BLDC speed and torque control

Discussion in 'The Projects Forum' started by vinodkaruvat, May 23, 2014.

  1. vinodkaruvat

    Thread Starter New Member

    Apr 3, 2012

    I have a doubt understanding the exact commutation of the BLDC motors. My motor has 7 pole pairs. Now assuming the 3 phases are - U,V and W ( so thats U+,U-,V+,V-,W+ and W- from the MOSFETS).
    Now, the BEMF of the floating phase , is the indicator to be measured. Now, the BLDC motor property states - 1000RP/V. So, as I increase the voltage the rotation/speed increases.This voltage is increased via the PWM duty cycle. Now, if the PWM duty cycle is increase , we increase also the current through the motor, thereby increasing the thrust.
    So, if I want the speed to remain at , say 3000rpm, and increase the thrust from , say 400g to 500g, I increase the current through the motor.
    I do this by increasing the PWM duty cycle. But, this means the speed will also increase from 3000rpm to a higher value,right. How can this be maintained then

    Next, i was moving my BLDC in a stepper motor like action. The disadvantages being that, it looses thrust as I am not firing my MOSFETs at the right interval. Now, I was studying the BEMF waveform for this stepper motor action in the BLDC. The BEMF is measured in the floating phase of the motor. For e.g - say U and V are supplied and W is floating. The duty cycle is 40%. I observed the following points in this stepper motor action -

    1) The BEMF (at the bottom part of the PWM) on the floating phase shows a distinct sine wave action.

    2) This sine wave keeps repeating (almost 3 complete cycles) with 3 peaks and 3 valleys.

    3) The speed is directly proportional to the amplitude of the BEMF. So , from point 2 I understand that the speed keeps rising and falling.

    4) After a series of rises and falls the control goes into the next state.

    My doubts -

    1) in the 1st cycle, when the speed is max (highest amplitude of BEMF), will commutation to next state ensure no loss in speed. I think that wld be fine.

    2) Regarding thrust. I need the thrust to be constant. Would proper commutation as stated in point 1 above ensure smooth thrust.

    3) So for greater thrust , I increase the PWM duty cycle. This will increase the speed and hence give me a higher amplitude BEMF. In my code via the ADC, I measure this amplitude and then commutate to next state at the highest ADC value. This will increase speed as the slope and amplitude will be greater with increased PWM duty cycle.

    Have I understood the points correctly and am I on the right track please.
    Kindly advice me pls.

  2. MaxHeadRoom


    Jul 18, 2013
    Are you trying to commutate as a 3 ph motor or a BLDC?
    Both are the same in construction but different in commutation, the AC is supplied a 3ph sinusoidal waveform the BLDC has only two windings energised at any one time, and is commutated by three sensors.
    They are both synchronous motors.
    There is much info out there on sites like picmicro and the semi manuf. on commutation methods.
    That is a high (14) pole count for a AC or BLDC motor
  3. vinodkaruvat

    Thread Starter New Member

    Apr 3, 2012
    Hello Sir,

    It's BLDC. It is sensorless.

  4. Experimentonomen


    Feb 16, 2011
    A sine wave with thee times the commutation frequency i'd say is the third harmonic.

    Is this a commercial or homemade bldc drive ?
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    To simplify this right down, the type of motor or PWM etc really don't matter.

    The speed will be maintained stable only when the motor torque equals the load torque.

    IF you increase motor torque from 400g to 500g, the motor will speed up UNLESS you also increase load by exactly the same amount.

    So you are kindof asking a silly question. :)
    vinodkaruvat likes this.
  6. vinodkaruvat

    Thread Starter New Member

    Apr 3, 2012
    thank u for ur responses.