Re: Does a magnet grow less magnetic over time?

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"P." <dit3_werkt_ook_niet@xxxxxxxxxxx> wrote in message
news:k850h49f70pooaroqakh8n50pt1jrgv39s@xxxxxxxxxx
On Tue, 4 Nov 2008 09:39:10 +0100, "Szczepan Bia³ek" <sz.bialek@xxxxx>
wrote:


"Don Kelly" <dhky@xxxxxxx> wrote
news:fNPPk.12465$kd5.3102@xxxxxxxxxxxxxxx
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Magnet motors (those with nothing but permanent magnets) do not
"consume"
energy except that the initial energy used to start them is eventually
dissipated as heat -and the motor stops.

So the permament magnet become weaker during working.

The magnet does not change. The energy you put into it by giving it a
swing is only converted from moving (kinetical energy) to heating up
materials by friction and eddy currents.

The magnet supplies no net energy to the process returning to the same
"energy point' at the end of each cycle. There may be some hysteresis
loss
but in modern magnets this is negligable - unless the magnet is pushed
beyond the linear region of the B-H curve-which is poor design.

If we need a magnet to convert energy we should be in "the linear region
of
the B-H ".
In magnet motors we are "beyond the linear region of the B-H".
The question was: "Hence, it seems to me that the magnetism must somehow
degrade over time...".
The answer is: Yes.
S*

You do not have to be in the linear region. The disadvantage of going
outside that region is that the iron saturates and cannot store much
more energy. It will still work though.
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The only problem with this is that permanent magnets are alreaduy in the
saturation region The linear region that I mentioned is the top of the
magnet's B-H curve where the permeability is mu0. The problem is the
opposite, where one drives the magnet to a high negative H and a lower B
where the magnet is no longer in saturation. Recovery in such circumstances
may not be complete so that the magnet is weakened when driven out of
saturation.

Part of the whole problem of concepts is the difference between ideal
characteristics of transformer steel and permanent magnet material. The
ideal for the former is to have as small a hysteresis loop as possible (low
coercive force) and for the latter, a big fat high coercive force hysteresis
loop. Transformer steel makes a lousy permanent magnet but Alnico makes a
lousy transformer core.
--

Don Kelly dhky@xxxxxxxxxxxx
remove the X to answer




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