Hardware Reference
In-Depth Information
Figure 4.95: EM torque produced by optimal commutation angle in CC-BLDC
drive mode.
T
em
is a function of
It is obvious from the above equation that the EM torque
the commutation angle α. This equation also shows that,
³
´
π
6
max[T (α)] = T
em
.
(4.96)
Therefore, the optimal commutation angle for the CC-BLDC drive mode
is α
opt
= π/6 as this angle can produce the maximum effective EM torque
for a given current. The average EM torque generated with this optimum
commutation angle is
√
³
´
π
6
=
3
3E
m
I
m
πΩ
max[T (α)] = T
em
.
(4.97)
The EM torque generated by the optimal commutation angle is shown in Figure
4.95. The idea of using constant current in the energized state comes from the
drive method for trapezoidal back-EMF motors. However, the spindle motors
used in HDD nowadays have sinusoidal back-EMF. If a constant current is used
for these motors, the generated torque will contain ripples even for optimal
commutation angle. These ripples are caused by the drive mode itself and will
be called as intrinsic torque ripple for the CC-BLDC drive mode.
Let us de
fi
ne the following objective function to assess the amount of ripple
in the produced torque,
Z
2π
1
T
em
(α)
[T
em
(θ,α)− T
em
(α)]
2
dθ.
O(α)=
(4.98)
0
