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Musing about coilguns

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Musing about coilguns

Unread postAuthor: Ragnarok » Mon Apr 12, 2010 1:35 pm

There seems to be quite a lot of discussion of coilguns taking over DYI's thread here.

So, to try and save that thread, I'm starting another one here for discussion of the reluctance coilgun I'm trying to design.

Technician1002 wrote:
Ragnarok wrote: Anyway, I'm looking at the possibility of ferrosheathing the coils.

Confining the flux lines does increase the flux density. High flux density provides high force.

Well, the point here is more to achieve the same flux density for lesser applied power.

Obviously, the steel projectile has a magnetic saturation point (depending on the alloy, usually in the range of 2 Tesla), and beyond that, efficiencies go down. But with those flux densities, you're not exactly lacking forces.

Matching your cap discharge time with inductance to limit the duration to just the interval where the projectile is within the driving coil's influence will transfer the maximum energy.

That's only really for when you're using an unswitched system.
I'm looking at using the "Dump-and-Quench" topology, which gives control over when you shut off and kill the current in the coil, which means such optimisation isn't needed. As long as the inductance of the coil doesn't actually result in a negative voltage that destroys the capacitors, that's about all that has to be considered.

Energy is lost deforming the projectile and exploding the work coil. A softer longer discharge with the same energy content can provide higher velocities.

We're not looking at anything like the rise time needed to deform the projectile here, and the coils will be built pretty solid as well. I'm not expecting notable energy losses to such things.

The flash capacitors you are using are designed for discharges lasting about .1 ms to 1 ms.

Well, I wasn't talking about flash capacitors (that was 245Tommy), although I may use them in prototype versions.

But I'm not actually sure they'd be a problem. My research has led me to favour higher resistance coils with more turns than what's usually recommended. Still capable of the necessary field strengths, and while they will have a higher inductance, overcoming that is the whole point in the dual bank system!

The important bit is that it means is lower currents - I'm looking at no more than about 100 Amps. That means less power needed to sustain the field in the coils, less expensive switching, less resistive heating, less concern about resistance elsewhere in the circuit, etc.

~~~~~

The aim with this project is to generate a coilgun that can give an air rifle reasonable competition. I'm hoping for 30 Joules kinetic, 120 m/s, 10 shots a minute, 5 MOA grouping and perhaps 20% efficiency. I might better those goals, but those alone would be more than enough - if I met them, I'd have a pretty reasonable claim to owning the world's best amateur reluctance coilgun.
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Unread postAuthor: 245Tommy » Mon Apr 12, 2010 1:44 pm

The aim with this project is to generate a coilgun that can give an air rifle reasonable competition. I'm hoping for 30 Joules kinetic, 120 m/s, 10 shots a minute, 5 MOA grouping and perhaps 20% efficiency. I might better those goals, but those alone would be more than enough - if I met them, I'd have a pretty reasonable claim to owning the world's best amateur reluctance coilgun.
You'll have some competition. http://sites.google.com/site/futureexpe ... tems/svpar
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Unread postAuthor: Ragnarok » Mon Apr 12, 2010 3:11 pm

245Tommy wrote:You'll have some competition.

Well, that depends on how you define best. I know I'm never going to beat any kind of energy record unless I hit on some kind of incredible revelation while I'm working on it.

But better means more than just "most energy".
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Unread postAuthor: rp181 » Mon Apr 12, 2010 8:36 pm

By dump and quench, do you mean like a SCR V-switch type deal? Or half bridge IGBT regenerative shooting?

Forcing the SCR into a closed state with another smaller capacitor is a mix of the two ideas.
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Unread postAuthor: Technician1002 » Mon Apr 12, 2010 9:54 pm

@ Ragnarok 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.

Could you post some of your proposed math? Items of interest are Time Constant for the discharge, Stored energy value in either Watt Seconds or Joules, Turns in the coil, Inductance, Capacitance, and expected peak Ampere turns in the work coil.

High resistance in the work coil is not desirable. It is energy loss. A low value non inductive resistor in series with a fast recovery diode on your capacitor bank to limit reverse voltage and quickly dissipate coil ringing is a better option. Try to limit the reverse voltage to less than 10 percent of the forward voltage rating.

A diode without a resistor would extend the duration of the reverse polarity and dissipate most of the energy in the diode at high current. The resistor becomes the primary energy absorption item. It limits the peak current. The resistor value can be found using Ohm's law to provide a value that will limit the reverse polarity to no more than 10 % of the forward applied pulse by sinking 10% the pulse current over a TC of 10 times the LC constant.

For example if your peak current is 100 amps at 5KV, a resistor that quenches the reverse polarity at 10 amps will quench the coil at 500 V peak reverse voltage at 10 times the forward pulse time. This is worst case peak values assuming no work was transferred to the projectile. The projectile will in reality absorb much of the pulse energy, hopefully as Kinetic Energy.
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Unread postAuthor: rp181 » Mon Apr 12, 2010 10:50 pm

I think what people forget most is that resistance takes energy, decreasing the energy delivered to the intended target. If you design your circuit for low current, that means you need a high resistance.

This is also the common problem with railguns, and people putting their capacitors in series.
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Unread postAuthor: Technician1002 » Tue Apr 13, 2010 2:24 am

Peak current is limited by the rise time of the inductor. Resistance is not the only factor to limit the current. When the projectile moves, and has an induced current, the movement creates a back EMF that also limits the current. You can get low current with high inductace. No high resistance is needed.

Capacitors in series is to increase the voltage to cause faster rise time in the coil for fast flux change, which is what makes the projectile move. Higher voltage produces higher current into higher inductance coils resulting in higher rates of flux change which induce higher currents in the projectile resulting in higher acceleration forces.. and that is the goal for high speed.

Dumping the energy into a resitor to simply make heat is not the goal.
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Unread postAuthor: Ragnarok » Tue Apr 13, 2010 6:38 am

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.
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