power factor

In an AC circuit, the voltage and current waves can fall out of step with each other. Power factor quantifies this misalignment. It is the cosine of the phase angle between voltage and current, expressed as a decimal between 0 and 1, or sometimes as a percentage. A power factor of 1.0 (or 100%) means voltage and current are perfectly synchronized; every ampere flowing through the circuit contributes to real, usable power. A power factor of 0.8 means only 80% of the current you are drawing actually performs work; the rest is reactive current that sloshes back and forth without delivering energy.

The distinction matters because utilities bill you for apparent power (measured in volt-amperes), but only real power (measured in watts) does useful work. If your plant runs motors, transformers, and fluorescent ballasts, your load is inductive, pushing current ahead of voltage. Current leads voltage in capacitive loads. In both cases, the phase angle widens, power factor drops, and your meter reads higher apparent power than the actual work being done. A manufacturing facility pulling 100 amps at 0.75 power factor is drawing the same apparent power as one pulling 75 amps at unity factor, yet producing the same output.

Induction motors are the primary culprit in industrial settings. Their magnetizing current creates a reactive component that lags the voltage by 20 to 40 degrees under normal load, typical power factors ranging from 0.85 to 0.90. Transformers contribute additional reactance. Many utilities impose penalties on customers whose power factors fall below 0.90, or even 0.95, because low power factor consumes extra capacity in transmission and distribution lines. Some sites install banks of capacitors at key points in the electrical distribution to offset the inductive lag, raising power factor toward unity and reducing both utility charges and system losses.

Measurement and correction

Power factor is measured with a power analyzer that samples voltage and current waveforms simultaneously and computes their phase relationship. Mechanical power factor meters, based on moving-iron or electrodynamic principles, are less common now; digital meters provide direct readouts and trend logging. The sign of the phase angle matters: inductive loads show lagging power factor; capacitive loads show leading power factor. Utilities typically care only about lagging power factor in industrial customers.

Raising power factor saves money and reduces heating in conductors. Shunt capacitors are installed in parallel with the load and tuned to cancel the inductive reactance of motors and transformers. Series capacitors are rare in industrial AC systems due to resonance risks. Active power factor correction, using solid-state converters to inject harmonic current that cancels the reactive component, is gaining ground in facilities with variable or nonlinear loads such as variable frequency drives and switch-mode power supplies, which themselves distort power factor.