On 3 Feb 2005 09:53:47 -0800, jim rozen
wrote:

In article , Don Foreman says...

The flux density in those alnico household magnets is considerably
less than that of saturated alnico. It became so almost instantly
when removed from the magnetizer at the factory, before it ever
arrived at the store.

Anyone who works on magnetos knows that the rotors should
always be stored with 'keepers' on the pole pieces.

I always wondered what would happen if I took one of those
old mags, and set it up with some of the new supermagnets.

Jim


The keeper discussion still seems to be simmering
gently so a few extra comments might be helpful.

For the moment consider only the iron alloy based
magnets. This includes Alnico, Alni and the earlier alloys
using cobalt,tungsten or carbon additives.

These alloys, when magnetised to saturation in a
fully closed magnetic circuit magnetising jig, all retain
about the same flux density after the magnetising field is
removed. This is quoted in the manufacturers literature as
the B remanence figure. It is typically about 12 Kilogauss.

If, now, a small air gap is introduced there is
an immediate drop in flux density which is only partially
recovered when the air gap is returned to zero.

Provided that any later experimentally
introduced airgap is smaller than this first gap there is no
further degradation.

If however a subsequent air gap is greater than
any preceding gap it ratchets the closed magnetic circuit
flux density down further and establishes a new range of
airgaps that the magnet can accept without further
degradation.

This is the behaviour that we
observe when we disassemble a magnetic base or remove the
armature from a servomotor that uses this type of magnet.

It doesn't explain the role of keepers commonly
used to protect magnets when not in use. Once a magnet has
been exposed to the demagnetising influence of a large
airgap, putting the keeper back on will NOT restore it to
it's previous state.

The keeper is there for a different reason.
Permanent magnets exhibit their magnetism as a result of the
saturating magnetising field forcing their randomly oriented
internal magnetic domains into a nice orderly additive
arrangement. As noted above this is fairly easily disturbed
by the demagnetising effect of an airgap.

It is also disturbed by mechanical shock and it's
sensitivity to shock or stray external magnetic fields is
increased if it's also fighting with the demagnetising
influence of an airgap. The older permanent magnet materials
which have low intrinsic coercive force are particularly
sensitive to this problem and this is why you see the old
schoolboy bar magnets and ancient magneto horshoe magnets
religiously stored with keepers on when not in use.

A keeper is also used occasionally to provide a
temporary alternate flux path when it is necessary to remove
part of the normal main flux path. This avoids exposing the
magnet to the demagnetising effect of a large disasembled
airgap. The keeper must, of course, be installed BEFORE the
main flux path is removed.

The mechanical shock effect is easily
demonstrated in the workshop. Magnetise a screwdriver or a
piece of hardened carbon steel by stroking it against a
decent permanent magnet. Left undisturbed on the bench it
will retain it's newfound magnetism indefinitely. Bash it
hard against any sustantial lump of metal and much of its
magnetism is immediately destroyed.

Similar effects, but to a much smaller degree
occur with ferrite and the rare earth supermagnets. With
most designs, additional airgap induced degradation is not
enough to matter.

Designs using the old magnet materials can be
uprated by changing to rare earth magnets but the vastly
different optimum length to diameter ratio of the new
material means that a pretty major mechanical redesign is
necessary.

Jim