Power Factor Correction
Use this page when you need to understand poor power factor, check whether correction is required, or verify that an existing correction system is working as expected.
Power factor correction decisions should be based on measured site data. The important question is not just whether power factor is low at one moment, but how it behaves over time as the site load changes.
A three-phase power logger records the relationship between voltage, current, kW, kVA and kVAR, giving the proof needed to size or verify correction equipment.
What to measure
- Power factor over the working cycle of the site.
- kW, kVA and kVAR trends, including peak demand periods.
- Whether poor power factor is steady or linked to particular equipment or operating periods.
- Before-and-after proof where power factor correction equipment is installed or serviced.
- Whether load profile or voltage behaviour suggests a wider power-quality issue.
Recommended loggers
| Product | Logger | Best fit | Measures |
|---|---|---|---|
 |
EC-7VAR-RS Electrocorder EC-7VAR-RS Three Phase Voltage, Current & Power Factor Recorder |
Best for direct three-phase power factor, kW, kVA, kVAR and energy analysis. | Three-phase voltage, current, power factor, kW, kVA, kVAR and energy. |
Detailed guidance
What power factor means
Power factor is the ratio of real power to apparent power flowing from the supply to the load. It is measured from 0 to 1.0, where 1.0 is unity power factor. Power factor is usually described as leading or lagging depending on the position of the current waveform relative to the voltage waveform.
A load with a low power factor draws more current than a load with a high power factor for the same useful power transferred. That higher current increases losses in cables, transformers and distribution equipment, and may require larger electrical infrastructure. Because of those losses and capacity demands, utilities may charge industrial or commercial customers more where poor power factor is present.
Why power factor should be measured over time
Power factor changes as site load changes. A single reading may not show what happens during production peaks, motor starts, light-load periods, night shifts or equipment cycling. Logging allows the power factor, kW, kVA and kVAR behaviour to be reviewed across the normal working cycle of the site.
The EC-7VAR-RS measures power factor using the L1 voltage and A1 current channels, allowing a study of power factor on that part of a three-phase system. It is the correct choice when power factor, real power, apparent power, reactive power and peak demand need to be measured directly.
How power factor correction works
Linear loads with low power factor, such as induction motors, can often be corrected using passive networks of capacitors or inductors. The correction equipment supplies or absorbs reactive power close to the load, reducing apparent power and improving the power factor seen by the supply.
For inductive motor loads, locally connected capacitors are often used to offset the inductive effect. If a load is capacitive, inductors, also known as reactors in this context, may be used. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the energy is moving back and forth each AC cycle.
Non-linear loads, such as rectifiers and some electronic equipment, distort the current drawn from the supply. In these cases, active or passive correction may be needed to address distortion as well as power factor.
Types of correction equipment
An automatic power factor correction unit normally contains several capacitor stages switched by contactors. A regulator measures power factor and switches capacitor blocks in steps so the power factor stays above the selected value as the site load changes.
An unloaded synchronous motor can also supply reactive power. In this role it is known as a synchronous condenser. It operates at a leading power factor and puts vars onto the network as required to support system voltage or maintain the required power factor.
For high-voltage systems or large, rapidly changing industrial loads, power electronic devices such as static VAR compensators or STATCOM systems may be used. These respond faster than contactor-switched capacitor banks and require less maintenance than synchronous condensers.
Engineering caution
Power factor correction equipment should not be added without engineering analysis. Reactive elements can create voltage fluctuations or harmonic noise when switched on or off. They may also supply or absorb reactive power when there is no corresponding local load, increasing no-load losses.
In the worst case, reactive elements can interact with the electrical system and with each other, creating resonant conditions, instability or severe overvoltage fluctuations. Logged data helps show the real load pattern before correction equipment is specified or adjusted.
Benefits of power factor correction
- Reduced electricity charges where the utility applies reactive power penalties.
- Lower reactive power demand, reducing avoidable stress on the site electrical system.
- Reduced I2R losses in transformers, cables and distribution equipment.
- Reduced heat in cables, switchgear, transformers and alternators, helping equipment life.
- Reduced voltage drop in cables, which can improve motor starting at the end of long cable runs.
- Reduced carbon emissions where lower losses translate into lower energy consumption.
How to approach the investigation
- Record long enough to include normal load variation, not just a short spot check.
- Look at kVAR and power factor together so that correction decisions are based on actual demand.
- Compare periods when large inductive loads, drives, compressors or motors are running.
- After correction, repeat logging under comparable load conditions to confirm the result.
Related guides and products
These pages and products give the next level of detail for this application.
Application guides:
- Power Factor and Three Phase Power Loggers: for kW, kVA, kVAR, power factor, peak demand and three-phase power analysis.
- Energy Audit Loggers: for energy audits, load profiles, peak demand and before/after savings proof.
- Load Survey and Current Loggers: for feeder checks, current logging, load surveys and capacity planning.
- All application guides: choose an Electrocorder by the job or problem you need to investigate.
Relevant products:
- EC-7VAR-RS: Electrocorder EC-7VAR-RS Three Phase Voltage, Current & Power Factor Recorder.