3-Phase BLDC Motor Sensorless Control Using MC56F8013

System Blocks ConceptAfter several successive back-EMF zero crossings, the exact commutation times can be calculated. Thecommutation process is adjusted. The control flow continues to the running state. The BLDC motor is thenrunning with regular feedback and the speed controller can be used to control the motor speed bychanging the PWM duty cycle value.For starting, there are three possible zero crossing detection scenarios:• Normal operation — Zero crossing is detected between two commutation period. This is the idealoperation.• No zero crossing detected — No zero crossing is detected between two commutation periods.• Zero crossing missed — In the application, after every commutation period, zero crossing detectionis disabled for a time Toff, which is proportional to the commutation period. After time Toff hasexpired, the application checks the polarity of the back-EMF signal. If the polarity is not asexpected, it means that the back-EMF shape had a zero crossing when the application was waitingduring time Toff. In this case, the application decides that a zero crossing was missed.3-Phase BLDC Motor Sensorless Control using MC56F8013, Rev. 0Freescale Semiconductor 35