Power Factor in Thai Factories: What It Is, Why Utilities Charge for It and How to Fix It

Motors, transformers and other inductive loads draw two kinds of power: active power (kW) that does useful work, and reactive power (kVAr) that only builds magnetic fields. Power factor is active power divided by apparent power (kVA). Thai utilities apply a charge when reactive demand rises above a threshold that corresponds to a power factor of about 0.85 (lagging), which is about 62 kVAr of reactive demand for every 100 kW. The exact rule and rate are set in the PEA or MEA tariff for your customer category, so check your bill and the current tariff.

Diagram: power triangle for 100 kW at power factor 0.70 versus 0.95, showing apparent power falling from about 143 to 105 kVA

Tap or click the diagram to enlarge it.

The Basics

Three Kinds of Power

It helps to keep these three apart.

1
Active power (kW)The power that does useful work: turning shafts, making heat and light.
2
Reactive power (kVAr)Power that swings back and forth to build magnetic fields in motors and transformers. It does no useful work but still loads the system.
3
Apparent power (kVA)What cables and transformers must actually carry. Power factor = kW ÷ kVA.

Worked example (illustrative) A 100 kW load at power factor 0.70 needs about 143 kVA. Raise the power factor to 0.95 and it needs about 105 kVA, roughly 26% less current for the same useful work. This is the same pattern as the worked example in DOE's power factor guidance.

Why It Drops

Common Causes of Low Power Factor

Lightly loaded or oversized motors. DOE notes that oversizing and underloading lead to low power factor and extra losses.
Inductive equipment such as transformers, arc welders and induction furnaces.
Fluorescent lighting with older ballasts.
Many motors running at part load at the same time.

The Fixes

How to Raise Power Factor

Start with the cause, then correct what remains.

1
Right-size motorsAvoid motors that run far below their rated load.
2
Install power factor correction capacitorsCapacitors supply the reactive power locally. DOE guidance suggests fitting them at the motor, at the motor control centre, or at the main supply when economically justified.
3
Use automatic capacitor banksWhen load falls, capacitors should switch out. Over-correction causes a leading power factor and possible overvoltage.
4
Have harmonics checkedPlants with drives or other harmonic sources can resonate with plain capacitors. Have an engineer check the system before adding them.
5
Monitor it every daySmart meters report power factor and kVAr continuously, so you can see whether your correction works throughout the month.

Not the same as saving kWh Power factor correction cuts reactive charges, current and upstream losses. It does not by itself reduce the kWh your machines use. Treat it as one part of an energy program.

This article is general guidance. Thresholds, rates and calculation rules differ by tariff and change over time — always use your own bill and the current PEA or MEA schedule. Correction equipment must be designed and installed by qualified people.

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Power Factor in Thai Factories: What It Is, Why Utilities Charge for It and How to Fix It

Three Kinds of Power

Common Causes of Low Power Factor

How to Raise Power Factor

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