Hall Effect Sensors in BLDC Motors

A digital Hall effect sensor (also called buffered Hall effect sensor) detects the strength of the currently present magnetic field, and has a single output wire with two possible output states: High and Low. BLDC motors are typically equipped with Hall effect sensors to provide information about the orientation of the rotor relative to the stator. When the rotor and its permanent magnets move, the magnetic flux detected by the Hall sensor changes. This can be used for commutation by a motor controller, as it can tell the controller when to activate which coil in the motor in order for it to turn.

For a simple BLDC motor with one Hall sensor, the frequency of the sensor signal will be twice the rotation frequency of the motor, toggling the level four times per motor rotation. This is illustrated in the figure below.

Figure 1. Fan Rotor and Hall Sensor Signal