Power factor measuring device using microcontroller for single-phase consumers

Abstract

The paper presents a device with microcontroller for measuring the power factor used for single-phase low voltage electrical consumers. Voltage and current are measured, and the paper presents two constructive variants of pulse circuits (TTL compatible) used to measure the phase-shift between current and voltage. In the first constructive variant, without galvanic separation, the voltage measurement is performed with a voltage divider connected in parallel with the consumer, and the current measurement is made by a shunt resistor connected in series with the consumer. In the second construction variant, with galvanic separation, the voltage is measured by a step-down voltage transformer, and the current is measured with a current transformer that has a load resistance. The signals coming from the voltage and current measuring equipment are limited by a group of diodes, then amplified-separated, and the resulting signals are connected to the input of a XOR TTL logic gate. The signal from the XOR TTL gate output has constant amplitude pulses and time durations proportional to the phase-shift value between current and voltage. The signal after the XOR TTL gate is applied to the digital input of a microcontroller development board that, also, has an LCD shield. The program used to calculate the power factor according to the pulse length (proportional with phase-shift value) is presented. Experiments are performed with RL consumers to measure the power factor and the obtained measurements are compared with those measured with a precision device (power quality analyser).