Table of Contents
AC Power Fundamentals
In AC circuits, voltage and current waveforms may not be in phase due to reactive components. This phase difference means the simple product of voltage and current does not give actual power consumed. AC power has three components: real power doing useful work, reactive power oscillating between source and reactive components, and apparent power which the source must supply.
The power factor is the ratio of real power to apparent power and indicates how efficiently the electrical system delivers energy. A power factor of 1.0 means all power does useful work; lower values mean more current is needed for the same real power output.
Power Triangle
Where P is real power in watts, S is apparent power in VA, Q is reactive power in VAR, V is RMS voltage, I is RMS current, and PF is power factor.
Typical Power Factors
| Load Type | Power Factor | Nature |
|---|---|---|
| Resistive heater | 1.00 | Unity |
| LED driver | 0.90-0.99 | Varies |
| Induction motor (full load) | 0.80-0.90 | Lagging |
| Induction motor (no load) | 0.10-0.30 | Lagging |
| Fluorescent lighting | 0.50-0.95 | Lagging |
FAQ
Why does power factor matter?
Low power factor means the utility must supply more current for the same useful power. Industrial customers with low power factors pay penalty charges. Improving power factor with capacitor banks reduces costs and frees electrical capacity.
What is the difference between watts and VA?
Watts measure real power doing actual work. Volt-amperes measure apparent power including both useful and reactive components. UPS systems and generators are rated in VA because they must handle total current regardless of power factor.
How do I improve power factor?
The most common method is adding power factor correction capacitors in parallel with inductive loads. Modern variable frequency drives also improve power factor. Target above 0.95 to avoid utility penalties and reduce energy costs.