On Sun, 22 Mar 2020 22:02:36 +0000 (UTC), Cydrome Leader
wrote:
wrote:
On Sat, 14 Mar 2020 21:25:20 +0000 (UTC), Cydrome Leader
wrote:
Has anybody remagnetized a rotary on-off mag base before? I was thinking
about a large capacitor and some turns of wire on EI transformer core
with the Is removed and the indicator base across the center and one side
leg.
How are these magnetized at the factory?
Probably your best bet is to replace the old alnico magnet with a new
rare earth magnet. This is because if the alnico magnet is removed
from the magnetic circuit it loses a lot of its magnetism. The alnico
magnets in old and/or inexpensive mag bases were/are magnetized after
assembly. This takes a LOT of current. I have seen the setups for
doing this and they are not trivial.
Eric
Any details on these magnetizers?
https://www.coolmagnetman.com/Magnetizing.pdf
See below from the link above.
8
5. Pulse Analysis
All of the capacit
ive
-
discharge magnetizer circuits shown may be modeled as a series
combination of a capacitor, a resistor, and an inductance. The
electrical resistance must include the
resistance of the source as well as that of the fixture (especially
including the ESR,
equivalent series
resistance, of the capacitors), and also includes components from
eddy
-
current conduction in surrounding
structures, in the magnet itself, from “skin effect” in the
conductors, etc. In addition, the resistance may
increase during the
period of the pulse (by perhaps 30%) due to heating in the fixture
(the resistance of
copper and most other metals increases with temperature). The
inductance of a fixture containing steel
pole material is dramatically affected by whether the fixture is
below or above magnetic saturation (the
inductance dropping greatly at currents above saturation). Other
effects may be of importance too, such as
the retention of energy by the electrolyte of the capacitors, the
absorption of energy by the magnet, and
other nonlinearities. Nonetheless, in many cases the overall system
behavior of the magnetizer and fixture
is modeled to sufficient accuracy by assuming constant values for the
resistance, inductance, and
capacitance. Even where the assumption of const
ant values of these parameters is not justified for final
design, the linear analysis may provide a good first approximation and
a check on the calculations. Where
the linear approach using fixed values is not accurate enough,
however, a computer simulat
ion including all
nonlinear effects may be used. The method is described in detail in
the bound notes (reference 7).
6. Design of Fixtures
6.1 General
There are five types of conditions which must be met in the
design of a magn
etizing fixtu
(1) The fixture, in combination with the magnetizer,
must provide a magnetic field of sufficient
strength and in the proper direction to saturate the magnet. The
directional requirement is usually not much
of a problem in m
agnetizing anisotropic materials, which can only be magnetized along a
particular
directional line (although with either sense, i. e. from right to
left, or from left to right, along that line).
This is because the field component in the required directio
n varies as the cosine of the angle between the
two, which does not change much for angles up to ten degrees of arc or
so. If the material is isotropic,
however, meaning that it can be magnetized in any direction, the
direction of field may be of much gre
ater
concern.
The magnet domains themselves align in a very short time (on the
order of 10
-
8
to 10
-
9
seconds). The
field may have to be maintained for a significantly longer time,
however, in order to overcome electrical
eddy currents, which may oc
cur in the fixture, the magnet itself, or in associated structure.
(2) The part must be held in the fixture in the proper
orientation, accurately but without imposing
stresses on the part during magnetizing (and possible thermal cycling
as we
ll) without breaking it. The part
must also not be damaged, chipped, or broken as it is being removed
from the fixture, or as it is loaded.
(3) The windings must be strong enough, or must be
reinforced to be strong enough, to withstand
the m
echanical forces on them during the magnetizing pulse, either to fail
due to ultimate stress limits or, at
a much lower level, in fatigue (after a number of cycles). Fields
high enough to magnetize high
-
energy
magnets often cause forces which could pull
apart copper conductors in a single pulse, if they are not
strengthened by other means. These forces are also more than strong
enough to bend, crush, or extrude out
epoxy potting plastics.
(4) The thermal requirements must be met. the near
-
i
nstantaneous temperature rise in the windings
during the pulse occurs too quickly for much of the heat to escape
across even a single thin layer of
electrical insulation. If this rise is too great, the insulation will
fail, on a single pulse. A thermal t
ime
constant exists for this effect, and a thermal mass, which is often
significantly different from that of the
fixture as a whole (that is, the time constant is shorter than that of
the fixture, and the mass is less). Both
must be taken into account. A
n extreme example is a large “C” frame fixture, used for automatically