Power Factor Demystified: Understanding and Correcting Reactive Power in Electrical Systems

Power Factor Essentials

Power factor is the ratio of true power to apparent power, PF = KW / KVA. A higher PF means more useful work per unit of energy. The beer analogy helps: KW is the beer, reactive power (KVAR) is the foam. PF tells you how much value you’re getting from the power you buy.

Why PF Matters

Bad PF increases current, causes voltage drops, and can trigger reactive power charges on commercial and industrial bills. The goal is PF close to 1, though a perfect PF is practically impossible. The video outlines typical PF categories for large buildings and explains penalties that arise when PF falls below supplier-defined thresholds.

Introduction to Power Factor

Power factor (PF) is a unitless ratio in alternating current circuits that indicates how much of the drawn power is actually doing useful work. It is defined as PF = KW / KVA, where KW is the true or real power, and KVA is the apparent power. An intuitive analogy compares the glass of beer to the electric system: the beer represents useful power and the foam represents reactive power. The more beer in the glass, the better the value for money; more foam means more reactive energy that does not do work but still adds to the cost.

In electrical engineering terms, the power triangle shows true power (KW) as the adjacent side, reactive power (KVAR) as the opposite, and apparent power (KVA) as the hypotenuse. The angle theta represents the phase difference between voltage and current. As reactive power grows, the apparent power grows, increasing current and losses. While the video keeps this part brief, it links the geometry to practical calculations for PF.

Why PF Is Important on Bills

Residential electricity bills often do not penalize poor PF, but commercial and industrial invoices commonly include charges or penalties when PF falls below a threshold, typically around 0.95. The reason is that a low PF increases current, causing voltage drops and reducing the supplier's distribution capacity. Cables, transformers, and other equipment must carry higher currents, potentially limiting new connections and increasing losses.

PF Levels in Real Buildings

In large commercial buildings, PF is often categorized as good (roughly 1 to 0.95), poor (0.95 to 0.85), and bad (below 0.85). Office buildings usually sit around 0.92–0.98, while industrial facilities can be as low as 0.7, illustrating the impact of inductive loads like motors and other equipment.

Worked Examples: Induction Motors

Two induction motors, each delivering 10 kW on a 415 V three-phase supply, are compared: one with PF = 0.87 and the other with PF = 0.92. Both provide 10 kW of useful work, but the first motor draws 11.5 kVA from the grid, while the second draws 10.9 kVA. The corresponding reactive powers are about 5.68 kvar for the first motor and 4.34 kvar for the second. This demonstrates how PF directly affects the apparent power drawn from the grid and thus the energy cost even when real work is identical. The underlying relationships are PF = KW / KVA and KVA^2 = KW^2 + KVAR^2.

Leading vs Lagging PF

A purely inductive load causes a lagging PF as current lags voltage, while a purely capacitive load causes leading PF as current leads voltage. Real systems often have a mix, and some reactive power is necessary to maintain magnetic fields in devices like motors; the aim is to minimize reactive power while still enabling the required work.

Correcting PF: The Core Idea

To correct lagging PF caused by inductive loads, capacitors are added to realign current with voltage. If leading PF is caused by capacitive effects, inductors can be used to bring the PF back toward one. The video promises practical calculations for capacitor sizing in subsequent examples, noting that the corrections must be tailored to the three-phase load and supplier agreements.

Capacitor Sizing: A Concrete Example

Consider a three-phase building with a total load of 50 kW and an initial PF of 0.78, and you want to raise PF to 0.96 to avoid penalties. The current apparent power is 64.1 kVA (50 kW / 0.78). The reactive power is sqrt(64.1^2 - 50^2) ≈ 40.1 kvar. With a target PF of 0.96, the new apparent power should be 52.1 kVA (50 kW / 0.96), giving a reactive power of sqrt(52.1^2 - 50^2) ≈ 14.6 kvar. The capacitor bank must supply the difference, 40.1 kvar - 14.6 kvar ≈ 25.5 kvar. This illustrates the practical, arithmetic steps for reactive power correction using capacitor banks in a multi-phase system.

Why This Matters for Industry

Beyond penalties, fixing PF reduces current, which lowers the required cable size, reduces energy losses, and improves transformer efficiency and equipment life. PF correction is a standard practice in commercial and industrial facilities to optimize energy costs and grid interactions.

Wrap-Up

Power factor is a measure of how effectively electrical power is converted into useful work. A poor PF increases current, raises costs, and stresses electrical equipment, while capacitive or inductive corrections can bring the PF closer to unity. The approach outlined combines intuitive understanding with concrete calculations and a clear path to practical correction strategies.