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# Re: [femm] FEMM homepage and help

- To: femm@yahoogroups.com
- Subject: Re: [femm] FEMM homepage and help
- From: Keith Gregory <k.gregory@lboro.ac.uk>
- Date: Mon, 25 Mar 2002 10:07:10 +0000
- In-reply-to: <3C9E858A.6060608@ieee.org>
- References: <a7kl9m+ekl4@eGroups.com>

`I'm sitting here waiting for one of my project students to turn up so I though I would add to this discussion.`
`I really can't fault anything Dave has said and the idea of a lumped parameter model is a vary good one. I would attack it slightly differently however.`
`I would define the problem as a circuit with the circuit elements having resistance and inductance. I don't think that at audio frequencies capacitances are going to be important. The inductances of the coils, both selfs and mutual, would be a function of their position. In addition, equation elements would be required to deal with the effects of the magnet. This is not dramatically different to Dave's proposal just defined slightly differently. A circuit model may make it easier to identify the effects of things like induced emf due to motion. It is also relatively easy to produce a reasonable representation of the mechanical structure which can be solved in the same way and at the same time as the electrical problem. The electrical parameters of such a system could be determined using FEMM.`
`A few years ago there were several people at Imperial who were very conversant with the use of circuit models for modelling dynamic systems, however, they may have all retired now!`
`There is one point that I would comment on (and I'm in a really picky mood just now!) and that it the bit about`

    "Model a permanent magnetic field, to see the effect on this field
    that a voice coil carrying a current has (as it generates its own
    flux)."

`I believe it better to think of only one flux in the system that is produced by a combination of permanent magnet and current in coils. The coil doesn't "generate its own flux" it changes the distribution of the flux produced by the system as a whole. The idea of the coil producing its own flux comes from superposition theory which is very dangerous to use in magnetic systems where linearity can be questionable.`
`Keith.`
`At 21:03 24/03/2002 -0500, you wrote:`

> `emacoustics wrote:`

> > `Hey folks,`

> >     A couple of quickies:
> >     Firstly, where can I get FEMM from? I had it on my system but had a
> >     major crash and couldn't back it up so I need to download it again.

> `You can download it from the homepage at <http://femm.berlios.de/>http://femm.berlios.de/`

> >     Secondly, I am still trying to model a loudspeaker magnet structure
> >     (some of you may remember my posts from a month or so back) and
> >     wondered if any of you could help. Wat I need to do is this:

> >     Model a permanent magnetic field, to see the effect on this field
> >     that a voice coil carrying a current has (as it generates its own
> >     flux). Then, model a second voice coil in antiphase (and very
> >     similar in size) which should go a long way to cancelling the
> >     negative effects of the first coil. This is a final year university
> >     project. David Meeker sent me some info, but I have a feeling my
> >     email did not reach him as I had some other questions. Does anybody
> >     know how I can do this? It does not have to be hugely detailed, as
> >     the idea of this simulation in the project is to show the effect, and
> >     it will be proved or otherwise using practical experiments.

> `I've just been sort of busy the last few weeks on some work-related problems, and I haven't had too much time to devote to femm.`
> `Anyhow, from what you'd said, the idea of your configuration was "to reduce second harmonic distortion caused by the flux generated by a coil effecting the permanent field." This is one of those situations where having a simple, lumped parameter model might shed some insight into what is going on. If you have a good idea of the effect you are looking for, you know what finite element analyses to perform and have an idea about how to interepret the results. I was going to try to write up something more elaborate on this but I just don't have the time right now (and that's sort of your job, anyhow.....)`
> `Although I don't really design voice coils for audio applications, it seems like what is going on is this: "Second harmonic distortion" in a typical one-coil configuration would mainly be due to reluctance centering effects. That is, the coil tugs on the iron in the speaker whenever there is current in the coil (this would occur even if there wasn't a PM around). This force always has a squared dependence on the applied current. That means that if current is sinusoidal at frequency W, for example, i=Cos[W t], the force will be proportional to Cos[W t]^2. Now, one of those trig identities is Cos[W t]^2 = (1/2) + (1/2) Cos[2 W t]. The resulting force is the sum of a constant component and a 2X (i.e. "second harmonic distortion") component.`
> `There is another way that you could create a 2X force. If you have 2 wires, the force on one wire is the cross-product of the current in the wire with the field from the other wire. If both wires have current that varies at a frequency W, the resulting force again has the steady-state and 2X components.`
> `I guess that the idea here is that by having two coils arranged and connected electrically and mechanically in the "right" way, you can sort of make the various 2X parts of the force cancel out. If you make a lumped-parameter model, it would probably be possible to get some insights into how the cancellation actually "works". It could be difficult to slog through making this sort of lumped-parameter model all one your own, but I'd guess that your professor could probably point you in the right direction.`
> `(Here's a hint: I'd assume linearity (for starters) and build expression for stored magnetic energy via magnetic circuit theory so that there is an explicity dependence of energy on the position of the two coils and the current in the coils. Force on a coil is then the derivative of energy with respect to the position of the coil of interest. Do the 2X forces add up to zero? How do you have to set things up so that they do? What sorts of other design changes would have the same effect?)`

>     Dave.
>     --
>     <http://femm.berlios.de/dmeeker>David Meeker

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

    Dr. Keith Gregory
    Senior Lecturer
    Department of Electronic & Electrical Engineering
    Loughborough University
    Loughborough
    Leicestershire
    LE11 3TU
    UK

    Phone: +44 (0) 1509 227025 Fax: +44 (0) 1509 227014
    Department web: http://www.lboro.ac.uk/departments/el/

    --=====================_8340072==_.ALT
    Content-Type: text/html; charset="us-ascii"

`<html>
I'm sitting here waiting for one of my project students to turn up so I
though I would add to this discussion.<br><br>
I really can't fault anything Dave has said and the idea of a lumped
parameter model is a vary good one. I would attack it slightly
differently however.<br><br>
I would define the problem as a circuit with the circuit elements having
resistance and inductance. I don't think that at audio frequencies
capacitances are going to be important. The inductances of the coils,
both selfs and mutual, would be a function of their position. In
addition, equation elements would be required to deal with the effects of
the magnet. This is not dramatically different to Dave's proposal just
defined slightly differently. A circuit model may make it easier to
identify the effects of things like induced emf due to motion. It is also
relatively easy to produce a reasonable representation of the mechanical
structure which can be solved in the same way and at the same time as the
electrical problem. The electrical parameters of such a system could be
determined using FEMM.<br><br>
A few years ago there were several people at Imperial who were very
conversant with the use of circuit models for modelling dynamic systems,
however, they may have all retired now!<br><br>
There is one point that I would comment on (and I'm in a really picky
mood just now!) and that it the bit about <br><br>
<tt>&quot;Model a permanent magnetic field, to see the effect on this
field <br>
that a voice coil carrying a current has (as it generates its own <br>
flux).&quot;<br><br>
</tt>I believe it better to think of only one flux in the system that is
produced by a combination of permanent magnet and current in coils. The
coil doesn't &quot;generate its own flux&quot; it changes the
distribution of the flux produced by the system as a whole. The idea of
the coil producing its own flux comes from superposition theory which is
very dangerous to use in magnetic systems where linearity can be
questionable.<br><br>
Keith.<br><br>
At 21:03 24/03/2002 -0500, you wrote:<br>
<blockquote type=cite class=cite cite>emacoustics wrote:<br>
<blockquote type=cite class=cite cite><tt>Hey folks,<br><br>
A couple of quickies:<br>
Firstly, where can I get FEMM from?&nbsp; I had it on my system but had a
<br>
major crash and couldn't back it up so I need to download it
again.</tt></blockquote>You can download it from the homepage at
<a href="http://femm.berlios.de/";>http://femm.berlios.de/</a><br>
<blockquote type=cite class=cite cite><tt><br>
Secondly, I am still trying to model a loudspeaker magnet structure <br>
(some of you may remember my posts from a month or so back) and <br>
wondered if any of you could help.&nbsp; Wat I need to do is
this:<br><br>
Model a permanent magnetic field, to see the effect on this field <br>
that a voice coil carrying a current has (as it generates its own <br>
flux).&nbsp; Then, model a second voice coil in antiphase (and very <br>
similar in size) which should go a long way to cancelling the <br>
negative effects of the first coil.&nbsp; This is a final year university
<br>
project.&nbsp; David Meeker sent me some info, but I have a feeling my
<br>
email did not reach him as I had some other questions.&nbsp; Does anybody
<br>
know how I can do this?&nbsp; It does not have to be hugely detailed, as
<br>
the idea of this simulation in the project is to show the effect, and
<br>
it will be proved or otherwise using practical
experiments.</tt></blockquote>I've just been sort of busy the last few
weeks on some work-related problems, and I haven't had too much time to
devote to femm.<br><br>
Anyhow, from what you'd said, the idea of your configuration was &quot;to
reduce second harmonic distortion caused by the flux generated by a coil
effecting the permanent field.&quot;&nbsp; This is one of those
situations where having a simple, lumped parameter model might shed some
insight into what is going on.&nbsp; If you have a good idea of the
effect you are looking for, you know what finite element analyses to
perform and have an idea about how to interepret the results.&nbsp; I was
going to try to write up something more elaborate on this but I just
don't have the time right now (and that's sort of your job,
anyhow.....)<br><br>
Although I don't really design voice coils for audio applications, it
seems like what is going on is this: &quot;Second harmonic
distortion&quot; in a typical one-coil configuration would mainly be due
to reluctance centering effects.&nbsp; That is, the coil tugs on the iron
in the speaker whenever there is current in the coil (this would occur
even if there wasn't a PM around). This force always has a squared
dependence on the applied current.&nbsp; That means that if current is
sinusoidal at frequency W, for example, i=Cos[W t], the force will be
proportional to Cos[W t]^2.&nbsp; Now, one of those trig identities
is&nbsp; Cos[W t]^2 = (1/2) + (1/2) Cos[2 W t].&nbsp; The resulting force
is the sum of a constant component and a 2X (i.e. &quot;second harmonic
distortion&quot;) component.<br><br>
There is another way that you could create a 2X force.&nbsp; If you have
2 wires, the force on one wire is the cross-product of the current in the
wire with the field from the other wire.&nbsp; If both wires have current
that varies at a frequency W, the resulting force again has the
steady-state and 2X components.<br><br>
I guess that the idea here is that by having two coils arranged and
connected electrically and mechanically in the &quot;right&quot; way, you
can sort of make the various 2X parts of the force cancel out.&nbsp; If
you make a lumped-parameter model, it would probably be possible to get
some insights into how the cancellation actually &quot;works&quot;.&nbsp;
It could be difficult to slog through making this sort of
lumped-parameter model all one your own, but I'd guess that your
professor could probably point you in the right direction.&nbsp;
<br><br>
(Here's a hint: I'd assume linearity (for starters) and build expression
for stored magnetic energy via magnetic circuit theory so that there is
an explicity dependence of energy on the position of the two coils and
the current in the coils.&nbsp; Force on a coil is then the derivative of
energy with respect to the position of the coil of interest. Do the 2X
forces add up to zero?&nbsp; How do you have to set things up so that
they do?&nbsp; What sorts of other design changes would have the same
effect?)<br><br>
Dave.<br>
-- <br>
<a href="http://femm.berlios.de/dmeeker";>David Meeker</a><br><br>
<br>
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<x-sigsep><p></x-sigsep>
Dr. Keith Gregory<br>
Senior Lecturer<br>
Department of Electronic &amp; Electrical Engineering<br>
Loughborough University<br>
Loughborough<br>
Leicestershire<br>
LE11 3TU<br>
UK<br><br>
Phone: +44 (0) 1509 227025&nbsp; Fax: +44 (0) 1509 227014<br>
Department web:
<a href="http://www.lboro.ac.uk/departments/el/"; eudora="autourl">http://www.lboro.ac.uk/departments/el/<br><br>
</a></html>`
`--=====================_8340072==_.ALT--`

- References:
  - FEMM homepage and help
    - From: "emacoustics" <michael.wheeler@ic.ac.uk>
  - Re: [femm] FEMM homepage and help
    - From: David Meeker <dmeeker@ieee.org>

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