Hardware Reference
In-Depth Information
the intrinsic UMP [17]. The extrinsic UMP can be reduced by adopting strict
quality control of the motor components and the motor assembly process.
(3)
Armature reaction produced in the spindle motor operation
The conditions for generating the armature reaction are explained in sec-
tion 4.2.7. Though the stator core and rotor core are usually saturated in
the spindle motor, the armature reaction in the spindle motor is very weak as
the airgap
fi
eld is almost independent of the armature current. Therefore, the
torque constant K
t
and back-EMF constant K
e
(refer to sections 4.2.3 and
4.2.3) can be considered constant in spindle motor operation.
4.3.10 Electromagnetic Power, Electromagnetic Torque
and Motor Losses
Power loss is inevitable in operation of any motor. Losses of power in both
electrical components and mechanical components contribute to the overall
power loss. The electrical losses include copper loss and iron loss. The copper
loss is the loss of power taking place in the resitances of armature windings,
and can be calculated as
X
i
j
(t)R
j
,
p
cu
=
(4.73)
j=a,b,c
where, i
j
and R
j
are the current and resistance of the j
th
phase winding. The
in
fl
uence of the copper loss in a motor is taken into consideration by including
the circuit resistance in the equivalent circuit of Figure 4.71.
The electrical conductivity and magnetic hysteresis loop of the soft-magnetic
materials are major factors contributing to iron loss. As the magnetic
fi
eld ro-
tates in the stator of the spindle motor, the local
fi
elds alternate and induce
EMF in the stator cores. Since the conductivity of the core material is not
zero, eddy current is produced in the core and results in eddy current loss.
This loss can be reduced by using stator core which is laminated with silicon
steel sheets or some other soft-magnetic materials, as shown in Figure 4.51.
The magnetic pole-pair of spindle motor is typically not smaller than three
and rotational speed is usually few thousand RPM. So the frequency of the
local
fi
elds can be quite high (equation 4.41). Therefore, the lamination sheets
must be very thin, typically 0.35 mm or thinner. For a given stator core, the
eddy current loss is directly proportional to the frequency and the amplitude
of the magnetic
fi
eld.
Soft-magnetic materials exhibit hysteresis characteristics, i.e., there is hys-
teresis loop in the B-H curve of these materials. The hysteresis loop is much
smaller for soft-magnetic materials compared to that of permanent magnet ma-
terials (see section 4.1.5). When the magnetic
fi
eld varies and alters between
