Three Phase BLDC and Hall Effect Sensor Questions

Discussion in 'The Projects Forum' started by Jin Ming, Mar 1, 2016.

  1. Jin Ming

    Thread Starter New Member

    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.
  2. MaxHeadRoom


    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.
    Last edited: Mar 1, 2016