Power factor is one of the most misunderstood concepts in electrical engineering — and one of the most financially significant for any industrial or commercial electricity consumer in India. A low power factor does not mean equipment is damaged or wiring is wrong. It means the electrical system is carrying more current than the actual work being done strictly requires, and that excess current costs money in losses, in maximum demand charges, and — if DISCOM penalises low PF — in direct surcharges on your electricity bill. Understanding what power factor is, why it goes low, how to measure it, and how to correct it is essential knowledge for any electrical engineer or energy auditor.
What Is Power Factor?
Power factor (PF) is the ratio of real power (the power that does actual work) to apparent power (the total power the supply must deliver). It is a dimensionless number between 0 and 1 (or expressed as a percentage between 0% and 100%).
The Power Triangle
Pythagoras applies to the power triangle: S² = P² + Q². A unity power factor (PF = 1.0) means Q = 0 — all the apparent power is real power, doing useful work. A power factor of 0.8 means 80% of the apparent power is real, and 20% is circulating reactive power that contributes to losses but does no work.
What Causes Low Power Factor?
Inductive loads draw lagging reactive current — current that lags behind the voltage. This is the primary cause of low power factor in industrial and commercial electrical systems. Capacitive loads cause leading power factor (current leads voltage), which is less common in practice but occurs in lightly loaded cable networks and certain electronic equipment.
| Equipment / load type | Typical PF | Character |
|---|---|---|
| Resistive heaters, incandescent lamps | 1.00 | Unity — ideal |
| Synchronous motors (over-excited) | 0.95–1.00 leading | Leading — can self-correct |
| LED drivers, switch-mode PSU (modern, PFC) | 0.90–0.98 | Near unity with PFC |
| Induction motors at full load | 0.85–0.92 lagging | Lagging — common |
| Induction motors at half load | 0.70–0.85 lagging | Worsens as load decreases |
| Induction motors at no load | 0.20–0.40 lagging | Very poor — mostly reactive |
| Fluorescent lights (magnetic ballast) | 0.50–0.65 lagging | Old technology — poor |
| Arc furnaces | 0.65–0.85 lagging | Variable, depends on arc |
| Welding sets | 0.35–0.60 lagging | Poor — high reactive draw |
| VFD-fed motors (with PFC) | 0.95–0.98 | Near unity at input |
Low PF penalty from DISCOMs in India
How to Measure Power Factor
Power factor is measured by a power analyser or power quality meter that simultaneously captures voltage, current, and the phase angle between them. A basic multimeter cannot measure power factor directly — you need an instrument with a power measurement function.
On a three-phase system, use a three-phase power analyser to measure total PF (which may differ from individual phase PF if loading is unbalanced). Record PF at different load levels — light load PF is typically worse than full load PF for induction motor-dominated plants.
Power Factor Correction with Capacitors
The standard method of improving lagging power factor is to install shunt capacitor banks. Capacitors draw leading reactive current, which cancels the lagging reactive current drawn by inductive loads. The net reactive demand seen by the supply is reduced, bringing the apparent power (kVA) closer to the real power (kW).
Avoid over-correction
Accurate power factor measurement starts with the right instrument. CIE's power quality analysers and clamp meters provide the measurement foundation for a PF correction project. Visit our instruments range or contact our energy measurement specialists for guidance on power quality assessment and correction.