MS430F47187 / SD16_A: Phase Angle Calibration by using Conversion Delay

BasePointer,

The setup you posted is correct, as is the way you use the preload-registers. Changing the preload registers will compensate for any phase-shift introduced by the sensors or the system itself in the current path and it actually does not matter, if you change the preload values for either the current or the voltage conversions, as long as you do not change both. In your specific case, you change the preload register for the I1 (current 1) path, so that it samples 8 (modulator-)cycles earlier than the voltage path, compensating for an inductive phase-shift. This phase shift is most visible at 60 deg (power factor of 0.5) as this is the way the math works. Let's say, your CT introduces a shift of 4 deg (inductive), the difference between cos(0deg) and cos(4deg) would be 0.0025, while at 60 deg, the difference between cos(60deg) and cos(64deg), which still has a four deg phase shift added due to the CT, would be 0.0616, i.e. 25 times higher.

This is the reason, why phase compensation needs to be done with a PF of 0.5.

There is an application report from TI describing this in more detail than I can here: http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slaa409a . Here you can also find the formula to calculate how big the preload values needs to be for any given phase shift.

Hope this helps,

Richard

Hi Richard,

Thanks for the reply. I understood the calculations. I have another question. In my configuration, the master channel is channel 5. Let’s think on the worst case which is master channel preload value is 0 and all others preload values are near to maximum value. The question is, when I receive SD16A interrupt, all other channels except master will still be in conversion state due to delay. Is it safe to read all channels value in sd16a interrupt when the master channel conversion is done? If not safe, what is the proper way to handle this condition?

    SD16PRE0 = 128; // Voltage Ch R

    SD16PRE1 = 128; // Voltage Ch S

    SD16PRE2 = 128; // Voltage Ch T

    SD16PRE3 = 255; // Current Ch R

    SD16PRE4 = 255; // Current Ch S

    SD16PRE5 = 0;   // Current Ch T – master

#pragma vector=SD16A_VECTOR

__interrupt void SD16AISR(void)

{

    Sample.Ch0 = SD16MEM0;           // Save CH0 results (clears IFG)

    Sample.Ch1 = SD16MEM1;           // Save CH1 results (clears IFG)

    Sample.Ch2 = SD16MEM2;           // Save CH2 results (clears IFG)

    Sample.Ch3 = SD16MEM3;           // Save CH3 results (clears IFG)

    Sample.Ch4 = SD16MEM4;           // Save CH4 results (clears IFG)

    Sample.Ch5 = SD16MEM5;           // Save CH5 results (clears IFG)

    SAMPLE_READY = true;

}

Thank you,

BP.