Hardware Reference
In-Depth Information
Figure 4.3: B-H curve of a permanent magnetic material
2. Point b: B = 0 and the magnetic
fi
eld intensity is equal to H
c
which is
known as the coercive force, or coercitivity. H
c
shows the value of the
reverse
fi
eld needed to remove the magnetization of the material after
it is saturated magnetized. This parameter shows the capability of the
magnet to counteract the in
fl
uence of external
fi
eld.
3. Point m: Maximum product resulting from B and H on the demagneti-
zation curve, i.e., the largest rectangle which can be formed within the
hysteresis curve in the second quadrant. This point indicates the maxi-
mum energy that a magnetic material can supply to an external magnetic
circuit (see section 4.1.6) when operating at this point, that is,
(BH)
max
= Maximum(w
m
)=H
m
B
m
.
(4.8)
In the design of electromagnetic devices, the operating point of the per-
manent magnet material should be designed around the point M for utilizing
the material effectively. A high (BH)
max
implies that the required magnetic
fl
ux can be obtained with a smaller volume of the material, making the device
lighter and more compact.
In the spindle motor, the permanent magnet is made of bonded neumodium-
iron-boron (NdFeB). This kind of material is made by binding rapid-quenching
NdFeB powder. The powder is mixed with resin to form a magnet by com-
pressing molding with epoxy or infecting molding with nylon. The magnet
surface is coated with epoxy to prevent corrosion. This kind of material is
fi
ne
in B
r
, H
c
and (BH)
max
. Because the quenching powder is used, the magnetic
property of the bonded NdFeB is isotropic. Therefore, it can be magnetized
with multi-poles in radial direction. The magnet can also be made as a ring
with a thin wall, and the cost of the ring is low in mass production.
