POWER FACTOR CORRECTION (PFC) BASICS

WHAT's PFC?



In general, the power factor (PF) can vary between 0 and 1. The higher PF, the more effectively electricity is being used. Therefore power factor correction refers to any technique that increases PF with the goal being unity power factor. The two reasons of imperfect PF are current distortions and phase shift between voltage and current harmonics of the same frequency. Likewise, there are two main categories of PF correction techniques.


The harmonic distortions are caused by non-linear components, such as rectifier bridge in DC power supplies which is connected directly to a large energy storage capacitor. These distortions can be corrected on the power supply design stage by introducing various passive or active PFC circuits (see for example active PFC). The major source of the V-I phase shift is industrial induction motors that from the circuit standpoint present inductive loads. Their effect can be reduced by adding external PFC capacitors.

WHY IMPROVE YOUR POWER FACTOR?



There are several reasons for correcting PF depending on your application. We know that when PF<1, there are AC currents circulating in the line that do not transfer working power, but cause heat dissipation in the wiring, create extra load to the generators, and require larger electricity generating equipment. That's why electric utilities may charge large customers an additional fee when PF< 0.95, bill them for total kVA, or add surcharges for excess kVARs. So, for an industrial facility there may be energy cost savings benefits from PFC.

As for electronics, there are regulations such as EN61000-3-2, that limit harmonics that certain types of products (PCs, TV sets, etc) can inject into the mains. Even though there are no international standards that directly regulate PF, correcting it automatically lowers harmonic distortions. So, for PSU designers the main reason to use PFC is to meet a specific harmonic content requirement for their application, even though it may not provide any direct cost benefits neither to the manufacturer or the user.

Finally, in household applications low PF reduces outlets and circuit breaker capabilities. Other than that, contrary to common misconception, homeowners and consumers do not benefit from PFC (see Do I need power factor correction for more details).