JPS6443766A - Wattmeter - Google Patents

Abstract

PURPOSE:To make a two-wattmeter method accurate by digitizing two active power values measured from the detection outputs of sensors and the effective values of the respective detection outputs of the sensors and finding the sum of the apparent power of each phase of a three-phase cable run. CONSTITUTION:The line voltage and line current of the three-phase cable run are detected by the sensors 1 and 2 and inputted to multiplying circuits 3 and 4 to extract active power values W1 and W2, which are inputted to an A/D converter 7. Further, the sensor outputs are converted by effective value converting circuits 12 and 13 into effective values V12 and V32, and I1 and I3, which are inputted to the converter 7. Then, an arithmetic means 14 finds phase angles phi1 and phi2 from the effective values of the voltage and current and the effective power values W1 and W2 and an arithmetic means 15 calculates the phase angles theta1 and theta3 of the currents I1 and I3. Then, an angle thetaX containing an unmeasured current I2 is calculated 21 from the angles theta1 and theta3 and the current I2 is found. Then an arithmetic means 18 finds apparent electric power VA=V12(I1+I2+I3)/3<1/2>. Therefore, loads of the respective phases are unbalanced, the apparent electric power is measured with high accuracy.