A three-phase reactive current measurement technique suitable for automation

The author describes a technique for measuring the reactive current in three-phase circuits. The clock frequency for synchronizing and timing is produced from supply voltage signals. The line current is sensed with the help of Hall effect transducers. The analog value of the reactive current is measured at the zero crossing of the voltage wave using a sample-and-hole circuit. The signal is then converted into digital information at appropriate timings, using an A/D (analog-to-digital) converter and a latch circuit on every cycle, and a digital output is obtained. The technique has been implemented, and the typical waveforms from a laboratory prototype are presented. The various sources of error are identified, and the usefulness of the circuit in various applications is outlined