Glossary · Energy and utilities automation
Power factor
Also known as: PF, Cos phi
German: Leistungsfaktor
In electrical engineering, the power factor is the ratio of active power to apparent power in an AC circuit. For sinusoidal voltage and current it equals the cosine of the phase angle between them (cos φ); with harmonics, it is lower than cos φ.
- Energy
In one sentence
The power factor is the ratio of active to apparent power in an AC circuit; a low value means more current for the same useful power.
Example
Before correction, the plant's power factor is 0.78 because of many induction motors; capacitor banks raise it to 0.95.
How it applies
- Engineering: A low power factor increases current, which requires larger cables and transformers and causes higher losses. Motors at part load and some electronic loads are typical causes.
- Operation: Utilities may charge for reactive energy when the power factor falls below a contractual limit.
- Monitoring: Power meters display the power factor per feeder, which helps locate causes and size correction equipment.
- Documentation: Electrical data in machine documentation should state the power factor at rated operation where relevant, and whether the drive technology influences it. Terms should distinguish displacement power factor (cos φ) from the total power factor.
Power factor vs. cos φ
Cos φ, the displacement power factor, considers only the phase shift of the fundamental. The total power factor also includes the effect of harmonic currents, for example from rectifiers and frequency converters. The two are equal only for purely sinusoidal quantities.