Three Phase BLDC and Hall Effect Sensor Questions

Thread Starter

Jin Ming

Joined Oct 26, 2015
Hi all,

I have recently acquired a BLDC that has no datasheet or any in-depth wiring diagram.

I am new to the field of motor and motor control and I am trying to build a controller for this motor. So, I have some questions,

1) How do I determine the correct commutation sequence to the 3 phases based on the 3 hall effect sensor which has no label or their known location in the stator without guess work? (Meaning, I'd like to know which wires corresponds to Hall A, B and C so that my microcontroller can generate the commutation correctly)

2) Can a motor have odd-numbered poles? I know the teeth on the stator does not really imply how many poles it has and from my classes, motors seemed to have even-numbered pole pairs. This image is taken from the web, and can someone guide me on how to determine the number of poles of this motor.


3) How do I go about measuring the Back-EMF of each phase of a BLDC and maybe displaying it on an oscilloscope? (I'd like to see the trapezoidal BEMF of the motor on a oscilloscope for my understanding). Do I connect the probe (ie. Channel 1) to Phase A, and the ground to Phase B?

Many thanks.


Joined Jul 18, 2013
Even number of poles only on a BLDC motor.
Short all 3 stator connections, spin by hand and count number of 'bumps'/rev for pole count.
Here is the commutation using Hall effect equivalent tracks, shows the back fed wave form from the stator windings, sinusoidal not trapezoidal.
There is also a method that initially exercises the motor to acquire the relative pole positions and from then on use an encoder to track pole position.
BTW, you connect a 10k resistor to each stator lead and connect all 3 resistors together to provide a virtual Zero reference for the 'scope gnd lead.


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