Hardware Reference
In-Depth Information
Figure 4.81: Schematic diagram of the drive system for spindle motor operated
in BLDC mode.
When the back-EMF of one phase is at its zero crossing position (ZCP),
the terminal voltage of this phase equals to the neural point voltage; see Fig-
ure 4.1.a. From section 2.5, the ZCPs take place at positions where the coil
direction is aligned with, or opposite to, the magnet axis on the rotor. For the
surface-mounted spindle motors, the armature reaction is usually quite weak,
which means the back-EMF waveforms are not in
fl
uenced by the currents in
the motor operation, and the ZCPs of the phase back-EMF are not affected
either. Therefore, the ZCPs in the spindle motor are reliable in detecting the
rotor position and speed of the motor.
In the motor operation, as there are three phase windings, the number of
ZCPs in one revolution is 6 times the number of pole-pairs. Therefore, the
more pole-pairs, the more information of the rotor can be obtained, and the
more accurate speed could be realized. This is one of the reasons why the
HDDs prefer to use the spindle motor with multiple magnetic pole-pairs, and
4 and 6 pole-pairs are the most prevailing choices.
Detection of ZCP in CV-BLDC mode
The phase voltage means the voltage difference between the phase winding
terminal and neutral point; see VAN, VBN and VCN in Figure 4.72. However,
the actual neutral point of the spindle motor used in HDDs may not be avail-
able. Therefore, as shown in Figure 4.81, a Y-connected resistance circuit is
used to create a virtual neutral point of the three-phase armature windings.
A virtual neutral point M is created by the resistance circuit. Since the
back-EMFs in the armature windings of the spindle motor are symmetrically
and sinusoidal, the virtual neutral point M has the same voltage potential as
the real neutral point N of the motor (shown in Figure 4.81). The proof is
