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Registered Member #55052
Joined: Thu May 14 2015, 06:47PM
Location:
Posts: 12
How did that new coil work? I am currently constructing a ring launcher and i have some 2/0 cable that I am using to make a 4 turn coil that is about 5-6 inches in diameter. This is my first ring launcher and I wasn't sure if this coil would perform well. I have some 8 gauge wire that I could also use. Which coil would work best in your opinion? I know the 2/0 coil will have a lower inductance which allow higher peak currents, but will the smaller number of turns(only 4) compared to a coil with 8 gauge wire(around 10) hurt the strength of the magnetic field?
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
benbmw wrote ...
How did that new coil work? I am currently constructing a ring launcher and i have some 2/0 cable that I am using to make a 4 turn coil that is about 5-6 inches in diameter. This is my first ring launcher and I wasn't sure if this coil would perform well. I have some 8 gauge wire that I could also use. Which coil would work best in your opinion? I know the 2/0 coil will have a lower inductance which allow higher peak currents, but will the smaller number of turns(only 4) compared to a coil with 8 gauge wire(around 10) hurt the strength of the magnetic field?
Thanks for the info, Ben
That depends on what capacitors / switching device you are planning to use, although I think either way, 4 turns will lead to a greater proportion of leakage induction, wasting a lot of energy not coupled to the projectile as well as the lower resistance putting A LOT of stress on your caps/switch gear.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Will not the way of the <=4-turn, FASTest pulse, quicker discharge of capacitor energy (max if tuned properly), keep the disc 'inside' the -energy- field for much more field-absorption, and perhaps 'capturing' all, or most, of the B-field, just before (or just after) it transverses the disk thickness (as actually calculated in magneto-forming) during the 'lift-off / flux penetration' race; tend just a teeny-tiny "bit-more" to reach the moon? Also, my gut tells me, there -must- be something, -even trivial-, that can be done to the disk (or launcher), so that it is not at the **absolute maximum** worst launching path WRT the atmosphere. It's *ALL* those darn air molecules hitting so -fast- and -perpendicularly- at just the wrong time. Just a thought...I am GREATLY interested in disk-launching and have, somewhat recently, put considerable thought into frisbee-type launching (thrust and spin), disks from these Aluminum types, down to coins. The energy and power are there--you just know it--if you ever held a properly-shrunk quarter!!!
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Signification wrote ...
Will not the way of the <=4-turn, FASTest pulse, quicker discharge of capacitor energy (max if tuned properly), keep the disc 'inside' the -energy- field for much more field-absorption, and perhaps 'capturing' all, or most, of the B-field, just before (or just after) it transverses the disk thickness (as actually calculated in magneto-forming) during the 'lift-off / flux penetration' race; tend just a teeny-tiny "bit-more" to reach the moon? Also, my gut tells me, there -must- be something, -even trivial-, that can be done to the disk (or launcher), so that it is not at the **absolute maximum** worst launching path WRT the atmosphere. It's *ALL* those darn air molecules hitting so -fast- and -perpendicularly- at just the wrong time. Just a thought...I am GREATLY interested in disk-launching and have, somewhat recently, put considerable thought into frisbee-type launching (thrust and spin), disks from these Aluminum types, down to coins. The energy and power are there--you just know it--if you ever held a properly-shrunk quarter!!!
By that reasoning a 1 turn coil ought to be the best; it isn't because the wiring, switch gear and caps together radiate more energy in the form of heat, light, sound and uncoupled magnetism. There is a sweet spot, but from my experience, it's somewhere above 4 turns.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Im back and its time to give a small hint about what will happen in 2016
I broke the old coil. So new one was made. This beast is wound with two parallel 1x35mm Copper strips, insulated (between turns) with duct tape and held together with copious amount of car body adhesive and epoxy. The coil alone has so much birch plywood and copper in it that it weights over 5kg. It has 11 turns and measures about 8µH inductance. Other than being super annoying to wind there is another very good reason why this coil is so beefy...
8700J test shot and results. Measured 90m/s speed which is really good for being fired at only 450V. As some of you may notice my capacitor bank is now quite a bit larger. And next time there may be 3 of those. I am seriously aiming for +1kJ projectile energies soon.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Interesting. I would like to know at what distance the effective acceleration happens. Care to do en experiment? First of all we need to think about the same thing the same way. I see a ring launcher as a core-less transformer. Your secondary (ring) is short cirucited. Due to the short circuit, the field, repells the disk. (so much for the basics) So.. a basic property of a transformer is the coupling factor k. It tells you about the Leakage inductance and stuff.. the less leakage the more force on the secondary (ring). So lets measure the coupling factor in dependence of the distance of the ring. When its goes close to 0 or 1/e or whatever we can call that particular distance the "effective range".
So lets measure the coupling. For that.. please build a small circuit:
Put your oscilloscope in singleshot mode. Capture the oscillation and measure its frequency. Change the distance from the coil to the disc. Without disc you will observe the lowest oscillation frequency, and the closer the disc comes, the higher the frequency will become. If you use manual cursors, make sure you use multiple oscillations to minimize measurement errors. The capacitor (1uF) must be the same for all measurements. If the frequency with the disc directly at the coil is way too high to seem reasonable (>500kHz) try to use 10uF then... but it must be a foil
Coupling factor is k = sqrt[ 1 - f_noDisk^2 / f_withDistk^2 ]
Oh.. and make sure you have at least one measurement without any disc close by. (should be the lowest frequency). The measurement precission of that frequency will impact the whole measurement.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
If a good one is available, yes. The problem is that with disc close by the L-Meter measures a "L parallel to a R" effectively. the smaller the R, the worse the resolution on L.. and any ESR will change the measurement too. I havent done a K-measurement for a long time, so i never have done it with my LCR-Meter. Maybe its irrational fear on my side.. Anyway... K is then sqrt(1-L_withDisc / L_noDisc)
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
And results are in from the first test. It turned out i did not have any paper at the workshop so i made a rough test with aviation magazines as spacers.
Here is the data. Shot #16 is the no disk shot
]disktest1.zip[/file]
Schematic worked as expected but i did add 1µF capacitor to the mosfet gate as debounce capacitor. Without it the switch bouncing was serious problem. Resonant frequency was very repeatable at each step so no problems there. I triggered the scope from mosfet gate so the data capture point in time was same every time.
Do we need higher resolution test?
EDIT: Swapped in fixed charts. EDIT2: Still wrong, argh!
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Really cool measurement result. Wouldnt have thought its working SO well ^_^
Its interesting to see that it seems to reach ~0 at the coil diameter (extrapolated). I wouldnt have thought that the field is expanding so far at all! What i see as disapointing here is that the best case coupling is only 0.63. It would be intersting if you could add one solid plate directly below the coil. (I know its not working for a real shot due to mechanical reasons, but just for the sake of measurment) My idea (based on a Post of BigBad in an other trhead) is that the field of the coil can not be pushed away from the disk, when its confined below. If the overall coupling increases that would be interesting. You basically repeat just the first measurement... your reference frequency should be close to 69kHz then and with the disk, its should even be higher.. hopefully in >150kHz-region.
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High power EM projectile launcher
