Some (Brushless DC) BLDC motors for sale are equipped with three internal hall-effect sensors that provide feedback to external circuits that facilitate precise control of the magnetic coils in the stator. Some types of BLDC controllers use the motor’s intrinsic Back EMF leaving the hall-effect sensors unused. In either case, the hall sensors can also be used for accurate position sensing.
BLDC Anatomy
The BLDC hub motor used in this experiment utilizes 27 electro-magnetic stator coils and 30 permanent magnets (also referred to as 15 pole pairs) (Figure 2). Many diagrams show the Hall effect sensors labeled as U, V, and W spaced equidistant (120 degrees) around the stator coils. Sensors are located equidistant from each other, but most are located on one side of the stator (Figure 3).
Note: The sensor labels (U, V, W) are assigned based on internal wire color code. For this experiment, sensor labeling is arbitrary.
The magic of 3 in BLDCs
As seen in Figure 3, the Hall sensors are centered in the coil faces. The center-to-center span between any two sensors is three coils, which results in 40 degrees of separation.
2 full coils + 2 half coils = 3 coil span
360 degrees / 27 coils * 3 coil span = 40 degrees
This configuration yields the same output values as if the sensors were physically 120 degrees apart. One third of the magnets will pass by each of the sensors resulting in 10 pulses from each sensor. Together, the sensors will deliver 30 pulses per 120 degrees or 90 pulses in one complete revolution.
9/27 (Coils) = 10/30 (Magnets) = 120/360 (degrees) = 30/90 (pulses) = 1/3 (of one rotation). Neat!
Summary
No matter which single sensor output square wave is examined following a transition, one of the remaining sensors is trailing while the other is leading (one is high while the other is low). It is for this reason that it does not matter which arrangement of sensor outputs you use when reading values. The only effected calculation is direction of rotation.
The animated illustration shows the sensor output at each transition and the relationship between the ten permanent magnets and the three sensored coils. Non-sensored, intermediate coils are omitted for visual clarity.