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Calculating current consumption

Patrick Schneider1
Intellectual 655 points

Hello Chris,

we got a problem with an answer you provided me some time ago.

In this thread you confirmed the overall current consumption to be the Is_A value 

https://e2e.ti.com/support/microcontrollers/c2000/f/902/t/344125  

Perhaps it was a misunderstanding but the Is_A value is simply not representing the real overall current consumption, as you are stating yourself in this thread:

https://e2e.ti.com/support/microcontrollers/c2000/f/902/t/423108   

Unfortunately neither the calculation provided in this thread is giving us proper results.

So the question arises, is it possible to calculate the overall current consumption? If so could you please provide us with a working formula?

Regards,

Patrick

  • 0
    TI__Guru** 119340 points
    Patrick,
    yes, my first post is not clear, my apologies.

    please review these
    e2e.ti.com/.../1660335
    e2e.ti.com/.../1676944

    you may need to add a sum current bus measurement to achieve what you really want
  • 0 in reply to ChrisClearman
    Intellectual 655 points

    Chris,

    that is most unfortunate, because changing the layout is costly at this point of the project.

    So what - in your opinion - seems to be the most accurate calculated aproximation of the total sum bus current?

    Regards,

    Patrick

  • 0 in reply to Patrick Schneider1
    TI__Expert 4760 points

    Hello Patrick,

    I went back to view your original post, and I see that  you asked for current consumption of the motor:

    Is it somehow possible to get the overall current consumption of the motor out of the delivered values in gMotorVars?

    The equation that Chris provided (sqrt(Id^2 = Iq^2) ) will yield correct results for motor current.  In other words, if you look at the current of any motor phase with a current probe, the calculation will correspond to the PEAK of the current waveform.

    But from this post, it now it appears you are asking for Bus current.  Just to be clear, Bus current is NOT EQUAL to motor current.

    Could you please confirm which current you are looking for?

    Thanks,

    Dave

     

  • 0 in reply to Dave (Wisconsin) Wilson
    Intellectual 655 points
    Hello Dave,
    yes, as I said it was probably a misunderstanding. English is not my native language so I probably misexpressed myself on that one.

    I am looking for the overall current consumption which has to be provided by my power source (neglecting minor stuff on the circuit board), is that what you mean with bus current?

    So am I correct that the motor current is in fact higher than the current from my power source because as Chris said some of the motor current is recirculating through other phases?

    Because as of now for example the (sqrt(Id^2 = Iq^2) ) provides 0,8A at 300rpm and we are measuring 0,2A (which is the value we would need) output from the battery.

    I hope I made myself a bit clearer and appreciate the help!

    Regards,
    Patrick
  • 0 in reply to Patrick Schneider1
    TI__Expert 4760 points

    Hello Patrick,

    If you don't have a buss current sensor in your design, I believe there are two alternatives to consider:

     

    1.  Assuming you already have three shunts in your design, you could add the shunt voltages together and average the results.  Unfortunately, this would have to be done using analog circuitry.  You can't simply add the sampled shunt readings together, since the addition of the shunt samples will always be zero.

     

    2.  This technique can be done completely in software.  It is based on the "conservation of energy" principle.  In other words,

     

     motor power + inverter losses = buss power.

     

    Since InstaSPIN uses the amplitude invariant form of the Clarke Transform, we can calculate motor power (in watts) using the following formulae:

     

    P_motor = (Vd * Id + Vq * Iq) * 1.5

     

    Let's make an assumption that the inverter losses (including shunt i^2R losses) are somewhere between 2% and 5%.  Then we can calculate buss power using the following formulae:

     

    P_buss = P_Motor * 1.04 (assuming 4% inverter losses)

     

    Now that we have buss power, and since we are also measuring Vbuss, we can calculate average buss current:

     

    I_buss(avg) = P_buss / V_buss

     

    Best Regards,

    Dave

     

     

  • 0 in reply to Dave (Wisconsin) Wilson
    Intellectual 655 points

    Hello Dave,

    thank you very much for your answer. I will try your second suggestion as soon as I'm getting access to our test rig.

    Just one additional question because I'm not that much into your pu calculated values.

    Do I have to add the Voltage and Current Scaling factors somehow or can I calculate with the raw values in the data fields?

    Regards,

    Patrick

  • 0 in reply to Dave (Wisconsin) Wilson
    Prodigy 105 points

    hello Dave,

    would you please explain the first solution to get current buss more clearly?

    i have used OP-AMP of TI sch to add 3-phase voltage,but the out of OP-AMP is almost zero.

    what is the correct way to add 3-phase voltage?

    op-amp circuit and it's output diagram attached:

  • 0 in reply to even mi
    TI__Genius 17175 points
    Even, please make a new thread for any questions you may have, as we prefer not to work on threads that are this old.

    Thank you
    Sean