Hardware Reference
In-Depth Information
Therefore, the virtual neutral point M is at the same voltage as the real neutral
point N and can be used to obtain the phase winding voltages,
V AN = V A −V M
V BN = V B −V M
V CN = V C −V M
(4.85)
As discussed in section 4.2.1, the phase back-EMF can be detected in the
silent interval of the phase winding. In this interval, there is no current in the
winding and the phase back-EMF can be written as
e A (t)=V AN (t)=V A (t) −V M
e B (t)=V BN (t)=V B (t) −V M
e C (t)=V CN (t)=V C (t)−V M
(4.86)
Therefore, the zero-crossing points of a phase back-EMF can be obtained us-
ing a zero-crossing detection circuit by observing the difference between the
voltages of winding terminal and the virtual neutral point in the silent state.
In the practical drive circuit, the three terminal voltages (V A , V B and
V C ), neutral point voltage V N (if available), and virtual neutral point voltage
V M , all with respect to the ground, are connected to the phase voltage ZCP
detection circuit shown in Figure 4.81. Three phase voltages can be restored
using equation 4.86 if the neutral point N is not provided. Otherwise, if
V N is available, we can simply replace V M with V N in equation 4.86. The
obtained phase voltages of the spindle motor in Y connection are illustrated
in Figure 4.82.
Finding zero-crossings in the back-EMF signal using the circuit above may
introduce errors due to the in fl uence of commutation of current. However, such
errors can be detected by exploiting our knowledge of the logical relationships
between the drive voltages and the ZCPs [102]. After eliminating the false
ZCPs, we are left with the pulses corresponding to the correct ZCPs of the
back-EMF of all three phases, and the interval between any two consecutive
ZCPs is equal to 60 . Figure 4.83 is an example showing the phase voltages
and their correct ZCPs.
Detection of ZCP in CC-BLDC mode and PWM-BLDC mode
Detection of ZCP in the CC-BLDC and PWM BLDC mode is more com-
plicated than that in the CV-BLDC mode because of superposition of high
frequency PWM signal on V N for these modes. Two possible ways of detecting
theZCPsinthesemodesare
1. Detection of ZCP during the ON state of PWM, and
2. Detection of ZCP at the end of the OFF state of PWM,
implementations of which are shown in Figure 4.84 and 4.85, respectively.

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