AN1913 Motorola / Freescale Semiconductor, AN1913 Datasheet - Page 9

• Manufacturer Part Number: AN1913
• Description: AN1913 3-phase BLDC Motor Control with Sensorless Back-EMF ADC Zero Crossing Detection using DSP 56F80x
• Manufacturer: Motorola / Freescale Semiconductor

3.5 Back-EMF Sensing
The internal torque of the motor itself is defined as:
where:
T
x
It is important to understand how the Back-EMF can be sensed and how the motor behavior depends
on the alignment of the Back-EMF to commutation events. This is explained in the next sections.
The Back-EMF sensing technique is based on the fact that only two phases of a Brushless DC motor
are energized at a time (see
the Back-EMF voltage.
Let us assume the situation when phases A and B are powered and phase C is non-fed. No current
passes through this phase. Assume the following conditions are met:
The branch voltage u
Figure 3-5
natural voltage level, can be sensed. Thus the Back-EMF voltage is obtained and the Zero Crossing
can be recognized.
The general expression can be found by:
where:
i
x
-
-
-
-
illustrates that the branch voltage of phase C, between the power stage output C and the
internal motor torque (no mechanical losses)
rotor speed, rotor position
phase index, it stands for A,B,C
magnetic flux of phase winding x
VC
BLDC control with BEMF Zero Crossing using ADC
can be calculated when considering the above conditions:
S
u
i
u
A
Figure
Ab
VA
iA
=
T
+
i
=
S
–
u
=
Bt
i
B
iB
performingPWMswitching
1
-- - u
2
1 --- -
i
+
C
3-2). The third phase is a non-fed phase that can be used to sense
x
d
u
=
C
=
iC
u
A
u
VB
0
u
VC
=
ix
d
=
i
0
C
=
i
u
x
=
1
-- - u
2
3
-- - u
2
Vx
=
0
d
iC
=
x
C
=
3
-- - u
2
A
d
d
ix
x
i
x
Target Motor Theory
(EQ 3-3.)
(EQ 3-4.)
(EQ 3-5.)
(EQ 3-6.)
9