AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 26

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

Control Technique
26
Stator magnetic field
Rotor movement
during one
commutation
Border of
stator pole
Zero crossing
edge indicator
with regular Back-EMF feedback
Motor is Running
at steady-state condition
DSP56F80x MC PWM Module in Motor Control Applications
Phase winding
Direction of
Phase current
Freescale Semiconductor, Inc.
Figure 5-9. Vectors of Magnetic Fields
Rotor magnetic
field
(created by PM)
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Motor is Starting
Alignment Stage
The rotor position is stabilized by
applying PWM signals to only two
motor phases
Starting (BEMF Acquisition)
The two fast (faster then the rotor can
move) commutations are applied to
create an angular difference of the
stator magnetic field and rotor
magnetic field.
The Back-EMF feedback is tested.
When the Back-EMF zero crossing
is recognized the time of new
commutation is evaluated. Until at
least two successive Back-EMF zero
crossings are received the exact
commutation time can not be
calculated. Therefore the
commutation is done in advance in
order to assure that successive
Back-EMF zero crossing events
would not be missed.
Running
After several Back-EMF zero
crossing events the exact
commutation time is calculated. The
commutation process is adjusted.
Motor is running with regular
Back-EMF feedback.