Importance and improvement of power factor

Importance: Power factor (cos φ = P/S) shows how effectively the supplied current is converted into useful work. For a given power, a low power factor needs a larger current, so cables, transformers and generators become larger, losses (I²R) rise and the voltage drop increases. A high power factor means smaller kVA demand, lower loss and better regulation.

Example

A 5 kW single-phase motor at 230 V:

• At cos φ = 0.5: I = 5000/(230 × 0.5) = 43.5 A, S = 10 kVA.
• At cos φ = 0.95: I = 5000/(230 × 0.95) = 22.9 A, S = 5.26 kVA.

The current is nearly halved and the line loss (∝ I²) falls to about one quarter, so the same line and transformer can carry more load.

How power factor can be improved

1. Static capacitors connected in parallel with the inductive load. They supply the leading reactive power needed by the load. This is the most common method (capacitor banks near the load or at the substation).
2. Synchronous condenser: an over-excited synchronous motor running at no load, which takes a leading current.
3. Phase advancers: used with large induction motors. A phase advancer is an AC exciter on the motor shaft that supplies the exciting ampere-turns to the rotor circuit at slip frequency, so the motor draws less magnetising current from the supply.
4. Avoiding lightly loaded motors and transformers, and using correctly sized equipment.

Capacitor needed to improve the power factor from cos φ₁ to cos φ₂ for load P:

C = P (tan φ₁ − tan φ₂) / (ω V²)