Sensorless BLDC Motor Control

Sensorless BLDC Motor Control

 However, sensors cannot be used in applications where the rotor is in a closed housing and requires minimal electrical entries, such as a compressor or applications where the motor is immersed in a liquid. Therefore, the BLDC sensorless driver monitors the BEMF signals instead of the position detected by Hall sensors to commutate the signal. The relationship between the sensors’ output and the BEMF is shown in Figure 15. The sensor signal changes state when the voltage polarity of the BEMF crosses from positive to negative or from negative to positive. The BEMF zero-crossings provides precise position data for commutation. [6] However, as BEMF is proportional to the speed of rotation, this implies that the motor requires a minimum speed for precise feedback. So under very low speed conditions—such as start-up— additional detectors—such as open loop or BEMF amplifiers—are required to control the motor (This is beyond the scope of this application note). The sensorless commutation can simplify the motor structure and lower the motor cost. Applications in dusty or oily environments that require only occasional cleaning, or where the motor is generally inaccessible, benefit from sensorless communation.