In a purely resistive AC circuit, voltage and current waveforms are in step, changing polarity at the same instant in each cycle. Where reactive loads are present, such as capacitors or inductors, energy storage in the loads result in a time difference between the current and voltage waveforms. Since this stored energy returns to the source and is not available to do work at the load, a circuit with a low power factor will have higher currents to transfer a given quantity of power than a circuit with a high power factor.
Real power is the capacity of the circuit for performing work in a particular time. Due to reactive elements of the load, the apparent power, which is the product of the voltage and current in the circuit, will be equal to or greater than the real power. The reactive power is a measure of the stored energy that is reflected to the source during each alternative current cycle.
AC power flow has the three components: real power (P), measured in watts (W); apparent power (S), measured in volt-amperes (VA); and reactive power (Q), measured in reactive volt-amperes (VAr).
The power factor can be expressed as:
In the case of a perfectly sinusoidal waveform, P, Q and S can be expressed as vectors that form a vector triangle such that:
If φ is the phase angle between the current and voltage, then the power factor is equal to , and:
By definition, the power factor is a dimensionless number between 0 and 1. When power factor is equal to 0, the energy flow is entirely reactive, and stored energy in the load returns to the source on each cycle. When the power factor is 1, all the energy supplied by the source is consumed by the load. Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle.
The power factor is determined by the type of loads connected to the power system. These can be:
- Resistive
- Inductive
- Capacitive