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Registered Member #321
Joined: Wed Mar 15 2006, 03:33AM
Location:
Posts: 14
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
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
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?
Registered Member #194
Joined: Fri Feb 17 2006, 07:52PM
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Posts: 19
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?
Registered Member #321
Joined: Wed Mar 15 2006, 03:33AM
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Posts: 14
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)
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Cheaper induction CG?
