AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 25

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

Freescale Semiconductor, Inc.
Control Technique
In order to start the BLDC motor the adequate torque must be generated. The motor torque is
proportional to the multiplication of the stator magnetic flux, the rotor magnetic flux and the sine of
angle between these magnetic fluxes.
It implies (for BLDC motors) the following:
1. The level of phase current must be high enough.
2. The angle between the stator and rotor magnetic fields must be 90deg±30deg.
The first condition is satisfied during the Alignment state by keeping the DC Bus current on the level
which is sufficient to start the motor. In the Starting (Back-EMF Acquisition) state the same value of
PWM duty cycle is used as the one which has stabilized the DC-Bus current during the Align state.
The second condition is more difficult to fulfill without any position feedback information. After the
Alignment state the stator and the rotor magnetic fields are aligned (0deg angle). Therefore the two fast
(faster then the rotor can follow) commutations must be applied to create an angular difference of the
magnetic fields (see
Figure
5-9).
The commutation time is defined by start commutation period (Per_CmtStart).
This allows starting the motor such that minimal speed (defined by state when Back-EMF can be
sensed) is achieved during several commutations while producing the required torque. Until the
Back-EMF feedback is locked the Commutation Process (explained in
Section
5.4.2) assures that
commutations are done in advance, so that successive Back-EMF zero crossing events are not missed.
After several successive Back-EMF zero crossings the exact commutation times can be calculated. The
commutation process is adjusted and the control flow continues to the Running state. The BLDC motor
is then running with regular feedback and the speed controller can be used to control the motor speed
by changing the PWM duty cycle value.
DSP56F80x MC PWM Module in Motor Control Applications
25
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