This paper describes the sensorless control and back electromotive force (BEMF) sampling methods for brushless DC motors, as well as the capability of ST7MC in implementing these topologies. The advantages and drawbacks of these topologies are also discussed.
This document explains the principles of sensorless drive for Permanent Magnet Brushless DC (BLDC) motors, focusing on the detection of Back Electro Magnetic Force (BEMF) to synchronize phase commutation without external sensors. It highlights the advantages of sensorless drives, such as lower cost and suitability for applications with closed housings or liquid immersion.
The note details four specific sampling methods implemented in the ST7MC microcontroller: sampling at the end of the PWM OFF state (the patented "3 resistor" method), sampling during the PWM ON state using the industry standard classical method, sampling during the PWM ON state using a digital filter, and a mixed sampling approach. It provides practical implementation details, component sizing, and practical results for each method.
The rotor must turn at a minimum speed to generate sufficient Back Electro Magnetic Force (BEMF) for detection.
The method requires only three resistors to limit input current in the I/O.
The maximum available duty cycle should be limited because the method cannot reach 100%.