Causes of poor power factor and improvement

Power factor is the ratio of real power to apparent power, cos φ = kW/kVA. A pf well below 1 (lagging) is called poor; it means the load draws extra reactive current that does no useful work.

Causes of poor (lagging) power factor

1. Induction motors – the main cause in industry. They need magnetising current; at full load pf is about 0.8–0.85, and at light load it falls to 0.2–0.3. Oversized or lightly loaded motors make it worse.
2. Transformers – magnetising current, especially when lightly loaded or on no load (e.g. at night).
3. Arc and induction furnaces, welding sets – operate at very low pf (about 0.3–0.6).
4. Discharge lamps with magnetic ballasts – fluorescent, mercury and sodium lamps with choke have pf around 0.5.
5. Higher supply voltage than rated increases magnetising current and lowers pf.
6. Harmonics from rectifiers, drives and converters reduce the true (distortion) power factor.
7. Reactors and chokes in the system.

Effects (why it must be improved)

Higher current for the same kW → larger kVA rating of generator, transformer and switchgear, larger cables, higher I²R losses, poorer voltage regulation, and penalty / higher kVA charges in the tariff.

Methods of improvement

Leading kVAR is supplied near the load to cancel lagging kVAR:

1. Static capacitors – most common. Connected in star or delta across the supply, either fixed or automatically switched (APFC panel with relay). Low loss, little maintenance, no moving parts; but life is shortened by over-voltage and harmonics.
2. Synchronous condenser – an over-excited synchronous motor on no load; gives smooth, stepless control of kVAR; used at large substations, costly and needs maintenance.
3. Phase advancers – used with large induction motors; supply exciting ampere-turns to the rotor circuit at slip frequency so the motor runs at higher pf.
4. Synchronous motors driving plant loads, run over-excited to supply leading kVAR.

 Supply ──┬──── Motor load (lagging)
          │
         === Capacitor bank (leading kVAR)
          │

Placement of capacitors: individual (at motor terminals – best for loss reduction), group, or central (at main bus with automatic control).

Good practice: correct motor sizing, avoiding idle running of motors and transformers, and electronic ballasts also help keep pf high. Target pf is usually 0.95–0.99; going to unity is not economical.