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MSP430F67641: The error of large current

beixian du
Prodigy 40 points
Part Number: MSP430F67641
Other Parts Discussed in Thread: EVM430-F67641, MSP430F449,
Hello:
     I have a question and need some help about the chip of msp430f67641,   The meter specification is 10(100)A,    const is 500 k
     
     When the meter power on with large current (USE  60A  TEST),   the meter start with a big error show about -11.234%,  and  later change to -0.197%,   and last change to 0.083%(Stable) (watch big current.mp4)
     When the meter power on with small current (USE 10A TEST),  the meter runs Stable with error 0.08 % ~  0.09%。  (small current.mp4)
    I have test many meters use Metering chip, and not no problem of large current start。   So  how can I solve the large current  start error of 67641 ?   Thanks!
  • 0
    TI__Mastermind 49692 points
    Hello,

    Are you using the EVM430-F67641 or a custom board for your tests? From my experience, it takes a few reports before the results stabilize (most likely due to the filters, buffers, etc. used by the Energy library software). Looking at your video, I would be concerned if the error never stabilized and was stuck at 11%. In this case, I would recommend that you re-calibrate the system, but since it's stabilizing by the 2nd or 3rd report, I wouldn't be concerned with this behavior.

    Regards,

    James
  • 0
    TI__Mastermind 49692 points

    beixian du said:
    const is 500 k

    As a follow-up, when you say "const", are you referring to impulses per KWH? If you're using 500k, then this is most likely the issue. Typically, the legacy Energy library uses 6400k as the default value which is much less than 500k. Also, the Energy library leverages the ADC samples to trigger the energy pulses.

    At 220V, 60A, 500k impulses/KWH, and 4096 ADC samples/second, you'll have approximately 2.234 ADC samples/impulse. This means that if there's a 50% duty cycle, you'll need the one ADC sample to turn on the pulse and the other ADC sample to turn off the pulse. However, for these inputs, you'll need to turn the pulse back on at the very next ADC sample. Thus, you're at 2 ADC samples/impulse and not 2.234. I'm not sure if it's coincidental or not, but 0.234/2 is approximately 11.7% error, which is similar to what you're observing.

    My suggestion would be to change 500k to 6400.

    Regards,

    James

  • 0 in reply to James Evans
    Prodigy 40 points
    I use emeter-metrology-67641.ewp and emeter-toolkit-67641.ewp which download in www.ti.com
  • 0 in reply to James Evans
    Prodigy 40 points
    Sorry about that, I make a mistake by edit this question, the impulses per KWH is 500, not 500k , so the result may be change to 1.8333, does it have any problem?
  • 0 in reply to beixian du
    TI__Mastermind 49692 points

    Hello,

    Have you read through the Implementation of a Low-Cost Three-Phase Electronic Watt-Hour Meter Using the MSP430F67641 user's guide? Section 4.3.2 LED Pulse Generation (per_sample_energy_pulse_processing) specifically mentions that the impulses per KWH is 1600. Our other F6779A and F6736A designs use 6400, but this specific design uses 1600. Your high voltage test equipment should also be set to 1600.

    Regards,

    James

  • 0 in reply to James Evans
    Prodigy 30 points
    Yes, I just change 3 value
    ACTIVE_ENERGY_PULSES_PER_KW_HOUR 500
    TOTAL_ACTIVE_ENERGY_PULSES_PER_KW_HOUR 500
    TOTAL_REACTIVE_ENERGY_PULSES_PER_KVAR_HOUR 500
    which define in Metrology-template.h.

    The only problem is the power off change to on for a little big current, the first, second value of error not right, and later will good all the time , so I thing that is not the problem of set the value 500, if set 1600 or 6400, I think it will have the save problem too。
  • 0 in reply to user4197873
    TI__Mastermind 49692 points
    Fewer pulses means less resolution, which means less reaction to changes (such as startup). Try using the default software and 1600.

    Regards,

    James
  • 0 in reply to James Evans
    Prodigy 30 points

    I have try , but also have the same result of the problem

  • 0 in reply to user4197873
    TI__Mastermind 49692 points

    If you've changed the constant to 1600 on your equipment (to match the default pulse rate in the EVM430-F67641's code) and you're still seeing around 10% accuracy on the first report, I don't consider this to be an issue. As you've observed, after the first three reports, the accuracy stabilizes around 0.1%. This is completely normal based on my experience due to filters, buffers, etc. settling in the code. Is this behavior violating any of your specifications or is it just different than what you're accustomed to seeing?

    Regards,

    James

  • 0 in reply to James Evans
    Prodigy 40 points
    Because other meters in our company do not have this problem, the first report shows very good. So the customer said there was a problem with the meter error
  • 0 in reply to beixian du
    TI__Mastermind 49692 points

    beixian du said:
    Because other meters in our company do not have this problem

    Do the "other meters" use the MSP430F67641? Please give us more details. From my side, I don't view this as an issue.

    Regards,

    James

  • 0 in reply to James Evans
    Prodigy 30 points
    No, We do not use MSP430F67641 before, we use MCU(msp430f449) + Metering chip (ATT7022)
  • 0 in reply to user4197873
    TI__Mastermind 49692 points

    user4197873 said:
    No, We do not use MSP430F67641 before, we use MCU(msp430f449) + Metering chip (ATT7022)

    Okay, this context makes sense. Thanks for sharing these details. As I mentioned before, this behavior on MSP430F67641 is normal and is due to the filters, buffers, etc. getting initialized during the first report. The software was probably done differently on the MSP430F449. If you're going to use MSP430F67641 in your design, I would recommend using the A version, MSP430F67641A.

    Regards,

    James

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