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Re: [femm] FEMM homepage and help
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/
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.
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