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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Lets's Play: Model the Multistage-Coilgun

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Author Post
Fri Apr 22 2016, 04:22PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 533
d) A choice of a large L/R causes a considerable backwards force initially in the eddy simulation. I can't see that in the reference force coming from FEM. You said there is a negative force. I'm puzzled.
Hmmh. Its there, but its only numerical present. Its in the Nano-Newton-Range and really its just the first few timesteps. So its ok, if you dont see it. But having the effect present is ok.

I am still convinced that the time constant changes over time, since the projectiles exposure to the magnetic field changes and therefore the region where eddy currents happen changes. this should increase the time constant when the projectile is inside the coil.

Additionally, please note that the dataset already consideres saturation effects.
ur plays a big role in the skin effect. What values are you using in FEM and FEMM?
Its the BH-Curve of some Iron supplied with FEMM

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Tue Apr 26 2016, 08:35AM
Registered Member #3988
Joined: Thu Jul 07 2011, 03:25PM
Posts: 660
I've looked at the FEMM.cir. It became quite obvious, that saturation plays a big role, which I had neglected in my math for simplicity purposes. I'm working on incorporating this. You're right about the time constant, but I believe it changes due to saturation effects. Skin effect becomes much less prominent for low ur. From your B-H curves ur drops from values around 10000 to 10. Projectile L also drops, but not as strongly. I'll be back once I've reworked my simulation.

Nano Newtons for the initial backwards force seems awfully low. Not really consistent with even low eddy currents.
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Wed Apr 27 2016, 02:36AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 533
Uspring, i think in the ZIP i delivered there is a circuit thats called "RawDataTestbench.asc"
This should give you an impression on the content of the FEMM extracted data. (also the saturation | current dependence and so on)

i hope that helps.
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Tue May 24 2016, 03:30PM
Registered Member #3988
Joined: Thu Jul 07 2011, 03:25PM
Posts: 660
I've been working on incorporating saturation in the eddy simulation. Involved, but doable. The main problem turns out to be the shape of the eddy force time dependency curve.

Generally the eddy force is initially repulsive. When the coils input voltage changes sign, the induced voltage in the projectile also changes sign with some (short) delay due to the projectiles L/R time constant. This will cause an attractive force later on, i.e. some time before the projectile reaches the center of the coil.

This doesn't agree, though, with a comparison of the dynamical FEM simulation and the statical FEMM simulation. The difference between both should be due to eddy effects. The force in the dynamical simulation is always less than in the static simulation. The reversal of the eddy force simply doesn't happen there.

Possibly this is caused by the length of the projectile, which would smear out the force curve and thus hide the small attractive part in the eddy force curve. How long is the projectile? Also, do you have any info on the projectile current from the dynamical simulation? Does it reverse sign and when?
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Sun Aug 14 2016, 02:54PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 533
Hi Uspring, sry i wasnt around so long. I wasnt working on electronics or simlation for a long time so i neglected the form here for a while Sry.
I still think this kind of commnication is not suited for a complex problem such as this.
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