Power factor and the power triangle
CERT 3 · Commercial & Industrial Systems · ~11 min
AC circuits with motors and transformers carry two kinds of power. Real power (kW) does the work. Reactive power (kVAR) shuttles back and forth building and collapsing magnetic fields, doing no work but occupying capacity. Their vector sum is apparent power (kVA) — the power triangle, where kVA is the hypotenuse.
Power factor is the ratio PF = kW ÷ kVA, or equivalently the cosine of the angle between voltage and current. A purely resistive load (heaters, incandescent lighting) has PF 1.0. Induction motors typically run 0.8 lagging, and worse when lightly loaded — an oversized motor idling is a power-factor disaster.
Low power factor costs real money and real capacity. A facility drawing 100 kW at PF 0.7 pulls 143 kVA of current-carrying capacity from conductors, transformers, and the utility; correct it to 0.95 and the same 100 kW needs only 105 kVA. Utilities bill demand penalties for poor PF, and capacitor banks are the standard fix because capacitors supply leading reactive power that cancels the motors' lagging draw.
Exam focus: PF = kW/kVA in both directions, recognize that motors lag, and know that capacitors are the correction device. Three-phase power questions (Unit 3.1) frequently hide a power factor inside them.
Quiz — 3 questions
1. A load draws 80 kW at 100 kVA. Its power factor is:
2. Capacitor banks correct power factor because capacitors:
3. An induction motor running lightly loaded typically has: