Environmental Engineering Reference
In-Depth Information
Current is injected into the BDCM in one phase and extracted from a second
phase. This 2-phase excitation means that at any given time, one of the motor
phases is available for use as a position sensor. Sensing back-emf is the most
common form of sensorless BDCM control. Also, because of 2-phase excitation, a
single current sensor in the bridge return path is all that is needed to regulate the
currents in block mode.
A+
B+
C+
A-
B-
C-
V
ab
V
bc
V
ca
V
an
V
bn
V
cn
I
a
I
b
I
c
Figure 5.12 Brushless dc motor commutation signals (120
conduction)
Figure 5.12 shows the 2-phase motor current conduction and the corresponding
inverter switch commands. The switches are labelled A
þ
,B
þ
,C
þ
across the top
in Figure 5.11 and correspondingly across the bottom. When the switch gate
command is logic 1, the switch is ON, connecting the mid-point of the phase leg to
either
þ
V
d
or 0. The line-to-line voltages can be used to flag the inert phase for
back-emf sensing.
