Power Factor Correction Explained
Power factor is the ratio of useful power to apparent power, and a low power factor means you draw more current — and pay for more capacity — than the useful work requires. This guide explains what causes low power factor, how correction works, and the pitfalls that turn a simple capacitor bank into a resonance problem.
What power factor actually means
Useful power does work: it heats, turns motors and lights lamps. Reactive power shuttles back and forth between source and load without doing useful work, but it still flows as current, heating cables and loading transformers. Power factor is useful power divided by apparent power; a factory full of lightly loaded motors and transformers will show a low power factor, high current, and voltage drop, even when its useful load is modest.
How correction works, in plain terms
Capacitors supply reactive power locally, so the supply no longer has to carry it. That reduces current, frees transformer and cable capacity, and improves voltage — which is why utilities penalise low power factor and reward correction. Correction is usually staged in steps that switch in as load changes, because a fixed bank sized for peak load will overcorrect at light load, pushing voltage up and power factor leading.
Harmonics and the resonance trap
The pitfall is resonance: capacitors together with system inductance can amplify harmonic currents from drives and rectifiers, overheating capacitors and distorting voltage. Where harmonics are significant, detuned reactors are fitted in series with the capacitors to shift the resonance away from harmonic frequencies. Never size a capacitor bank from a single power-factor reading without checking harmonic content and load variation first.
A sane correction sequence
Measure power factor and harmonics over a representative load cycle, not a single snapshot. Size correction in steps matched to load variation, include detuned reactors where harmonics require them, and verify after installation: power factor at peak and at light load, voltage, capacitor temperature, and harmonic levels. Correction that is only checked at commissioning and never again will drift out of step as loads change.
If you want a second pair of eyes on your own numbers or drawings before you decide, see the remote transformer services — documents only, fixed price, fixed turnaround, no site visit. For the wider testing picture, start with the transformer testing and commissioning guide.