Do VFDs Improve Power Factor? Displacement PF, True PF, and Harmonics Explained

Variable frequency drives (VFDs) are often described as having a high power factor. That can be correct and still be misleading. A VFD may have little phase shift but draw pulsed input current, creating harmonics that reduce true power factor.

The result is a common contradiction: one meter displays a cos φ close to 1.00, while another shows a lower total power factor. Both may be reporting correctly—they are measuring different parts of the same electrical behavior.

What Changes When a Motor Is Connected Through a VFD?

A motor connected directly to the mains draws magnetizing reactive current. Its displacement power factor can be poor at light load because it still needs magnetic flux while producing little mechanical work.

A VFD rectifies incoming AC to a DC bus, then produces controlled voltage and frequency for the motor. The utility sees the drive’s input stage rather than the motor’s phase angle directly.

This can improve supply-side displacement power factor, but it introduces a second issue: nonlinear input current.

Displacement Power Factor vs True Power Factor

Displacement Power Factor

Displacement power factor, often shown as cos φ, describes the phase angle between fundamental voltage and current. Capacitor banks correct this relationship by supplying reactive power locally.

Many standard VFD rectifiers draw their fundamental current broadly in phase with the voltage, so displacement PF may appear close to unity.

Distortion Power Factor

Distortion power factor describes nonsinusoidal current. A conventional six-pulse drive charges its DC bus during portions of the voltage waveform, creating pulses and characteristic harmonics.

Even with little fundamental phase shift, RMS current includes harmonics that do not deliver equivalent useful power. Total PF therefore remains below the displacement value.

True Power Factor

True PF is real power in kW divided by total apparent power in kVA. It accounts for displacement and distortion. With limited voltage distortion, it is approximately displacement PF multiplied by distortion factor.

This is why “PF,” “cos φ,” and “DPF” should not be treated as interchangeable values on a power-quality report.

What the Main Measurements Tell You

Always confirm which definition the meter uses. A display labelled only “PF” may report true PF, displacement PF, or a value configured by the manufacturer.

Does Installing a VFD Improve Power Factor?

It can improve displacement PF compared with a directly connected motor, particularly at light load. This does not guarantee high true PF at the facility incomer.

The actual result depends on:

• The VFD input topology

• Whether it includes an AC line reactor or DC choke

• Drive loading and operating speed

• The number and diversity of drives operating together

• Other nonlinear and inductive loads

• Transformer and cable impedance

• Harmonic filters or active front-end technology

A VFD’s energy benefit comes mainly when speed control reduces process power—for example, by slowing a pump or fan—not merely from changing power factor.

Why Can True PF Fall at Light VFD Load?

At light load, real power and fundamental current decrease while input current may remain strongly pulsed. THDi can therefore rise as a percentage.

This does not make every lightly loaded VFD a severe problem. Results at one drive and the point of common coupling can differ because of load diversity and source impedance.

Can a Capacitor Bank Correct VFD Power Factor?

A capacitor bank corrects lagging displacement PF; it does not cancel harmonic current. If the VFD already has high displacement PF, added capacitance may provide little benefit and create leading PF at light load.

Capacitors can form resonant circuits with transformers and cables, potentially amplifying drive harmonics. A detuned bank addresses a resonance risk but is not an active harmonic filter.

Do not connect ordinary power factor correction capacitors between a VFD and its motor unless the drive manufacturer explicitly approves the arrangement. The VFD output is a switched waveform, and conventional motor-side correction practices do not transfer directly to drive operation.

An APFC panel may still be useful for other directly connected inductive loads in the facility. Its CT location, target, step sizes, and harmonic suitability should be reviewed with the VFD population included.

How Should VFD Power Factor Be Evaluated?

At a defined point, record kW, kVA, kVAr, true PF, displacement PF, THDi, voltage THD, and the harmonic spectrum across minimum, normal, and peak production.

High displacement PF with lower true PF points toward distortion. Low displacement PF and low true PF indicate that reactive power is also present. High voltage distortion requires attention to harmonic current and network impedance.

What Can Improve True Power Factor with VFD Loads?

Options include AC line reactors, DC chokes, passive or active harmonic filters, multi-pulse arrangements, and low-harmonic drives. Selection depends on loading, system strength, and the applicable target.

These technologies do not provide identical performance. Compare them using measurements and a harmonic study rather than selecting equipment only from total drive kW. An active harmonic filter may treat several loads centrally, while a drive-level solution may be better when one large VFD dominates distortion.

FAQ

Do All VFDs Have a Power Factor Close to 1.00?

No. Many have high displacement PF, but true PF varies with input design, harmonic current, and loading. Check the manufacturer’s data and site measurements.

Why Does the Meter Show cos φ of 0.99 but PF of 0.90?

The first value probably represents displacement PF, while the second includes waveform distortion. Harmonic current creates the difference.

Will Adding Capacitors Reduce VFD Harmonics?

No. Standard capacitors do not cancel harmonic current and may create resonance. Use a solution selected from the harmonic spectrum and system impedance.

Is an Active Harmonic Filter Always Required for VFDs?

No. The choice depends on measured distortion, load diversity, network strength, applicable limits, and existing drive technology. Reactors, chokes, passive filters, or low-harmonic drives may be suitable in other cases.

Conclusion

A VFD can improve displacement power factor while harmonic current keeps true power factor below unity. Looking only at cos φ may therefore hide the additional RMS current created by waveform distortion.

Evaluate true PF, displacement PF, THDi, kVAr, and the harmonic spectrum together. Capacitor banks can correct remaining inductive demand, but they cannot replace harmonic mitigation and should not be added around drives without a system review.

YT Electric can assess your VFD load profile, power-factor measurements, harmonic spectrum, and existing capacitor equipment to recommend an appropriate power-quality solution.