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Back Emf of BLDC motors (trapezoidal or sinusoidal)

Rushi Jariwala
Expert 1225 points
Other Parts Discussed in Thread: DRV8301

Hi,

While working on DRV8301 kit, I checked the back emf waveform of my motor. 

And it put me into a lot of confusion. 

Setup: Rotated the motor externally to measure back emf.

This is the back emf waveform when measured between Line to Line (see channel 1 - yellow signal)

And this is the back emf waveform when measured from Line to Neutral. (created a neutral point by a star connected resistor network at the motor terminal - 100 ohms resistor) (see channel 1 - the yellow signal)

Why is the shape different?

My questions:

1. When we say a BLDC motor has trapezoidal or sinusoidal back emf waveform - how are they referenced. Phase to neutral or phase to phase. In my case, one looks trapezoidal in nature and other sinusoidal.

2. What impact does the shape of back emf have on the system while using FOC control? 

3. The back emf waveform that you see, are they good or bad? Does the motor need improvement? How to judge? 

  • 0
    TI__Guru 63190 points
    Rushi,

    The magnitude and zero-crossing is the most important factors of the waveform for FOC, I cannot say whether a waveform is "right" or "wrong". Is the motor working correctly?

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    Expert 1225 points

    Hi Adam

    I have not run the motor yet. I want to set the encoder offset first.

    1. I understand that a waveform may not be right or wrong - but I have 2 motors, both have similar specs in terms of power and torque capabilities. The only difference is in the back emf waveform. Not how do you compare the motors?

    2. In my case, why is phase to neutral back emf not equal to phase to phase back emf? Is it because the waveforms are not sinusoidal?

    3. Which type of motor would you call it based on back-emf- trapezoidal or sinusoidal?

      

  • 0 in reply to Rushi Jariwala
    TI__Guru 63190 points
    Hope you had a good holiday, Did this issue get resolved? Could you update me on the latest?

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    Expert 1225 points
    Hi Adam,

    My previous questions have not been answered yet.
  • 0
    Expert 1100 points
    Rushi,

    1) As far as I know (it could be wrong), the word 'BLDC' commonly refers to brushless motor with trapezoidal BEMF. Brushless motor with sinusoidal BEMF are called PMSM or BLAC.

    Look at page 125 of this document for more information about BEMF voltages.
    www.ti.com/.../spru485a.pdf

    2) I personally try to apply a voltage to the motor phases, that has a shape which is similar to my motor BEMF. This will help to decrease current fluctuation on the motor or in other words it decreases the torque ripple. But you can also apply FOC to a BLDC motor or a trapezoidal voltage(six step commutation to a PMSM motor. In both cases motor will turn (of course if you apply it right), but the torque ripple will be higher than other method.

    3) If you try to have a motor with perfect trapezoidal shape, try to plot the BEMF vs electrical motor angle on a polar coordinate system. If you have a perfect trapezoidal motor you will see a hexagon. But I am not sure that your aim is that or not. Does the motor need improvement? It totally depends your application and your requirements. I think there is no one answer this question except from you.

    Regards,
    Hakan
  • 0 in reply to Hakan B
    TI__Guru 63190 points
    Hakan,

    Thanks for your reply!

    Rushi,

    Please let me know if this is clear now.

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    Expert 1225 points

    Yes. I have got my answers to some extent.