Re: drag force with FEMM

["tano1938" <rgarritano@xxxxxxxxxxx>]

Thanks Dave, you're the greatest!!! We are all very fortunate to 
have you as a moderator of this group.

Ron 








--- In femm@xxxxxxxxxxxxxxx, David Meeker <dmeeker@xxxx> wrote:
> tano1938 wrote:
> 
> > Is it possible with FEMM to get the drag force on a copper or
> > aluminum cylinder in a voice coil type magnetics design. In other
> > words, if I replace the coil in a linear voice coil motor (which I
> > have already modeled in FEMM ) with a solid cylinder of the same 
size
> > as the coil, I would like to find the drag force on the cylinder.
> 
> Yes, you can look at this sort of problem with femm. In an 
axisymmetric 
> problem, the magnitude of the currents that you induce due to 
motion are
> J=conductivity * v cross B = conductivity*velocity*Br
> where "Br" denotes the radial component of the field and "velocity" 
> denotes the axial velocity of the coil. The force on a 
differential 
> volume of your cylinder is then:
> dF=J cross B = -velocity*conductivity*Br^2
> Now, we could make two key assumptions: The frequency of the 
motion is 
> "low enough" so that the contribution of the reaction currents in 
the 
> coil to the Br field are negligible; and the amplitude of the 
motion is 
> small. Then, we could get a single damping coefficient by 
integrating 
> conductivity*Br^2 over the volume of your cylinder:
> Damping Coefficient = Integral[conductivity*Br^2] over coil volume
> 
> Unfortunately for you, the volume integral of Br^2 isn't one of the 
> "canned" volume integrals. You could be lazy and just do the 
integral 
> of B over the block to get the average Br, which you could then 
square 
> and multiply by the coil volume--this would be OK if your coil is 
> immersed in a relatively homogeneous field. Otherwise, you could 
do a 
> more detailed quadrature, e.g. draw a line down the center of the 
coil, 
> dump the B.n line plot values to disk, and do the Br^2 integral "by 
hand".
> 
> However, there are some caveats. The frequency has to be "low 
enough" 
> for this approximation to be valid. You could get an idea of what 
is a 
> "low enough" frequency by treating your coil as a 1-turn coil 
carrying a 
> constant current density and computing the R and L of the coil. 
Your 
> frequency should then be low compared to Sqrt[R/L]--maybe at least 
an 
> order of magnitude less, also remembering that the units of Sqrt
[R/L] 
> are rad/sec and not Hz. If this condition is not met, you then 
have to 
> account for the inductances of the current loops in the cylinder, 
which 
> is possible, but requires a lot more work.
> 
> Dave.
> -- 
> David Meeker
> email: dmeeker@xxxx
> www: http://femm.berlios.de/dmeeker