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Per Unit with Current sense offset

Vincent Abe
Prodigy 130 points

Dear Experts,

With reference to the TI Motor Control Primer http://processors.wiki.ti.com/index.php/TMS320C2000_Motor_Control_Primer

              

If an application current sensing circuit (http://processors.wiki.ti.com/index.php/TMS320C2000_Motor_Control_Primer#PU_System_Model_and_Base_Values) is biased such that it assigns 3.3 V to say +15 A, and 0 V to say -10 A,

 

Questions:

  1. Would this cause a problem in the number system of the code? If there would be, is there a work around in software?
  2. How would the pu system work? I would still expect my +1.0 pu to be +15 A since this gives maximum output for the ADC. Is my thinking right?
  3. How would 0 A be interpreted, seeing that the system is not symmetric?
  4. Does that mean that my pu output is [-0.67 pu, +1.0 pu]

 

Thanks for your help!

Vincent

  • 0
    TI__Guru** 119680 points

    I would suggest you bias your 0A at 3.3/2 = 1.65V for a symmetric system

    Why do you want to give yourself an artificial offset?  What does your gain circuit even look like to produce these values?

    This will just make the software much more difficult....don't do that to yourself.

    In your case use a full scale of 30A (+15A = 3.3V, -15A = 0V, 0A = 1.65V)

     

     

  • 0 in reply to ChrisClearman
    Prodigy 130 points

    Thanks Chris,

    I completely agree with you.

    Unfortunately the driver electronics have already been designed with the offset. Is there any work around in software (we are developing around the floating point code)?


    Cheers

  • 0 in reply to Vincent Abe
    TI__Mastermind 27240 points

    I have not come across an asymmetrical current sensing in motor drive electronics. I cant appreciate the need for a higher positive current sensing than negative.

    In your case, 0A should correspond to 1.32V instead of 1.65V. From an algorithm standpoint, read the ADC value corresponding to zero current, which will be your offset, and subtract that during current sensing.

    I would use 10A as 1pu because it will help to normalise the max current of both polarities to 1pu.

     

    -ramesh