PFC

Power factor correction is a set of techniques that reduce the reactive power drawn by an AC circuit, bringing the phase angle between voltage and current closer to zero. In practical terms, it makes your electrical load more efficient by eliminating the penalty charge utilities impose on customers with poor power factors, and it reduces the heating losses in distribution infrastructure. A load with a power factor of 0.7 lagging requires substantially more current to deliver the same real power as a load with a power factor of 0.95, and that excess current wastes energy as heat in cables and transformers.

The need for PFC arises because inductive loads, particularly electric motors, fluorescent ballasts, and switched-mode power supplies, draw current that lags the applied voltage. This reactive current does no useful work but still flows through utility lines and transformers. Utilities measure power factor and charge industrial customers a penalty, typically in the form of a multiplier on their bill, when the power factor falls below a threshold like 0.9 or 0.95. Residential customers rarely face these charges because their overall reactive load is smaller, but the waste still occurs.

Hardware approaches to correction

The traditional method is to install shunt capacitors in parallel with the load. A 50 Hz or 60 Hz capacitor draws leading current that cancels some or all of the lagging current from an inductor, reducing the net reactive power. For fixed loads this works reliably; for variable loads, automatic switchable capacitor banks or continuously adjustable static var compensators (SVCs) are used. Modern switched-mode power supplies often include active PFC circuits, which use a boost converter topology to draw nearly sinusoidal current from the mains in phase with the voltage. These are particularly common in computer equipment, LED drivers, and industrial power supplies rated above a few hundred watts.

Active PFC stages typically operate at switching frequencies between 50 kHz and 200 kHz and can achieve power factors above 0.98 even with wide mains voltage variations. Passive PFC using a simple LC filter is cheaper but less effective and occupies more space. A third approach, soft-start devices and phase-angle controllers, reduces inrush current and can improve power factor modestly as a side effect, though they are primarily used for other reasons.

The nameplate on industrial equipment often specifies the power factor, and facility managers use power factor meters to determine whether correction is cost-effective. Installing PFC can reduce monthly utility charges by 5 to 15 percent depending on the baseline power factor and the facility's load profile, with payback periods typically between two and five years for industrial installations.