Figure 4.67: The spectrum of the MMF generated by the single phase winding
shown in Figure 4.65.
4.3.9 Cogging Torque, UMP and Armature Reaction
The spindle motor in HDD uses concentrated windings, multiple magnetic
pole-pair and surface-mounted magnet. During the operation of these motors,
cogging torque and UMP are generated which is explained in sections 4.2.3
Bonded NdFeB magnet, surface mounted on the rotor, is used in the HDD
spindle motors. As the relative permeability of the bonded NdFeB magnet
is close to 1, the surface mounted magnet makes the gap length between the
stator core and the rotor core big (see Figure 4.68).
Since the motor core is made of steel, the permeability of each core is much
higher than that of air. The main reluctance in the magnetic circuit of the
motor comes from the airgap between the stator core and the rotor core. This
makes it diﬃcult for the current in the armature to produce strong radial
magnetic fi eld in the airgap. Considering that the NdFeB magnet is powerful
in producing the magnetic fi eld, the dominant radial airgap fi eld is governed
by the magnet. Figure 4.69 shows an example, where, it is found that even
with an armature current 5 times the rated current, the effect on the airgap
radial fi eld is insigni fi cant.
(1) Cogging torque generated in the spindle motor operation
As the concentrated windings are used and installed in the slots of stator core,
the cogging torque is unavoidable in the spindle motor. Figure 4.70 shows the
cogging torque produced in a spindle motor with 9 slots and 6 pole-pairs.