Re: [femm] transient analysis

[William Brodie-Tyrrell <wfbrodie@xxxxxxxxxxxxxxxxxxxx>]

Stoned koala bears drooled eucalyptus spit in awe as Keith Gregory declaimed:

>If I was building a capacitor discharge induction launcher, i.e. one that 
>produces current in the projectile by induction (and I have built a few) I 
>wouldn't have iron anywhere near it. The performance of an induction 

<snip>

yes, I understand this. The armature current is also reduced by the increased
inductance of the armature. However, I think the same amount of energy is
transferred to the armature - the current builds up slowly to a lower level,
but B.H is higher due to the iron inside the armature. Net effect is a much
slower discharge, with force spread over a longer time (the L-R decay of
current in the armature takes longer). I am yet to verify this (see
below) but should have presentable results Real Soon Now (tm).

I spent much of yesterday running FEMM sims on different geometries, mostly
varying the amount of iron around the place: none, a core inside the coil, a
long central core that the armature travels down, fully shielded (coaxial core
& tube), etc. The results were nonsense (mA induced in the armature), but now
I realise I forgot to divide delta-flux by the time delta when calculating
armature EMF. So everything is out by 6 orders of magnitude; I'll fix that
tonight and see if I get something meaningful.

>I would suggest that you just use FEMM to work out the system inductances 
>and their variation with projectile position and then use those 
>characteristics in a MatLab simulation to work out the forces. The 

this is exactly what I've been doing. see below for something more explicit.

>simulation would be able to make a reasonable estimate of the projectile 
>current too. It might be possible to determine from AC FEMM simulations an 
>effective "shorted-turn" representation of the iron so that eddy-current 
>effects could be included in a MatLab simulation.

I've been ignoring eddy currents for now (the shielding will be laminated in
the appropriate direction); I realise that the numbers won't be exactly right
but they should be OK for comparison purposes.

>I have found in the past that static AC simulations - where the projectile 
>is assumed not to move - made at the approximate ringing frequency of the 

how do you find its ringing frequency? Do you mean the LRC time constant of
the power supply + coil or some magnetic time constant?


What I've done is this (please tell me if I appear to misunderstand something;
its 5 years since my eleceng magnetics classes and I haven't done any since
then):

blockintegral(A.J) in the power coil with varying armature position to get
power coil inductance.

blockintegral(Bz) over the armature and the region of iron (or air) inside it
with zero armature current and a non-zero test coil current to find the flux
linkage into the armature with varying armature position.

measured lorentz force on the armature with test current in it and the coil.

measured blockintegral(A.J) in the armature with a test current (no coil
current) to get armature inductance varying with armature position. uniform
J was probably a bad idea, but I'll re-run this part with a specified I so
most of it will flow nearer the centre of the armature.

measured resistive power loss in the armature with a test current to get
armature resistance.


In matlab, the coil current is simulated as an LRC system with zero initial
current and a specified capacitor size & voltage. R is chosen for very slight
underdampedness. Flux through the armature is determined and its rate of
change (when I remember to divide by time) gives me EMF in the armature. The
armature is simulated as a series L (position-varying), R and voltage source
(EMF).

Having figured out the armature current at each time step, it determines the
force (lorentz force numbers from the sim) and does motion simulation.

http://www.users.on.net/gbt/coilgun/*.fem has 3 examples of geometry that I'm
testing with. I'll put up all the code and data when it looks like its
working.


William Brodie-Tyrrell

--
"There is no God and Dirac is his prophet"
-- Wolfgang Pauli

<wfbrodie@xxxxxxxxxxxxxxxxxxxx>
http://www.cs.adelaide.edu.au/~william