Power Factor (PF)
Definition
Power factor (PF) is the ratio of real power, in watts, to apparent power, in volt-amperes, in an AC circuit. A PF of 1.0 is the ideal case. At a PF of 0.5 the circuit carries twice the current that the real power alone would require. In LED drivers and other switch-mode power supplies, a low power factor comes from harmonic distortion of the current as well as from any phase shift. The PF of an LED driver ranges from about 0.5 for simple designs to more than 0.9 for drivers with power factor correction. Circuits are therefore sized in volt-amperes (VA = watts / PF) rather than in watts alone.
In the field
A 600 W LED wash at a PF of 0.85 draws 706 VA, not 600. On a 230 V circuit this is 3.1 A rather than 2.6. On LED-heavy rigs, circuit loading is therefore calculated in volt-amperes, with the power factor taken from the fixture datasheet.
Sources
- IEC 60050-131 (Electropedia), IEV 131-11-46 « power factor » : rapport puissance active / puissance apparente · electropedia.org
- Vicor, What does power factor mean and why must we correct it? Facteur de déplacement et facteur de distorsion des alimentations à découpage · vicorpower.com
- STMicroelectronics, HVLED815PF : driver LED à facteur de puissance élevé (PF > 0,9) ou standard (PF > 0,5) selon le schéma d'application · st.com