Re: Does a magnet grow less magnetic over time?

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"Szczepan Bia³ek" <sz.bialek@xxxxx> wrote in message
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"Don Kelly" <dhky@xxxxxxx> wrote
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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 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*


If we are driving permanent magnets beyond the linear region (which I take
as the "top" of the B-H curve for a "flat-topped" permanent magnet material
where the effective permeability is mu0 )then there is negligable
degradation.
If we drive the permanent magnet out of this region- then we encounter
hysteresis and a net loss of energy is inevitably involved.

So we are in a state of
a) no hysteresis loss but mechanical losses
b)both hysteresis and mechanical losses.

In either case, we need some external energy source to have continuous
motion. In case a) the degradation is 0 (ideal case) and in case b) (real
world case) degradation occurs.
If we operate a "purely magnetic" motor- (by that I mean no external input
such as from an electrical source) then
In case a) the motor eventually comes to a standstill
In case b) it also comes to a standstill -but much faster.

Driving a permanent magnet out of the "top" of the B-H curve region makes no
sense unless your purpose is to make it weaker- which is also non-sensical.
For a motor, the magnetic field is not an energy source but acts as a "gear"
if you wish, converting electrical energy to mechanical energy (or vice
versa).

Actually, I think that we agree in many respects except for choice of words
and definitions.

--

Don Kelly dhky@xxxxxxxxxxxx
remove the X to answer


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