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Making a Magnet |
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A coil with no iron has a relitive permeability of one. u=ur u0, dB/dH=1
An air coil plots a straight line. The magnetizing force H is the same as the B.
A substance under study is placed inside a coil. A variable current is passed in the coils
that produces an H field, or source field, that is normally measured in amp-turns/meter.
The substance under study responds with a B field,
normally measured in Teslas.
The B-H Curve, a hysteresis loop
The magnetizing force H is horizontally displayed.
Sometimes refered to as the "current curve".
The flux density B on the verticle.
Sometimes refered to as the "flux curve".
A magnetic field meter is used to measure the two fields.
Br:
Residual magnetic field.
Remanent flux density.
Remanence
Retentivity
Remanent induction
Br is measured after saturation induction is reached to the extreem left or right.
Br is the same through out a closed magnetic circuit on the axial line;
And a closed loop is used in the definition.
Retentivity is a special Maximum-Residual-magnetic-field, or residual polarization, after saturation.
Br for AlNiCo:
Br= 7.2KG to 13.5KG, 720mT to 1350mT
Br=680mT AlNiCo-3
Br=1.28T 1280mT AlNiCo-5
Br= .82T 820mT AlNiCo-8
Also known as residual induction.
Anisotropic (native prefered direction) materials have higher Br.
Before magnetization, Anisotropic is "born" in a magnetic field and also cooled within that magnetic field.
Both Isotropic and Anisotropic are magnetized later in the manufacturing process.
Magnetization of an object is its magnetic moment per unit volume.
Denoted M and has the units A/m.
Bs, Bm:
Saturation flux density.
For iron it is a couple of Teslas.
Denoted as Bm in the graph and at the point Bm,Hm
Typical
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Hc:
Coercivity
The magnetic force H, in amp-turns/meter, or (One A-turn/m=4p x 10-3 Oe) oersteds,
required to completely display a demagnetized substance . (B=0 including the demag force).
Because the ease of demagnetising is the same as the ease of magnetizing,
Hc is a measure of vulnerability to stray magnetic fields.
It is the field strength to bring Br to zero. It includes the driving force
So it does not reflect the real zero-Br. This value is Hci or intrinsic Coercivity.
Hc for AlNiCo:
AlNiCo-3: Hc= 380 Oe, 30KA/m, 38mT
AlNiCo-5: Hc= 640 Oe, 52KA/m, 64mT
AlNiCo-8: Hc=1650 Oe, 165mT
AlNiCo-38 Hc=1500 Oe, 150mT
Rare Earth 15 KOe, 1500mT
FeriteCeramic 4 KOe, 400mT
Hci:
Intrinsic Coercivity
The magnetic force H, in amp-turns/meter, or (One A-turn/m=4p x 10-3 Oe) oersteds,
required to completely demagnetize a substance. (B=0). H is applied and then removed.
Any intrinsic measurements: H is applied then removed, then applied then removed...
The "native" or "intrinsic" B field of the materal only; without the driving force H.
H, Hm:
Magnetizing force
Hm is much higher than Hc, and is closer to Hci if not identical.
Field strength to completely saturate magnet.
Hm strength to completely magnetize (align) all domains.
AlNiCo-5 Hm 3000 Oe, 240KA/m, 300mT
Alnico 8, 7000 oersteds 560 KA/m 700mT.
Jagged B curve:
As the magnetising force increases or decreases, the tiny domains, also known as Weiss domains,
in various places, will
randomly flip. This spontaneous action will produce an unevenous in the B curve.
If the B curve is monitored acoustically on a speaker, you can easily hear hash and pops.
Domains form tenuous groups, and the alliances produce different size pops.
When magnetized strongly enough, all domains form only one single domain,
and the material is magnetically saturated.
The curve is more erratic than what I have depicted.
u:
Permeability
Several permeability dB/dH slopes:
Permeability is simply the slope; and is a highly non-linear quantity.
"u-INITIAL Slope", and "u-MAX slope"
Unmagnetized material starts off with the greatest permeability (greatist slope).
And after saturation is reached, you may as well have empty space in the coil.
In fact, chokes and transformers will fail in this region. I have employed several Saturable-Reactors
(transformers) DESIGNED to fail in this region. In my control and broadcast work,
they form great high power regulators!
What was I talking about? Oh yes, magnets...
Permeability, u
A property of a material that describes the ease with which a magnetic flux is established in the material.
High permeability substance will have narrow H swings, producing a high B.
Typical
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Energy
Energy=BH, Joules
Energy Density
Magnet Quality (BHmax):
Maximum Energy Product
Measured in MegaGauss Oersted (MGOe).
Stored energy in a magnet
Magnet performance
"magnetic energy product"
Measured in units of megagauss-oersteds (MGOe)
Units: megajoules per cubic meter
Bmax can go to 13000 joules/m3
(BH)=5.5 MGOe, 44 KJ/m3 AlNiCo-5
(BH)=5.3 MGOe AlNiCo-8
9 KJ/m3 AlNiCo-3
