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.