Figure 4.98: The A-phase current waveform produced by optimal commutation
angle with constant voltage drive mode.
4.4.5 Acoustic Noise in Spindle Motor Driven in BLDC
Many factors can induce acoustic noise in spindle motor operation. The bearing
is the major source of acoustic noise in the spindle motors using ball-bearing.
If fl uid dynamic bearings are used, this source of noise is eliminated and the
acoustic performance of the spindle and, therefore, of the HDD is greatly re-
duced. This issue was discussed in section 4.3. Since the noise contributed by
bearings is negligible in FDB spindles, the noise caused by the electromagnetic
sources becomes evident. The magnetic sources of the acoustic noise can be
1. the deformation of motor parts caused by the radial fi eld ,
2. unbalanced magnetic pull (UMP) , , and
3. torque ripples , .
For PMSM with surfaced PM ring, the airgap magnetic fi eld is almost
independent of the drive current (refer to section 4.3.9). Therefore, dependence
of the acoustic noise caused by the deformation of motor parts on the drive
current is insigni fi cant. It is not diﬃcult to distinguish this kind of noise during
experiments. The acoustic noise caused by the deformation of spindle motor
parts is very weak .
As explained earlier in section 4.3.9, both intrinsic and extrinsic UMPs of
the spindle motor depend on the EM structure of the motor, but they are not