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Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Many years ago, did some modeling of the effects of external iron and projectile eddy current/saturation effects. The first GIF movie shows an iron projectile with iron shielding with no eddy currents. Note two things: 1) the instant penetration of field lines into permeable material and 2) the saturation of the iron shield as seen in the form of field lines that "escape" the iron. Time elapsed during simulation ca. 2.5-3msec.
The second GIF movie shows a simulation where eddy currents are significant. The field lines require some time to penetrate the iron. In fact, careful observation of flux lines at the initiation of the pulse shows the field lines completely expelled from the conductive iron.
Another interesting effect (that I did not have much time to investigate) was the projectile flux-dragging effect when losses were present. This is seen in the second GIF as the projectile exits the coil.
The thing to take away from this is that for pulses of the order of milliseconds, eddy currents will be present, but iron sheaths will help performance more than it hurts. Once you start using pulse widths less than 1ms or so, the use of iron sheaths will deliver diminishing returns. One can use the skin-effect formula to give an order-of-magnitude estimate of penetration into a linear permeable material D = sqrt{delta_t / (sigma * mu_r * mu_0)}. This will likely be an underestimate because of the non-linear saturation effects, which effectively reduce permeability.
Also, when the core armature saturates, the force on the armature becomes almost linear with respect to applied coil current. This is because the core behaves as if you had used a permanent magnet (a body with a constant magnetization) as a projectile. The force is proportional to the _square_ of the current in a non-saturated permeable armature because armature magnetization depends linearly on the applied field.
Take a look at this for a detailed picture of what is going on.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
External iron can work, but you need to use something like high-mu powder cores that can take very high frequencies and flux density. Simply using laminated iron will only give you a few hundred hertz (down to about 1ms) or so.
Note, also though, that iron is destabilising, laterally, or longitudinally, or both. With care, it's only destabilising longitudinally, but then you can electronically restabilise it by measuring the position of the projectile optically or whatever.
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External Iron
