Line Reactor & Load Reactor
The complete guide to the line reactor and load reactor — the three-phase series inductors used with a VFD (variable frequency drive). A line reactor on the input side cuts harmonics and inrush; a load reactor on the output side cuts dv/dt and protects the motor. Learn construction, % impedance rating, sizing and where each goes.
Complete Learning Path — Line / Load Reactor
From what a reactor is and where it goes, to construction, harmonics, dv/dt, % impedance rating, sizing and applications
What is a Line / Load Reactor?
A line reactor or load reactor is a three-phase iron-core inductor connected in series with a variable frequency drive (VFD). It adds inductance that opposes fast changes in current — smoothing waveforms, damping transients and protecting equipment.
It is the same component in both roles — the name just tells you which side of the drive it is on. A line reactor (also called an input reactor or AC line reactor) sits between the AC supply and the drive; a load reactor (or output reactor / motor reactor) sits between the drive and the motor.
One component, two jobs
Input side → protect the drive and the supply from harmonics & inrush. Output side → protect the motor from fast switching spikes.
Construction: a 3-Phase Iron-Core Reactor
Physically, a reactor is a rugged three-phase inductor: three copper windings, one per phase, on a laminated iron core.
XL = 2πfL
The inductive reactance rises with frequency — so a reactor opposes fast (high-frequency) current changes far more than the slow fundamental
Because reactance grows with frequency, the reactor barely impedes the 50/60 Hz fundamental but strongly opposes the high-frequency harmonic and switching currents — exactly the ones you want to suppress.
Line Reactor (Input Side): Fewer Harmonics
A line reactor on the drive's input smooths the peaky, distorted current that a diode rectifier draws, cutting harmonic distortion (THD) and protecting the supply.
Cuts harmonics
Lowers input current THD to help meet power-quality limits.
Limits inrush
Softens the turn-on surge into the DC bus capacitors.
Buffers transients
Absorbs voltage spikes, sags and line notching from other drives.
Protects the drive
Reduces stress on the rectifier diodes and DC bus capacitors.
Load Reactor (Output Side): Lower dv/dt
A load reactor on the drive's output slows the ultra-fast IGBT switching edges, lowering dv/dt and the voltage overshoot that reaches the motor.
On long cables the drive's fast voltage edges reflect and can double at the motor terminals (the reflected-wave or transmission-line effect), stressing and eventually failing the winding insulation. A load reactor lowers the dv/dt, cuts the peak voltage, reduces motor heating and audible noise, and lets you run longer motor cables safely.
Longer cable = more risk
The reflected-wave problem gets worse as the motor cable gets longer. A load reactor (or a stronger dv/dt or sine-wave filter) is commonly needed once cable runs exceed a few tens of metres.
How a Reactor Is Rated: % Impedance
Reactors are specified by their percent impedance (%Z) — the impedance as a fraction of the drive's rated voltage and current. It sets both the smoothing you get and the voltage you lose.
%Z = (Irated × XL / Vphase) × 100
Higher % = more current smoothing and harmonic reduction, but a larger voltage drop
The 5% rule of thumb
A 5% reactor drops about 5% of the rated voltage at full load and gives good harmonic reduction — the most common choice. Use 3% where you mainly want protection with minimal voltage loss.
Line Reactor vs Load Reactor
Same component, opposite ends of the drive — here is the side-by-side of what each one does.
| Property | Line reactor (input) | Load reactor (output) |
|---|---|---|
| Location | AC supply → drive | Drive → motor |
| Main benefit | Reduces harmonics (THD) | Reduces dv/dt & overshoot |
| Protects | Rectifier, DC bus, supply | Motor winding insulation |
| Also helps | Inrush, transients, notching | Long cables, heating, noise |
| Frequency it faces | Line + harmonics | Drive PWM switching |
Sizing & Selection
Choosing a reactor is mostly about matching the drive's ratings and picking the right % impedance.
Current & voltage
Match the reactor's rated current and voltage to the drive/motor — it carries the full line or motor current.
% impedance
5% for harmonic reduction; 3% for light protection with less voltage drop.
Frequency
Rated for the supply frequency (50/60 Hz) and the drive's PWM carrier for output units.
Duty & cooling
Continuous current rating with margin, and adequate ventilation for the heat it dissipates.
Reactor vs DC choke vs dv/dt / sine filter
A DC choke (DC bus reactor) inside the drive also cuts harmonics and is compact. A dv/dt filter is a stronger output filter (reactor + capacitors) for longer cables, and a sine-wave filter removes the PWM entirely for a clean sinusoidal motor voltage — at higher cost and size.
Applications
Line and load reactors are standard accessories anywhere a VFD drives a motor and power quality or motor protection matters.
Pumps
Line reactors cut harmonics on pump-drive banks; load reactors protect submersible and long-cable motors.
Fans & HVAC
Reactors are common on building HVAC and cooling-tower fan drives.
Conveyors & machines
Protect drives and motors in factory automation and material handling.
Power quality compliance
Line/DC reactors help sites meet harmonic limits such as IEEE 519.
Multiple-drive systems
Reduce interaction and notching where many drives share one supply.
Retrofit protection
Cheap insurance added to existing drives against surges and motor failures.
Key Terms at a Glance
The essential reactor vocabulary engineers search for.
Line reactor
Input-side series inductor.
Load reactor
Output-side series inductor.
% impedance
Rating: typically 3% or 5%.
dv/dt
Voltage rate-of-rise the load reactor limits.
THD
Harmonic distortion the line reactor cuts.
Reflected wave
Cable overshoot that stresses the motor.
Frequently Asked Questions
Quick, expert answers to the questions people ask most about line and load reactors.
What is a line reactor?
A line reactor is a three-phase inductor connected in series on the input (line) side of a variable frequency drive, between the AC supply and the drive. It reduces the harmonic current the drive draws, limits inrush current, buffers voltage spikes and notching, and protects the drive's rectifier and DC bus capacitors.
What is a load reactor?
A load reactor is a three-phase inductor connected in series on the output (load) side of a variable frequency drive, between the drive and the motor. It slows the fast rising edges of the inverter PWM output, reducing dv/dt and voltage overshoot at the motor terminals and protecting the motor winding insulation, especially on long cable runs.
What is the difference between a line reactor and a load reactor?
They are the same type of component on different sides of the drive. A line reactor is on the input side and reduces harmonics, inrush and protects the rectifier. A load reactor is on the output side and reduces dv/dt and reflected-wave voltage to protect the motor. Both are three-phase series inductors rated by percent impedance.
Why do VFDs need a reactor?
A variable frequency drive draws peaky, non-linear current that creates harmonics, and its IGBT inverter produces very fast voltage edges. A line reactor smooths the input current to cut harmonics and protect the drive, while a load reactor softens the output edges to protect the motor. Reactors improve power quality and reliability.
What does percent impedance mean for a reactor?
Percent impedance is the reactor's impedance expressed as a percentage of the drive's rated voltage and current. A 5 percent reactor drops about 5 percent of the rated voltage across itself at full load. A higher percentage gives more current smoothing and harmonic reduction but a larger voltage drop.
Should I use a 3% or 5% reactor?
A 3 percent reactor gives light smoothing and a small voltage drop, suitable for basic drive protection. A 5 percent reactor is the common standard for meaningful harmonic reduction. Use 5 percent when harmonics are a concern and 3 percent when you mainly want inrush and transient protection with minimal voltage loss.
Does a line reactor reduce harmonics?
Yes. A line reactor smooths the peaky input current a drive rectifier draws, typically lowering the total harmonic distortion from around 80 percent down to roughly 35 percent with a 5 percent reactor. It is the cheapest and simplest way to cut a drive's input current harmonics.
How does a load reactor protect a motor?
The inverter's fast PWM edges combine with cable capacitance to create voltage overshoot (reflected-wave) at the motor that can reach twice the DC bus voltage and stress the winding insulation. A load reactor slows those edges, lowering dv/dt and the peak voltage, which protects the insulation and allows longer motor cables.
What is the difference between a line reactor and a DC choke?
A line reactor is an AC inductor on the drive input; a DC choke (DC bus reactor) is an inductor inside the drive in the DC link. Both reduce harmonics. A DC choke is often more compact and effective per size, but a line reactor also adds voltage-transient and surge protection on the AC side.
What is the voltage drop across a reactor?
At full load the voltage drop roughly equals the reactor's percent impedance, so a 3 percent reactor drops about 3 percent and a 5 percent reactor about 5 percent of the rated voltage. This drop must be allowed for so the motor still receives adequate voltage.
What is the difference between a load reactor and a dv/dt or sine-wave filter?
A load reactor is a simple series inductor that reduces dv/dt moderately. A dv/dt filter adds capacitors and damping for stronger edge slowing on longer cables. A sine-wave filter is a full LC low-pass that removes the PWM switching entirely and delivers a near-sinusoidal voltage to the motor, at higher cost and size.
Where are line and load reactors used?
They are used with variable frequency drives on pumps, fans, compressors, conveyors and HVAC systems, in industrial plants and buildings where power quality, harmonic limits or long motor cables are a concern, and wherever a drive's rectifier or a motor's insulation needs extra protection.
Conclusion & Key Takeaways
A line or load reactor is a simple, rugged three-phase inductor that quietly protects both your drive and your motor.
Same part, two sides
Line = input, load = output.
Line = harmonics
Smooths input current, cuts THD.
Load = dv/dt
Protects motor insulation.
Rated by %Z
Usually 3% or 5%.
XL = 2πfL
Opposes fast currents.
Cheap insurance
For VFDs on pumps, fans, HVAC.