Hardware Reference
In-Depth Information
Figure 4.90: Three phase voltages in twelve-step starting.
The changes in current vector between two successive switching states in
six-step and twelve-step are illustrated in Figure 4.91. Let's assume that the
current
fl
ows from A to B during a state and the corresponding current vector
is I
1
. In the next state of the six-step method, the current
fl
ows from A to C
with corresponding vector I
6−step
;thevariationinthevectoris∆I
6−step
.On
the contrary, current
fl
ows from both A to C and A to B in the next stage of
twelve-step starting, and the current vector is illustrated as I
12−step
, current
vector variation as ∆I
12−step
. We can see that both the magnitude and angle
of the vector variation are smaller in the twelve-step method than in the six-
step method. So the magnetic
fi
eld produced by the armature windings varies
smoother in the twelve-step method, making it less prone to exciting resonance
of the rotor during the starting process. The twelve-step method makes the
starting easier and smoother.
Both these schemes can be easily realized using electronics. Details are
not included in this topic, but the interested readers may refer to [131] for
elaborate analysis on the spin-up capabilities of these two methods. The results
introduced in the literature show that, the twelve-step starting is more robust.
4.4.4 Spindle Motor Driven in Sensorless BLDC Mode
The performance of a motor is dependent on its drive mode. What is the
most suitable drive mode for the spindle motor discussed in section 4.3.1?
The spindle motor used in HDD is compact in EM structure and mechanical
structure, weak in armature reaction, and has sinusoidal back-EMF in the
constant speed. What is a suitable drive mode for such motor so that the
requirements mentioned in section 4.3.1 can be met? It is shown using the
analysis presented below that the sensorless BLDC drive mode is a good choice
for HDD spindle motor.
