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MSP430F6779A: Calibration; Active Power scaling factor for 1PF is different compared to a 0.5C PF

Jack Swale86
Intellectual 810 points
Part Number: MSP430F6779A


I am using the MSP430F779A and the 'emeter' sample code (although mapped to my hardware configuration).

I am using the GUI to calibrate the meter. I have a set up of;

Voltage = 246V
Current = 24A
PF = 1
Apparent Power = 5.91kW
Active Power = 5.91kW
Reactive = 0kW

I have then calibrated against a reference meter and Active Power scaling factor came out at 2207 and the Phase scaling factor 397.

I then checked the values against a 6uH + 1K load and noticed that I would have to change the Active Power scaling factor to 4553 and change the phase to 253.

So my question is, what should I do to make sure the values are always accurate?

  • 0
    TI__Mastermind 49692 points

    Hi Jack,

    First, I'd recommend carefully following the calibration instructions in Section 6.3.3 in the Implementation of a Three-Phase Electronic Watt-Hour Meter Using the MSP430F677x(A) app note.

    Typically, you only need to calibrate the meter once. After following the calibration instructions, you'll want to capture the final calibration factors. Let's assume that you recorded these values and then reprogrammed the board, which should reset the calibration values to the default values. In the GUI, you can re-enter the recorded calibration factors, and the power values (e.g. V, I, PF, active power, etc.) should be nearly exactly the same as when you finished calibration earlier. When you're wanting to use the meter AFTER calibration has been completed, don't worry about the scaling factors. The power values are what's important.

    Also, your first load appears to be purely resistive (no reactive power), but your second load has some inductance and resistance that will introduce some reactive power. This would be why the scaling factors change, since the phase between the voltage and current changes and the active power is reduced.

    Hopefully this makes sense.

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    Intellectual 810 points

    Hi James,

    Thanks for the reply.

    Just so we are clear, I have followed the instructions with a 1K load, 246V, 24A and 0% phase shift. Then I have got it so that the Power values are correct by changing the error %. by entering them into the 'Manual Cal' section.

    Then if I changed the set up to show 1K + 3H load, the power factor, active power and reactive power are all wrong on the GUI compared to a reference meter that is hooked up to the same set up.

    Kind Regards,

    Jack

  • 0 in reply to Jack Swale86
    TI__Mastermind 49692 points
    Hi Jack,

    After the voltage and current gain calibration steps, did you go through the active power gain calibration step and then the phase correction step? When you introduce a reactive load, the results won't be right if the EVM hasn't been properly calibrated. Does your source allow you to specify a phase shift between the voltage and the current? If not, you'll need one that can do this.

    Regards,

    James

    MSP Customer Applications
  • 0 in reply to James Evans
    Intellectual 810 points

    Hi James,

    I have a set up that can incorporate phase shift.

    I do the following;

    1. Voltage and Current Calibration using the % error method.

    2. Active power Calibration using the % error method.

    Now I change the setup and look to see if it is correct.... a purely resistive load is correct, a purely capacitive load is correct.

    I then change to a inductive load... Voltage and Current are correct, active power is correct. However, Reactive power is too low and therefore power factor is too high by roughly 0.15.

    It is as if I need to calibrate Reactive Power too but can't.?

    Thanks,

    Jack

  • 0 in reply to Jack Swale86
    TI__Mastermind 49692 points

    Hi Jack,

    Thanks for the additional details. I see that you're performing voltage/current calibration and then active power calibration, but I don't see that you're performing the phase correction calibration in Section 6.3.3.2 described in the SLAA577G app note. This would explain why you're seeing wrong results for an inductive (reactive) load.

    Regards,

    James

    MSP Customer Applications

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