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Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
The acceleration is so violent i have little to no hope that anything attached to the disk would stay on.
And reason for going to triggered gap is it being simple in mechanical mind. At these energy levels magnetic forces and electrode wear are serious. Being relatively low voltage switching it makes the switch erosion even worse. My new triggered gap design uses brass electrodes since brass turns instantly to vapor instead of spattering molten metal blobs everywhere covering my insulator materials with conductive particles.
Just some time ago i realized that commercial spark gap have shot life depending on capacitor charge in Coulombs, not energy or peak currents. And this is where it kicks my butt:
My setup is 31.6 mF at 1450 V
E = 0.5 * C * U^2 0.5 * 31.6*10^-3 F * 1450V^2 = 33220 J
But charge in Coulombs is C * V
31.6*10^-3 F * 1450 V = 45.82 C
It would have to be *really* large commercial spark gap to have charge transfer rating larger than that. If my capacitor bank was lets say 50kV it would be a whole lot easier to any kind of mechanical or spark gap switching:
33.22kJ at 50kV equals about 26.58µF of capacitance.
Depends. Parallel conductors with currents going in the same direction are attracted, opposite currents are repelled. For a helical coil, that implies an axial contraction and a radial repulsion. Adjacent conductors in a spiral coil like Kizmos will be attracted to each other, but they will be repelled by the conductors on the other side of the coil. It might well be, that the forces are of the same order of magnitude as those on the disk, i.e. several tons.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Kizmo wrote ...
The acceleration is so violent i have little to no hope that anything attached to the disk would stay on.
And reason for going to triggered gap is it being simple in mechanical mind. At these energy levels magnetic forces and electrode wear are serious. Being relatively low voltage switching it makes the switch erosion even worse. My new triggered gap design uses brass electrodes since brass turns instantly to vapor instead of spattering molten metal blobs everywhere covering my insulator materials with conductive particles.
Just some time ago i realized that commercial spark gap have shot life depending on capacitor charge in Coulombs, not energy or peak currents. And this is where it kicks my butt:
My setup is 31.6 mF at 1450 V
E = 0.5 * C * U^2 0.5 * 31.6*10^-3 F * 1450V^2 = 33220 J
But charge in Coulombs is C * V
31.6*10^-3 F * 1450 V = 45.82 C
It would have to be *really* large commercial spark gap to have charge transfer rating larger than that. If my capacitor bank was lets say 50kV it would be a whole lot easier to any kind of mechanical or spark gap switching:
33.22kJ at 50kV equals about 26.58µF of capacitance.
26.58*10^-6 F * 50000 V = 1.33 C
See the difference?
Interesting...shot life dependent upon "only?" cap charge! Do you have an equation or reference to calculations? At an auction, I picked up four (4) never-used brass HEMIspheres: 3" in dia. and 2lbs each with a 4" low R electrode, into which screwed 12" bars (these were magnetic and I will keet then out of the circuit. They were outfitted with what looked like a brass orcordian--my guess--an air trigger for slamming the electrodes together. To bad the guy had started peeling them off
I am planning to use them on my quarter shrinker that currently consists of two 70uF @ 12kv in parallel--I use only 10kV, so that comes to E=7kJ. Q=CV=(140EE-6)*10kV=1.4 Coulombs (close to your 1.33Q).
I remember getting some not-to-bad results with these caps in series...forgot the voltage, but using 10kV here (Which preserves the same 7kJ energy, that required 20kV@ the series 35uF). That's (35x10^-6)*(20kV)=0.7Q. 1/2 of the parallel (low V) arrangement--same energy...and capacitor pair...don't know about Ip.
The best part--I got all this bras for 25 cents
Does anybody know how this kind of "low surface area" power switching compares with the much larger contact area on those electrodes that slam large 'flat' areas together? Maybe they (spheres) were intended to be used in a vacuum or gaseous environment??? BTW: Those loud discharge caps from the disposable cameras barely make a "pop" when shorted through these spheres!!!
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Signification wrote ...
Interesting...shot life dependent upon "only?" cap charge! Do you have an equation or reference to calculations?
Here is one app note:
app note wrote ...
Lifetime The lifetime of the triggered gap can be approximated in terms of the cumulative charge, in coulombs (Q), that can be passed through the device without changing its main static breakdown voltage by more than 20 percent. Expressing the height of the current pulse in amperes and the duration in seconds, the area under the pulse is the coulomb of charge contained in it.
Example 1: A triggered gap is required to pass a 20,000 peak amp pulse having a 10µs pulse half-width. Approximating the pulse shape by a triangle, the charge contained in the pulse is:
q = 20,000 amps x 10-5 sec = 0.2 coulombs If the Coulomb Rating (Q) is given as 100 coulombs, the life under these conditions is approximately: Life = Q/q = 100/0.2 = 500 discharges
Example 2: A 10kV main static breakdown triggered gap must dump a 2.0µF capacitor charged to 8kV. The charge stored in the capacitor is: q = CV = 2.0 x 10-6 farads x 8,000 Volts q = 0.016 coulombs
If the Coulomb Rating (Q) is given as 100 coulombs, the life under these conditions is approximately: Life = Q/q = 100/0.016 = 6,250 discharges
And research paper: (This is why i chose brass)
Here is my set of electrodes. I call this gap as the "final solution"
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Nice work there. Can't wait to see your velocity measurements. Are you planning to try laser beams in the first few mm or cm of motion, to refine the linear induction motor model?
Who knows what's inside regular ballistic chronographs? Are the background strips retroreflective? Are the optical sensor signals AC-coupled?
Remember when high-speed photography didn't mean high frame-rate videography? Your launcher appears to be very robust and repeatable. Got any interest in playing with microsecond flashlamps, taking one variably-delayed picture per shot, to study the acceleration & even make movies? Or multiple flashes to get multiple time points per shot?
Registered Member #4762
Joined: Sun May 06 2012, 05:59PM
Location: Russia
Posts: 93
I could suggest something from the USSR old stock things (not sure if similar devices are available worldwide): This is IRT-6 (ИРТ-6), mercury pulse valve, and it commutates up to 30 Сoulombs at up to 25 kV and 100kA (numbers are manufacturer rated, so I think it can be pushed 1.5x times up from that). Unfortunately I need mine for my own can crusher, but this is the ultimate solution to all commutation, brass evaporation, loud sounds and other problems with high energy capacitor banks. If you manage to find someone in Russian Internet who sells and ships one to you (which may be tricky since it has ~500g of liquid mercury inside), I highly recommend it.
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High power EM projectile launcher
