Practical Coilgun Design 

FEMM

Coil CurrentNow that we've looked at a few coil geometries, let's take a step back. I started FEMM modelling with an important assumption. Is it true that force scales neatly with coil current? The answer to this question will suggest if there's an advantage to an extremely high magnetic impulse or not. It tells us whether to pursue kilovolt capacitors, or to build many stages of modest capacitor banks. FEMM ModelA reader once suggested that exciting things happen when you discharge capacitors above 500v or so. I want to find out if it's as good as they think. This model approximates my actual parts, using nice round numbers:
The projectile is made of iron welding rod:
Like my other FEMM models, this model has some important assumptions:
FEMM Coil CurrentWe'll use Lua scripting to explore a range of coil currents from 10  10,000 amps. This is a wider range than I need  I expect my actual coilgun's peak current to fall somewhere above a mere 100 amps. But we're interested to see what happens at really high current. FEMM accepts "current" as a sheet density. Here's how to convert wire current w to coil current c. First compute ampturns: N*I = (layers)*(turns)*(current) = 4 * 31 * w Next compute crosssectional area of coil windings: Area = (outer radius  inner radius)*(coil length) Finally compute current density: J = (ampturns)/(area) = 124w / (240mm^{2}) * (10^{6} mm^{2} per
m^{2}) This equation will be used in the Lua script below. Preprocessor Lua Program
Postprocessor Lua Program
ResultsYou can see the detailed Excel spreadsheet here. You can get the raw results file here as it was written by FEMM. For each coil, it computes the force on the projectile at many positions and sums the result. This is a numerical solution to integrating force as it acts over a distance, yielding "work". We ignore friction and losses, and assume the work is converted completely into kinetic energy.The graph shows the total work done for the given continuous coil current. ConclusionThe graph shows how much kinetic energy (blue) is absorbed by the projectile from various coil currents. It also shows the final velocity (red), assuming that KE = 1/2 * m * v^{2}. The kinetic energy graph is practically a straight line. The energy transfer is not very sensitive to coil current. This is surprising, since I expected the magnetic saturation to reduce the efficiency with large current. The result is surprising enough that I tend to discount the outcome. For now, I assume there is a flaw in the modelling procedure. The modelling should be repeated with a close eye to saturation. The next model should review the ferrous material and check that the BH curve accurately represents the material at very high magnetic fields. It probably needs adjustment, since FEM warns of accuracy errors if your BH curve does not include your operating region. The next model should run some tests manually so the amount of saturation can be examined and reviewed. This will tell you where the expected curve should depart from linearity. The bottom line... There is more modelling and analysis needed here. Comments and suggestions are welcomed. 



Last Update 20080614
©19982019 Barry Hansen 