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SA2005FPA Datasheet(PDF) 6 Page - Sames |
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SA2005FPA Datasheet(HTML) 6 Page - Sames |
6 / 10 page sames sames SA2005F 6/10 http://www.sames.co.za TYPICAL APPLICATION In figure 5, the components required for the three phase power/energy metering section of a meter, is shown. The application uses current transformers for current sensing. The 4-wire meter section is capable of measuring 3x230V/80A with precision better than Class 1. The most important external components for the SA2005F integrated circuit are the current sense resistors, the voltage sense resistors as well as the bias setting resistor. BIAS RESISTOR R7 defines all on-chip and reference currents. With R7=24kW, optimum conditions are set. Device calibration is done on the voltage inputs of the device. CT TERMINATION RESISTOR The voltage drop across the CT termination resistor at rated current should be at least 20mV. The CT’s used have low phase shift and a ratio of 1:2500.The CT is terminated with a 2.7W resistor giving a voltage drop across the termination resistor 864mV at rated conditions (Imax for the meter). CURRENT SENSE RESISTORS The resistors R1 and R2 define the current level into the current sense inputs of phase one of the device. The resistor values are selected for an input current of 16µA on the current inputs at rated conditions. According to equation described in the Current Sense inputs section: R1 = R2 = (I / 16µA ) x R / 2 LSH = 80A /2500 / 16µA x 2.7W / 2 = 2.7kW I = Line current / CT Ratio L The three current channels are identical so R1 = R2 = R3 = R4 = R5 = R6. VOLTAGE DIVIDER The voltage divider is calculated for a voltage drop of 14V. Equations for the voltage divider in figure 5 are: RA = R16 + R19 + R22 RB = R8 || (R13+P1) Combining the two equations gives: ( RA + RB ) / 230V = RB / 14V A 5k trimpot is used in each of voltage channel for meter calibration. The center position of the pot is used in the calculations. P1 = 2.5k and values for resistors R13 = 22k and R8 =1M is chosen. Substituting the values result in: RB = 23.9k RA = RB x (230V / 14V - 1) RA = 368.9k. Resistor values of R16, R19 and R22 is chosen to be 120k, 120k and 130k. The three voltage channels are identical so R14= R15= R16 = R17 = R18 = R19 and R20 = R21= R22. |
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