AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 10

• Manufacturer Part Number: AN1914
• Description: 3-Phase BLDC Motor Control with Sensorless Back EMF Zero Crossing Detection Using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

Target Motor Theory
3.5 Back-EMF Sensing
10
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 DC Brushless motor
are connected at a time (see
voltage.
Let us assume the situation when phases A and B are powered and phase C is non-fed. No current is
going through this phase. This is described by the following conditions:
The branch voltage C can be calculated when considering the above conditions:
AS shown in
C and the zero voltage level. Thus the Back-EMF voltage is obtained and the zero crossing can be
recognized.
The general expressions can also be found:
There are two necessary conditions which must be met:
The
Shaded rectangles designate the validity of the equation (EQ 3-6.). In other words, the Back-EMF
voltage can be sensed during designated intervals
i
x
•
•
Figure 3-6
Top and bottom switch (in diagonal) have to be driven with the same PWM signal
No current is going through the non-fed phase used to sense the Back-EMF
-
-
-
-
internal motor torque (no mechanical losses)
rotor speed, rotor position
phase index, it stands for A,B,C
magnetic flux of phase winding x
Figure
shows branch and motor phase winding voltages during a 0-360°electrical interval.
DSP56F80x MC PWM Module in Motor Control Applications
3-5, the branch voltage of phase C can be sensed between the power stage output
Freescale Semiconductor, Inc.
For More Information On This Product,
T
Figure
i
u
=
S
Vx
u
Ab
i
1 --- -
A
VA
=
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u
x
=
S
iA
C
=
=
Bt
–
3
-- - u
2
3-2), so the third phase can be used to sense the Back-EMF
A
+
u
i
B
u
VC
ix
1
-- - u
2
u
ix
where x
i
iB
C
are energized
d
=
+
i
=
x
u
u
3
-- - u
2
VB
0
=
iC
iC
d
x
=
i
=
=
C
=
C
A
A B C ,
0
1
-- - u
2
=
d
d
,
0
d
x
i
x
(EQ 3-3.)
(EQ 3-4.)
(EQ 3-5.)
(EQ 3-6.)