rp181 wrote:By dump and quench, do you mean...
The "dump" will be a little bit more complex than normal because of the dual-bank system, but it'll probably be IGBT based. I'm not expecting very high currents, so it wasn't too hard to find something up to the job. I found some discrete IGBTs which will handle 1200V and a peak current of 135 A.
The other advantage of these current levels is that they're within the current limit of relatively cheap discrete components - so that means either no need for parallel operation (which, with the negative temperature gradient of most IGBTs, is usually problematic) or more expensive single components.
The "quench" is nothing more complex than a diode/resistor set-up designed to burn off the energy left in the coil after the power is shut off.
Not as sophisticated as a diagonal half-bridge, but a half-bridge takes time to channel the energy back to the capacitors (not that there's a huge amount of energy to recover), and that means you've got to shut the pulse off earlier.
In many cases, dump-and-quench can actually be more effective. However, I haven't ruled out the possibility of a system that channels some of the energy from one coil into the next, saving a certain amount on energizing the next stage, which, if it can be made to work, could actually be a better solution.
Technician1002 wrote:100 amps is way too low. Rework the math. Unless you are doing a huge number of turns on your work coil, there will be little force relatively speaking.
A coil 3cm long, with 462 turns, running 100 amps gives a current density of ~1,550,000 Amps per meter.
Multiply by the relative permeability of air, and that gives you about 1.95 Tesla. And 1.95 Tesla will result in forces about 100 Newtons, which is not exactly lacking. (Equivalent "magnetic pressure" is ~3.5 MPa or ~510 psi)
Better relative permeability (i.e. Ferro-sheathing) will mean that lesser currents again are required.
Obviously, I'll need to burn a bit more current than that to sustain the appropriate flux density at the projectile when it's only partly in the coil, but studies I've seen suggest that in a sheathed coil, force on the projectile is pretty constant with respect to displacement from the centre of the coil (provided, of course, that the projectile is at least partly within the coil).
High resistance in the work coil is not desirable. It is energy loss.
Resistance is energy loss, but if you actually look at the numbers...
Take a 0.5 ohm coil that needs 200 amps to generate the same field strength: I<sup>2</sup>*R = 20 kW
And the 1 ohm, 100 amp coil: I<sup>2</sup>*R = 10 kW
Same field, half the power needed to sustain it.
It's true that the nature of a multi-turn solenoid means that the turns on the outer layers cost more resistance (as they take more wire), so the maths above is slightly simplified, but the basic principle still applies.
Strictly, a coil wound to the same length and internal diameter as mine, but with half the resistance would only need 157 Amps to sustain a 1.95 Tesla field, but that's still 12.3 kW that are being burnt.
23% more input for the same output? I'll take the higher resistance coils, given that the topology can overcome their higher inductance without trouble. (I'd take it further, except for the fact that there's only so much inductance I can accept.)
Could you post some of your proposed math?
It depends on the stage. I'm looking at 10 stages or more, and each one will need different amounts of energy (the later coils, where the projectile is faster will need to be energised for less time) - so capacitance, time constant and stored energy will vary depending on the velocity each stage is tuned for.
However, I am planning on using the same coil dimensions for each stage. 20 gauge wire, 8mm internal diameter (although the projectile will be 6mm, the barrel walls take up the difference), 30mm long (so 33 turns per layer), 14 layers of turns (so 462 total).
Inductance should be in the 1550 μH range before ferro-sheathing, resistance will be a smidgeon under 1 ohm.
Peak current is limited by the rise time of the inductor.
Which is why I've designed a topology that has two capacitor banks for each coil. One high voltage, but lower capacitance bank that has the brunt to overcome the coil's inductance - and a lower voltage one that has the capacitance to maintain the desired current levels.
Basically, while you'd normally get a sine wave as your current/time graph, I'm flattening off the top of the sine wave, giving a squarer pulse.
It lets me get the desired current levels in the coil very fast, and maintain them at that level. No waiting around for slow rise times, and no energy wasted on oversaturating the field.
Saturation is an important point in reluctance coilguns. Up until the saturation point of the projectile, the attractive force increases (roughly) quadratically with current. Beyond that, it increases (roughly) linearly.
So, the ideal is to keep the field strength as near the saturation point as possible, because that's where you'll get the best combination of efficiency and kinetic energy*.
*Oversaturated fields can increase kinetic energy, but at a MAJOR cost to efficiency.
And that's what this system is designed to do. Bring reluctance coilgunning as close to that limit as possible.
One other point on efficiency. I'm using a smaller projectile and higher velocity to further improve it. Much of the energy burnt is because coils have to be kept energised for a longer time. A faster projectile means that each coil needs to be kept energised for less time, and should therefore have lesser energy demands.