dont leak and I take certain precautions. It was a great way to start of the hobby cheap. I scored several used NST's for free from local neon sign shop (asked for donation for college project) and some HV diodes for $2 each mailorder. I wrapped coils out of regular household wire (14awg-8awg) and used a simple spark gap out of 2 bolts. For under $20 I was flinging disks and copper rings in the air
Re: Cheaper induction CG?
Plasmaniac, Wed Mar 15 2006, 02:51PM
I scored this cap for only 5€ on a hamfest:
Also, my railgun caps (15x2200µF 400V) are from an other hamfest some time ago: 2DM (1€) each.
So if you have some luck and/or connections to some guys that go there every month, you can get really cool stuff on hamfest
I use non-pulse-rated caps in my induction coilgun, also. If you can't find some HV caps, you can make a electrolytic capbank, just like FastMHz.
[Edit: Fixed oversized picture]
Plasmaniac, Wed Mar 15 2006, 02:51PM
I scored this cap for only 5€ on a hamfest:
Also, my railgun caps (15x2200µF 400V) are from an other hamfest some time ago: 2DM (1€) each.
So if you have some luck and/or connections to some guys that go there every month, you can get really cool stuff on hamfest
I use non-pulse-rated caps in my induction coilgun, also. If you can't find some HV caps, you can make a electrolytic capbank, just like FastMHz.
[Edit: Fixed oversized picture]
Re: Cheaper induction CG?
Dr. Shark, Wed Mar 15 2006, 03:34PM
I suppose some success could be achieved with microwave caps, if they are overdriven hard, like Steve Ward does it.
Or you could try the Plate/Al-foil cap that is described in another thread. It could give you the required energy, however the impedance would be hard to match due to the absurdly high voltage (40kV).
Dr. Shark, Wed Mar 15 2006, 03:34PM
I suppose some success could be achieved with microwave caps, if they are overdriven hard, like Steve Ward does it.
Or you could try the Plate/Al-foil cap that is described in another thread. It could give you the required energy, however the impedance would be hard to match due to the absurdly high voltage (40kV).
Re: Cheaper induction CG?
GimpyJoe, Wed Mar 15 2006, 10:44PM
I've seen the ring launchers, but the reason I want to recharge the cap is to use a multistage design like Pulslaser's.
Yeah, I'm hoping to find some good deals, but as far as I know there are only two or three hamfests in the Atlanta area each year
GimpyJoe, Wed Mar 15 2006, 10:44PM
I've seen the ring launchers, but the reason I want to recharge the cap is to use a multistage design like Pulslaser's.
Yeah, I'm hoping to find some good deals, but as far as I know there are only two or three hamfests in the Atlanta area each year
Re: Cheaper induction CG?
mike0t4ever, Thu Mar 16 2006, 06:08PM
you mean to use one cap to be used for firing multiple stages?
and recharged in between the firing of each stage?
just think of it this way:
if you rechage the cap in such a short time the pulse energy going in is equal (assuming no losses) as the energy that you will be dumping into the coil.
which would mean that the cap charger would be capable of being connected directly to the coil.
now how do you build such a charger? most likely with caps (unless you have some knowledge of compulsators or similar devices)
mike0t4ever, Thu Mar 16 2006, 06:08PM
you mean to use one cap to be used for firing multiple stages?
and recharged in between the firing of each stage?
just think of it this way:
if you rechage the cap in such a short time the pulse energy going in is equal (assuming no losses) as the energy that you will be dumping into the coil.
which would mean that the cap charger would be capable of being connected directly to the coil.
now how do you build such a charger? most likely with caps (unless you have some knowledge of compulsators or similar devices)
Re: Cheaper induction CG?
GimpyJoe, Thu Mar 16 2006, 07:03PM
That's exactly what I said in my first post
A nice pulse-rated cap is charged multiple times by energy storage caps.
GimpyJoe, Thu Mar 16 2006, 07:03PM
That's exactly what I said in my first post
A nice pulse-rated cap is charged multiple times by energy storage caps.
Re: Cheaper induction CG?
Quantum Singularity, Thu Mar 16 2006, 09:30PM
Funky, I think what Mike is trying to say is if you can recharge your caps fast enough between firing, then you can just use your 'recharging circuit' to pulse the coils directly. The time between each coil firing isnt much longer than the pulse is, it can even be shorter. Unless you space your coils out about like 20 feet or something, your charger would have to pulse just as fast as your caps the drive the coils. Even spaced out that far you still talking a charge time in the millisecond range or so. So what you propose probably would give much if any benefit and make it alot more complicated. Just look at the graph that pulslaser put in his post "Amateur coil gun approach sound barrier" and youll see just how tight the pusles are. I might be missing something here though, if I am please key me in.
P.S. If you are merely trying to avoid having to buy multiple caps for multiple stages then you can probably use a lower voltage and a switching device that can be shut off like IGBT. That way you only fire a portion of the caps energy during each stage. For example if you can get a cap that holds 1000 joules, and trigger quick pulses around 250J net, then you could run 4 stages. This is a very simplistic example though. Good luck.
P.S.S. As many have pointed out in these forums you dont have to have pulse caps. Many have done well with electrlytic banks. If you can get something cheap enough that you dont care if you eventualy ruin it, then try it!
Quantum Singularity, Thu Mar 16 2006, 09:30PM
Funky, I think what Mike is trying to say is if you can recharge your caps fast enough between firing, then you can just use your 'recharging circuit' to pulse the coils directly. The time between each coil firing isnt much longer than the pulse is, it can even be shorter. Unless you space your coils out about like 20 feet or something, your charger would have to pulse just as fast as your caps the drive the coils. Even spaced out that far you still talking a charge time in the millisecond range or so. So what you propose probably would give much if any benefit and make it alot more complicated. Just look at the graph that pulslaser put in his post "Amateur coil gun approach sound barrier" and youll see just how tight the pusles are. I might be missing something here though, if I am please key me in.
P.S. If you are merely trying to avoid having to buy multiple caps for multiple stages then you can probably use a lower voltage and a switching device that can be shut off like IGBT. That way you only fire a portion of the caps energy during each stage. For example if you can get a cap that holds 1000 joules, and trigger quick pulses around 250J net, then you could run 4 stages. This is a very simplistic example though. Good luck.
P.S.S. As many have pointed out in these forums you dont have to have pulse caps. Many have done well with electrlytic banks. If you can get something cheap enough that you dont care if you eventualy ruin it, then try it!
Re: Cheaper induction CG?
GimpyJoe, Thu Mar 16 2006, 11:26PM
Thanks! I didn't realize the time the coil and cap spend ringing down was so long compared to the charging time. I guess I'll just have to see what I can find at the hamfest.
GimpyJoe, Thu Mar 16 2006, 11:26PM
Thanks! I didn't realize the time the coil and cap spend ringing down was so long compared to the charging time. I guess I'll just have to see what I can find at the hamfest.
Re: Cheaper induction CG?
mike0t4ever, Fri Mar 17 2006, 04:33AM
QS: thanks for clarifying my response
Funky: in the ieee papers on coilguns (most of which are induction guns) alot of guns use one capbank/coil (at least those powered by caps)
keep in mind that the current levels can be much larger than in reluctance(iron slug) guns
the reason for that is that a conductive projectile in a coil can be thought of as a shorted n:1 transformer
mike0t4ever, Fri Mar 17 2006, 04:33AM
QS: thanks for clarifying my response
Funky: in the ieee papers on coilguns (most of which are induction guns) alot of guns use one capbank/coil (at least those powered by caps)
keep in mind that the current levels can be much larger than in reluctance(iron slug) guns
the reason for that is that a conductive projectile in a coil can be thought of as a shorted n:1 transformer
Re: Cheaper induction CG?
Quantum Singularity, Fri Mar 17 2006, 04:19PM
I am no expert when it comes to the engineering principles of coilguns, but I would think by design the induction cg utilizes quicker pulses than a reluctance based cg. And thats probably why I get blazing speed from a 120uF cap and only 8 turn coil. I doubt using those parameters I could make as efficient of cg using the reluctance method and a ferro-magnetic projectile. So then an induction cg might demand a pulse rated cap more so than the reluctance variety. I guess most cg's using a cheaper electrolytic bank are probably reluctance. I guess I am not sure if I have ever seen an induction design that used electrolytics... has anyone used or seen this? Or are all of them pulse-cap or similarly powered?
Quantum Singularity, Fri Mar 17 2006, 04:19PM
I am no expert when it comes to the engineering principles of coilguns, but I would think by design the induction cg utilizes quicker pulses than a reluctance based cg. And thats probably why I get blazing speed from a 120uF cap and only 8 turn coil. I doubt using those parameters I could make as efficient of cg using the reluctance method and a ferro-magnetic projectile. So then an induction cg might demand a pulse rated cap more so than the reluctance variety. I guess most cg's using a cheaper electrolytic bank are probably reluctance. I guess I am not sure if I have ever seen an induction design that used electrolytics... has anyone used or seen this? Or are all of them pulse-cap or similarly powered?
Re: Cheaper induction CG?
Yohan, Fri Mar 17 2006, 06:26PM
It all depends on the current you wish to obtain and that is regulated by the esr and esl in your system.
Make no mistake, reluctance systems use very quick pulses too. Because the payload is a lot heavier for a SRLA system, it will require more stages to accelerate the armature to the speeds the induction CG can do with a few because the system will tear itself apart. As the slug reaches faster speeds toward the latter stages, those pulses need to be short as well.
Ultimately, you would want a power supply capable of delivering your desired pulse at a constant voltage based on an eventual steady state current you wish to achieve. This is impossible to do with SCR's and sparkgaps because you can't turn them off. So, you have to adjust your pulse lengths by circuit parameters. e.g. adding or removing caps and turns.
We used a bank of lytics for this very purpose in conjuction with IGBT's. They weren't pulse rated but they did the job just fine. I believe some of our pulses ended up no more than 500us in the last stages at roughly 1-2ka pulses.
EDIT: Where are the efficiencies at with induction CG's?
Yohan, Fri Mar 17 2006, 06:26PM
It all depends on the current you wish to obtain and that is regulated by the esr and esl in your system.
Make no mistake, reluctance systems use very quick pulses too. Because the payload is a lot heavier for a SRLA system, it will require more stages to accelerate the armature to the speeds the induction CG can do with a few because the system will tear itself apart. As the slug reaches faster speeds toward the latter stages, those pulses need to be short as well.
Ultimately, you would want a power supply capable of delivering your desired pulse at a constant voltage based on an eventual steady state current you wish to achieve. This is impossible to do with SCR's and sparkgaps because you can't turn them off. So, you have to adjust your pulse lengths by circuit parameters. e.g. adding or removing caps and turns.
We used a bank of lytics for this very purpose in conjuction with IGBT's. They weren't pulse rated but they did the job just fine. I believe some of our pulses ended up no more than 500us in the last stages at roughly 1-2ka pulses.
EDIT: Where are the efficiencies at with induction CG's?
Re: Cheaper induction CG?
mike0t4ever, Sat Mar 18 2006, 03:54AM
in most of what i've read they use pulse caps (non-polarized oil/film)
also using an approach where by having a very underdamped circuit you can reverse charge the cap and reuse the energy later (don't need a more complicated diag-half bridge)
another advantage of induction guns is no saturation effects
and the coil can be shorter than the projectile (in quite a few designs this was a common feature)
iirc some efficiencies are as high as 50% and higher,search in the ieee transactions on magnetics
(part of it is that larger guns are intrinsically more efficient but also out of our reach)
EDIT:
this was found here (i put the stars around the good part)
Abstract:
Recent advances in energy storage, switching and magnet technology
make electromagnetic acceleration a viable alternative to chemical
propulsion for certain tasks, and a means to perform other tasks not
previously feasible. Applications include the acceleration of gram-
size particles for hypervelocity research and the initiation of
fusion by impact, a replacement for chemically propelled artillery,
the transportation of cargo and personnel over inaccessible terrain,
and the launching of space vehicles to supply massive space
operations, and for the disposal of nuclear waste. The simplest
launcher of interest is the railgun, in which a short-circuit slide
or an arc is driven along two rails by direct current. ******The most
sophisticated studied thus far is the mass driver, in which a
superconducting shuttle bucket is accelerated by a line of pulse
coils energized by capacitors at energy conversion efficiencies
better than 90%. ******Other accelerators of interest include helical,
brush-commutated motors, discrete coil arc commutated drivers, flux
compression momentum transformers, and various hybrid electrochemical
devices. (Author)
CASI Accession Number: 80A51229 Pages: 00003
mike0t4ever, Sat Mar 18 2006, 03:54AM
in most of what i've read they use pulse caps (non-polarized oil/film)
also using an approach where by having a very underdamped circuit you can reverse charge the cap and reuse the energy later (don't need a more complicated diag-half bridge)
another advantage of induction guns is no saturation effects
and the coil can be shorter than the projectile (in quite a few designs this was a common feature)
iirc some efficiencies are as high as 50% and higher,search in the ieee transactions on magnetics
(part of it is that larger guns are intrinsically more efficient but also out of our reach)
EDIT:
this was found here (i put the stars around the good part)
Abstract:
Recent advances in energy storage, switching and magnet technology
make electromagnetic acceleration a viable alternative to chemical
propulsion for certain tasks, and a means to perform other tasks not
previously feasible. Applications include the acceleration of gram-
size particles for hypervelocity research and the initiation of
fusion by impact, a replacement for chemically propelled artillery,
the transportation of cargo and personnel over inaccessible terrain,
and the launching of space vehicles to supply massive space
operations, and for the disposal of nuclear waste. The simplest
launcher of interest is the railgun, in which a short-circuit slide
or an arc is driven along two rails by direct current. ******The most
sophisticated studied thus far is the mass driver, in which a
superconducting shuttle bucket is accelerated by a line of pulse
coils energized by capacitors at energy conversion efficiencies
better than 90%. ******Other accelerators of interest include helical,
brush-commutated motors, discrete coil arc commutated drivers, flux
compression momentum transformers, and various hybrid electrochemical
devices. (Author)
CASI Accession Number: 80A51229 Pages: 00003
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