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Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
It has been a while since I have done anything that is related to good old pops and bangs.
So here it goes: I want to see practical limits of "conventional" ring launcher and possibly something else in future. Starter setup is following: (note: this is just a small scale prototype )
12x 400VDC 6800µF nominal, about 5850µF measured S+M brand (Siemens, Epcos) capacitors. These have been sitting around and collecting dust for couple years so i gave them good cleanup and reform/leakage current test. All 12 turned out to be winners leaking less than 1mA at full rated voltage after ~5 minutes.
Bank was wired in 2s6p configuration. Busbars are 6x30mm 6061 aluminum. 17.583mF at 800V means whopping 5600J of electrical energy. And if we are naughty and charge these capacitors right to the surge voltage rating (440V), 6800J
2nd iteration of the work coil. About 10 turns of two parallel 2½mm^2 copper wires. Measures 10.6µH. Coil is heavily epoxied between two plywood disks and so far has been holding together ok. Here you can see the projectile I ended up using. It is cut from 6mm aluminum plate, 128mm outer diameter and 30mm inner diameter. Disk weights about 192grams.
800V shot with projectile. Current transformer ratio is 0.005V/A so 81.60V translates into 16.32kA
The bank is solid state switched of course. SCR used is single semikron SKKH330/16E (datasheet: )
I ran out of time with this setup and 4kJ was about maximum i wanted to try without proper projectile catcher. As you see the stack of old bed sheets does not work all that well at these energies :D (before bed sheet stack i had piece of foam mattress taped over the scale and the disk went right through it..)
Couple poor quality videos:
4kJ test, measured disk speed was 60m/s. Kinetic energy 346J and the efficiency was 8.65%. And yes, the disk with bed sheets did hit the ceiling pretty hard..
Casio EX-F1 pro 1200fps is used for speed measurement
Much more to come in near future. I have several work coils under work right now as well as different projectiles. With this coil it looked like the thick 6mm disk gave me by far best efficiency (but not all that great yet..).
UPDATE 09/02/2016
High speed camera is not ideal for measuring disk velocity. So I made a simple beam break chronograph.
Phototransistors with black plastic housings
650nm lasers
Small test by whacking beams with wooden stick. Beam to beam distance was 25mm so 0.025m / 524*10^-6s = 47.70m/s
Frame i made from scrap metal. Its very rigid and lasers keep their alignment even if I stand on the frame.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
...just started looking into your post details, and I will post some of my calculations when I have more time--right now, just a couple of things:
*1) Can you post the HWFM current according to your scope waveform?--I would like to check with what I get.
*2) concerning disc thickness, have you considered equations from parameters such as resistivity of disc and dB/dt values. This should give estimates of EDDY current and SKIN effect for determining field penetration into the disc for your parameters--in magnetoforming this is useful in calculating minimum disc thickness to 'contain' the field for greater efficiency i.e. this reveals min thickness, below which, the field lines would penetrate the disc--thus being wasted in 'lifting force'. Also, for thicker discs, the weight would be excessive. BTW, magnetoforming is done in a vacuum to eliminate serious air effects.
One more thing--just out of curiosity: If, somehow, the disc could be made to launch laterally, thus minimizing air resistance,(somewhat like a frisbee), what would be your guess as to the increased efficiency (or speed) as opposed to the common configuration which MAXIMIZES air drag?
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Signification wrote ...
...just started looking into your post details, and I will post some of my calculations when I have more time--right now, just a couple of things:
*1) Can you post the HWFM current according to your scope waveform?--I would like to check with what I get.
*2) concerning disc thickness, have you considered equations from parameters such as resistivity of disc and dB/dt values. This should give estimates of EDDY current and SKIN effect for determining field penetration into the disc for your parameters--in magnetoforming this is useful in calculating minimum disc thickness to 'contain' the field for greater efficiency i.e. this reveals min thickness, below which, the field lines would penetrate the disc--thus being wasted in 'lifting force'. Also, for thicker discs, the weight would be excessive. BTW, magnetoforming is done in a vacuum to eliminate serious air effects.
One more thing--just out of curiosity: If, somehow, the disc could be made to launch laterally, thus minimizing air resistance,(somewhat like a frisbee), what would be your guess as to the increased efficiency (or speed) as opposed to the common configuration which MAXIMIZES air drag?
1. Yes i can, but not untill next weekend
2. Not really, i can machine and test different size disks quicker than i can solve all that math/physics involved. If you are capable of doing that i will be more than happy to see how well theory backs up my experiments :)
Afaik i have seen disk launchers like this that do indeed launch disks like a frisbee. Someone on this forum even? But if my memory serves me right the efficiency of such launcher was somewhat poor.
My projects usually involve brute force instead of optimized design anyways. Eventually this project will be stepped towards several 10s of kJs..
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
loneoceans wrote ...
That looks like a fun project with a nice bank of capacitors. I'm a little bit concerned about your capacitor bus bars being so close together though
Yeah, I was wondering about those close bus-bars too! Was this design originally for some other special Low-L power project--a huge IGBT perhaps? You didn't design it this way for your launcher did you?
Does it make a significant different to place them *this* close? I ask because I have an application that will require very low inductance--an IGBT-based induction heater as I learn more about them.
Yeah, I was wondering about those close bus-bars too! Was this design originally for some other special Low-L power project--a huge IGBT perhaps? You didn't design it this way for your launcher did you?
Does it make a significant different to place them *this* close? I ask because I have an application that will require very low inductance--an IGBT-based induction heater as I learn more about them.
A laminated bus bar is the way to go for low-L bus projects like induction heaters, and they will have something like 1 or 2 orders of magnitude less bus inductance than even this setup with the bars so close together.
Kizmo, it might be fun to try making a linear ring launcher just for fun as well! If you make the ring small you can get some really amazing velocities, but I suspect the efficiency will be much reduced and the stress on your capacitors be increased.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Busbars being close together is just a fluke. I used 30mm wide bars that i drilled for another application years ago. They just happened to be right shape and size to work with minimum effort :)
Air gap between rails is about 2mm, which is fine for 400V. Im always open for ideas or suggestions so keep them coming. And im not too worried about stressing these junk capacitors. I have so many of these its not even funny. Only downside is that one shorted capacirtor in bank of many capacitors in parallel results explosive short circuit clearing
Thing like that happened couple years ago, single 420V 3300µ capacitor got hit by 12kJ while charging. It was mighty bang and capacitor element got launched like ETG projectile
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Signification wrote ... If, somehow, the disc could be made to launch laterally, thus minimizing air resistance,(somewhat like a frisbee), what would be your guess as to the increased efficiency (or speed) as opposed to the common configuration which MAXIMIZES air drag?
Who remembers "tracer" disk guns from the 1970's?
Attached picture is from first link; second link gives some history.
I was about to say that Kizmo's busbar spacing looks plenty wide for 800 volts. Even more so for 400 V, which is almost below the threshold for jumping across any air gap. Unless you drop a metal object on them. As loneoceans said, interconnection inductance would be much lower if the busbars were face to face, separated by a thin insulating spacer, instead of edge to edge. Of course that maximizes the Lorentz force trying to push them apart.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Ah...the Star Trek disk shooter--Oh YEAH! In fact, not to long ago I found one of the discs in the attic under some insulation! But my favorite was that Whamo air gun (can't recall the name) that you cocked by pulling back a stiff rubber membrane--it then fired a vortex through the air. I got it off ebay as broken and finally managed to fix it. By far the hardest part of the repair was just getting through the rivets and inside the gun. To my great surprise, the rubber disc was not rotten--in fact, like new!...don't make 'em like that anymore-for sure. I think they were banned due to kids injuring their eardrums with point blank shots! They were recently replaced by a big clumsy thing with some sort of air bag--comparatively, junk.
Around then, I remember spring-twist driven guns that fired flying propellers--always getting stuck in trees!
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High power EM projectile launcher
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