Variable frequency drives (VFDs) are everywhere in Australian facilities. They run HVAC systems, pumps, fans, compressors and manufacturing equipment. They improve motor control and cut energy use by matching speed to demand.
A site may see a poor power factor reading and reach for a capacitor bank. That can work for directly connected motors. VFD loads behave differently because harmonic distortion can pull down the true power factor even when the displacement power factor looks healthy.
Key Points
Variable frequency drives improve motor control but can distort the current waveform and lower true power factor.
Displacement power factor can look healthy while true power factor stays poor because of harmonic distortion.
Standard capacitor banks can resonate with network inductance and amplify harmonics rather than fix the problem.
In Australia demand is often billed on apparent power in kVA so poor power factor has a direct cost.
Correct diagnosis depends on measuring true power factor, displacement power factor and harmonic distortion before choosing equipment.
SATEC power quality meters and the Expertpower platform give facilities the measured data needed to select the right correction method.
Why Power Factor Matters
Power factor describes how effectively electrical power becomes useful work. A value near 1.0 means the available capacity is used efficiently. A lower value means more current is needed for the same real power.
Poor power factor raises current in cables, transformers and switchgear. It adds losses, reduces spare capacity and can lift demand charges. Many Australian distributors bill demand on apparent power measured in kVA. Typical network demand charges sit in the range of $8 to $12 per kVA each month so a low power factor carries a real cost. Some networks also set a minimum. New South Wales, for example, requires a minimum power factor of 0.9.
Traditional correction targets the reactive power drawn by inductive loads. Capacitors supply part of that reactive power locally. Trouble begins when the cause of a low reading is assumed rather than measured.
How VFD Loads Change the Electrical Picture
A VFD converts incoming AC power to DC through a rectifier. An inverter then creates a controlled output for the motor. This lets speed and torque adjust to demand, although the current drawn by the drive may not follow a smooth sinusoidal shape.
Many modern VFDs show a fairly good displacement power factor. The fundamental current often sits close to the voltage. The current waveform can still carry harmonics. These distorted components raise RMS current and apparent power. True power factor can then fall below displacement power factor.
A basic meter that reports only phase angle power factor may hide the real problem.
True Power Factor Versus Displacement Power Factor
Displacement power factor measures the phase relationship between the fundamental voltage and current. It describes traditional inductive and capacitive behaviour well.
True power factor includes both phase displacement and waveform distortion. That makes it the more useful figure for sites with nonlinear loads such as VFDs, UPS systems, LED lighting and switched mode power supplies.
A VFD heavy site might record a displacement power factor of 0.97 while true power factor sits lower. A standard capacitor bank could improve displacement while doing little about harmonic current. High RMS current, transformer heating and limited capacity may all remain.
Why Standard Capacitor Banks May Make Things Worse
Capacitors interact with the inductance of the network. Together they can form a resonant circuit at a particular frequency. On a site with strong harmonic current that resonant frequency may line up with a dominant harmonic order. Distortion can then be amplified rather than reduced.
Possible results include capacitor overheating, blown fuses, nuisance tripping and early failure. Harmonic current may also flow into the bank because capacitor impedance drops as frequency rises.
This does not mean capacitors have no place with VFD loads. It means they should not be chosen without measurement and a system assessment. Detuned capacitor banks can suit sites where reactive correction is still needed. Other sites may need line reactors, passive filters, active harmonic filters or low harmonic drives. The right answer depends on the electrical data.
Measure Before Choosing a Solution
Effective correction begins with measurement rather than assumption. The first task is to find where the low reading occurs and how it changes during operation. Harmonic levels and power factor can shift as motors accelerate, slow down or run at part load.
Useful measurements include true power factor, displacement power factor, current total harmonic distortion, voltage total harmonic distortion, the harmonic spectrum, RMS current and demand trends. Readings should be taken at relevant distribution points and at the point of common coupling where needed.
In Australia, harmonic voltage distortion at the point of common coupling is governed by AS 61000.3.6. A short snapshot can miss changing conditions. Continuous monitoring shows when the issue happens and which loads contribute.
Selecting the Right Correction Method
A site with poor displacement power factor and low harmonic distortion may still benefit from conventional or detuned capacitor correction. A site with heavy harmonic distortion may need harmonic mitigation instead.
Active harmonic filters respond dynamically as loads change. Passive filters suit stable and predictable load profiles. Line reactors reduce current distortion at individual drives although performance depends on drive design and system impedance. Some sites need a combined approach where reactive correction handles displacement and harmonic filtering improves waveform quality. Equipment should be chosen from measured data and an engineering assessment.
The table below compares common correction methods.
| Correction Method | Best Suited To | Improves Displacement Power Factor | Reduces Harmonic Distortion | Key Considerations |
|---|---|---|---|---|
| Standard capacitor bank | Sites with low harmonic distortion and mainly inductive loads | Yes | No | Risk of resonance where harmonics are present |
| Detuned capacitor bank | Sites needing reactive correction with some harmonics present | Yes | Limited | Tuning reactor helps avoid resonance |
| Line reactors | Individual VFDs | Minor | Partial | Performance depends on drive design and system impedance |
| Passive harmonic filter | Stable and predictable load profiles | Some | Yes | Less effective when loads vary widely |
| Active harmonic filter | Variable loads and changing conditions | Some | Yes | Responds dynamically as loads change |
| Low harmonic drive | New installations or drive replacement | Yes | Yes | Harmonic mitigation built into the drive |
SATEC Metering for VFD and Power Factor Analysis
Accurate correction starts with accurate data. SATEC provides energy metering and power quality analysers that reveal what is happening around VFD loads. These instruments measure true power factor, displacement power factor, harmonic distortion, demand, current and voltage. Facilities can then separate conventional reactive power problems from distortion related issues.
The PM180 is a Class A power quality analyser suited to investigations at the incomer and the point of common coupling. It captures harmonic spectra, waveforms and power quality events, which helps engineers study resonance, excessive current and voltage distortion.
For distribution boards, motor control centres and individual feeders the PM130EH PLUS measures true power factor, THD and individual harmonics. Both meters give the evidence needed to choose correction equipment with confidence.
Data from these meters can flow into Expertpower for dashboards, reports and trend analysis. Engineers and facility managers gain a clear view of how power factor and harmonics move through the day. This metering does not replace detailed electrical design. It supplies the accurate information that good design decisions depend on.
A More Reliable Approach to Power Factor Correction
Power factor correction for VFD loads takes more than adding capacitance. The site must understand why power factor is low. Capacitors can be the right choice where phase displacement is the main issue.
Harmonic mitigation is often needed where distortion is the primary cause. Accurate metering helps avoid costly assumptions and reduces the risk of treating the wrong problem.
FAQs - Power Factor Correction for VFD Loads
What is the difference between true power factor and displacement power factor?
Displacement power factor looks only at the phase relationship between the fundamental voltage and current. True power factor also accounts for harmonic distortion so it gives a fuller picture on sites with VFDs.
Can a capacitor bank fix low power factor caused by a VFD?
A capacitor bank can improve displacement power factor but it does little for harmonic distortion. On sites with strong harmonics it may even cause resonance so measurement should always come first.
Why do VFDs create harmonics?
A VFD rectifies AC to DC which draws current in pulses rather than a smooth sine wave. Those pulses contain harmonic components that raise RMS current and lower true power factor.
How can I tell which correction method my site needs?
Measure true power factor, displacement power factor and harmonic distortion at the relevant points before deciding. The results show whether reactive correction, harmonic mitigation or a combination is required.
