User Manual

Alignment
Starting
(Back-EMF Acquisition)
Running
Alignment time
expired?
Start motor
Minimal correct
commutations done?
No
Yes
Yes
No
Control Technique
3-Phase BLDC Motor Control with Sensorless Back-EMF, ADC, Zero Crossing, Rev. 3
20 Freescale Semiconductor
Preliminary
Figure 5-5. Commutation Control States
5.4.1 Alignment
Before the motor starts, there is a short time (which depends on the motor’s electrical time constant) when the
rotor position is aligned to a known position by applying PWM signals to only two motor phases (no
commutation). The Current Controller keeps the current within predefined limits. This state is necessary in
order to create a high start-up torque and recognized the rotor position. When the preset time-out expires, this
state is finished.
• The Current Controller subroutine (with PI regulator) is called to control the DC-Bus current. The
subroutine sets the right PWM ratio for the required current.
The current is sampled and the Current Controller is calculated in every PWM cycle.
The BLDC motor rotor position (with flux vectors during alignment) is shown in Figure 5-6.