The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load, to the apparent power in the circuit. In a purely resistive AC circuit, voltage and current waveforms are in step (or in phase), changing polarity at the same instant in each cycle. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit.
In a simple alternating current (AC) circuit consisting of a source and a linear load, both the current and voltage are sinusoidal. If the load is purely resistive, the two quantities reverse their polarity at the same time. At every instant the product of voltage and current is positive, indicating that the direction of energy flow does not reverse. In this case, only real power is transferred.
If the loads are purely reactive, then the voltage and current are 90 degrees out of phase. For half of each cycle, the product of voltage and current is positive, but on the other half of the cycle, the product is negative, indicating that on average, exactly as much energy flows toward the load as flows back. There is no net energy flow over one cycle. In this case, only reactive energy flows—there is no net transfer of energy to the load. (Reactive power does not do any work, so it is represented as the imaginary axis in a vector diagram. Real power does do work, so it is the real axis.)
In the case of a perfectly sinusoidal waveform, P, Q and S can be expressed as vectors that form a vector triangle.
|S||The apparent power (W)|
|P||The active power (V*A)|
|Q||The reactive power (V*A)|