AN1913D Freescale Semiconductor / Motorola, AN1913D Datasheet - Page 24

• Manufacturer Part Number: AN1913D
• Description: 3-phase BLDC Motor Control with Sensorless Back-EMF ADC Zero Crossing Detection using DSP 56F80x
• Manufacturer: Freescale Semiconductor / Motorola

Control Technique
24
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 both 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 in 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, both the stator and the rotor magnetic fields are aligned (0deg angle). Therefore, the
two fast (faster then the rotor can follow) commutation must be applied to create an angular difference
of the magnetic fields (see
The commutation time is defined by the start commutation period (Per_CmtStart).
This allows to start the motor the way that minimal speed (defined by state when Back-EMF can be
sensed) is achieved during several commutation while producing the required torque. Until the
Back-EMF feedback is locked into the Commutation Process (explained in
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. 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.
BLDC control with BEMF Zero Crossing using ADC
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure
5-9).
Go to: www.freescale.com
Section
5.4.2) assures that