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How to determine supply current from single motor shunt current measurments

tom prohaszka
Expert 1240 points

I have a 3 phase motor with a current sum shunt, as well as shunts on A and B phases of the motor.  Since this is a PMSM sensorless control, the SVM continuously changes the PWM.   The resulting "Sum" current is a small slice out of the entire pwm period (20KHz).  I believe I can get the measurement with the ADC.   The problem I have is how to translate this slice of current into an equivalent supply current.    I did something similar on a BLDC sensored motor, and it was dependent on the duty cycle at the time of measurement.  The problem with PMSM is that the duty cycle is all over the place.

Any ideas or alternatives that can be suggested?

  • 0
    Genius 5170 points

    I believe TI example systems use two shunts to obtain three phase current information ... if you have two shunts then is it not possible to apply the same technique for your application? Becuase of two shunts some approximation will be made to obtain all three phase current infomration but depending on application the accuracy should be good enough for many applications. If possible post a block diagram of your system showing the current you need to measure ...

  • 0 in reply to Arefeen
    Expert 1240 points

    Thanks for the comment, but the question is related to finding the RMS supply current based on a measured ATD value from a sum current shunt, not the 2 shunts on the motor legs.  The current measurment relation is not a 1 to 1. 

     

  • 0 in reply to tom prohaszka
    Genius 5170 points

    Sorry .. I may have totally missed your question ... where is your total sum current shunt located ? I guess I am not sure about supply current either...