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# Re: [femm] transient analysis

- To: femm@yahoogroups.com
- Subject: Re: [femm] transient analysis
- From: Keith Gregory <k.gregory@lboro.ac.uk>
- Date: Tue, 17 Jun 2003 13:40:37 +0100
- In-reply-to: <5.1.0.14.0.20030616074745.01dedd50@staff-mailin.lboro.ac.u k>
- References: <Pine.GSO.4.21.0306161008490.3258-100000@mulder.smug.adelai de.edu.au> <5.1.0.14.0.20030613130909.00ae4b28@staff-mailin.lboro.ac.uk>

`Please take my last posting as an object lesson - don't try to write e-mails when students are bending your ear about exams. I hope it sort of makes sense.`
`Keith.`
`At 08:16 17/06/2003 +0100, you wrote:`

> `William,`

>     This is in reply to your last but one post I'll have a look at the web
>     pages asap.

>     I suppose it depends on what you are trying to do. If you are trying to
>     produce a fairly gentle acceleration up to not too high speeds then the
>     machine is effectively a linear induction motor and iron would certainly
>     make the thing work better. These things have been built ( a colleague of
>     mine has built several as student projects) and there are journal papers
>     describing them. However, if you are trying to produce very high
>     accelerations (like a gun) then the presence of iron is questionable. That
>     said I have used steel projectiles and found that they work at high
>     energies when the electrical accelerating forces are significantly bigger
>     than the ferromagnetic forces. In this situation steel is just another
>     conducting medium.

>     The launchers than I have built where all intended to move a solid
>     conducting mass to the highest speed possible in a single stage of
>     acceleration. One of the most difficult problems I had to contend with was
>     making the current change fast enough; often the projectile is long gone
>     before the current hits its peak. In the fastest launchers I was getting
>     accelerations such that the projectile is transonic within the first
>     millimetre of travel - I wouldn't have been able to do that with a tubular
>     iron cored device.

>     I tried steel projectiles and they worked ok at high energies where the
>     accelerating forces

>     Tubular launchers are by their nature inefficient, mostly because they
>     expend a lot of their energy trying the crush the projectile rather than
>     push it. If you do the sums I think you will find that the forces radially
>     inward can be substantially bigger than the axial accelerating forces.

> `At 11:01 16/06/2003 +0930, you wrote:
> >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 suppose it depends on what you are trying to do. If you are trying to
>     produce a fairly gentle acceleration up to not too high speeds then the
>     machine is effectively a linear induction motor and iron would certainly
>     make the thing work better. These things have been built ( a colleague of
>     mine has built several as student projects) and there are journal papers
>     describing them. However, if you are trying to produce very high
>     accelerations (like a gun) then the presence of iron is questionable. That
>     said I have used steel projectiles and found that they work at high
>     energies when the electrical accelerating forces are significantly bigger
>     than the ferromagnetic forces. In this situation steel is just another
>     conducting medium; its conductivity is not great though and we have usually
>     backed steel projectiles with aluminium or copper.

> `
> >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?`

>     Yep. If you can get an estimate of the system inductance an resistance and
>     you know the capacitance then just use the standard equation. If the coil
>     inductance is low then the power supply parameters (things like capacitor
>     equivalent series inductance and resistance and the cable parameters are
>     vastly important).

> `>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).`

>     I'm not sure that I would calculate any forces in FEMM other than for
>     comparison. I would do the force calculation in a separate simulation, in
>     this case the MatLab one. To calculate the forces you need to know the
>     derivatives of the self and mutual inductances with respect to armature
>     position. These can be got from FEMM pretty easily. Also, I'm not sue why
>     you would want to calculate the armature emf directly, it should come out
>     of the MatLab calculations. We clearly do this stuff differently, I could
>     send you some descriptive stuff if it would be of any interest, but it
>     might be better off list.

> `Keith.`
> ``
> ``
> `Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/`

``

- References:
  - Re: [femm] transient analysis
    - From: Keith Gregory <k.gregory@lboro.ac.uk>

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