--- title: "Causes of poor power factor and improvement" subject: "Industrial Power Distribution and Illumination" chapter: 10 key: "semester_6/industrial_power_distribution_and_illumination:poor-pf-causes-improvement" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/poor-pf-causes-improvement site: BCT Engineering --- # Causes of poor power factor and improvement Industrial Power Distribution and Illumination · Chapter 10: Electrical Energy Audit in Industry - Asked in: 2068 Bhadra (old course), 8 marks - Video: [Power Factor Explained - The basics what is power factor pf](https://www.youtube.com/watch?v=Tv_7XWf96gg) — The Engineering Mindset - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/poor-pf-causes-improvement - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-10 ## Question Write the causes of poor power factor in an electrical power system and discuss the method of improving it. ## Short answer Poor power factor is caused by induction motors, lightly loaded transformers, arc furnaces, discharge lamps; it raises current and losses. Improvement methods: static capacitors (most common), synchronous condensers, phase advancers, and over-excited synchronous motors to supply leading reactive power. ## Answer **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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main cause of poor lagging pf in industry? A. Resistive heaters B. Incandescent lamps C. Induction motors, which need magnetising current, especially when lightly loaded D. Capacitor banks Answer: C. Induction motors draw magnetising lagging current; pf is 0.8-0.85 at full load and drops to 0.2-0.3 at light load. (Key point: Main cause of poor pf) 2. Why must a poor pf be improved? A. It lowers the voltage level to useful values B. For the same kW it needs higher current and kVA, causing larger equipment, I^2R loss, poor regulation and penalties C. It reduces the kW consumed D. It improves voltage regulation Answer: B. Low pf raises current for the same real power, which increases equipment size, losses, voltage drop and kVA charges. (Key point: Effects of poor pf) 3. Which statement about static capacitors is correct? A. They absorb lagging kVAR from the motor B. They need a rotating shaft C. They supply leading kVAR near the load, with low loss, but their life is shortened by over-voltage and harmonics D. They give stepless control at no cost Answer: C. Static capacitors supply leading kVAR to cancel lagging kVAR; they have no moving parts but are affected by over-voltage and harmonics. (Key point: Static capacitor method) 4. What is a synchronous condenser? A. An under-excited motor on load B. A bank of fixed capacitors C. A rotor circuit device for induction motors D. An over-excited synchronous motor running on no load, giving smooth stepless kVAR control Answer: D. It supplies leading kVAR with smooth control, but is costly and needs maintenance, so it is used at large substations. 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