Line Reactor Symbol
Definition: The Line Reactor symbol represents a series-connected three-phase or single-phase inductor (choke) inserted in the AC supply or drive output line to reduce harmonic distortion, limit fault current, and suppress electromagnetic interference, drawn in schematics as a series inductor symbol (IEC 60617-04: rectangle; ANSI/IEEE 315: looped coil) between the In and Out pins, with the reference designator L followed by a number.
Also known as: AC line choke, input reactor, output reactor, harmonic filter inductor, drive reactor, VFD reactor, line choke.
What the Line Reactor symbol means
The Line Reactor symbol identifies an iron-core or air-core inductor connected in series with a power line, most commonly on the AC supply input or output of a variable frequency drive (VFD), rectifier, or active front end converter. The reactor's impedance is proportional to frequency, so it presents low impedance to the fundamental power frequency (50/60 Hz) while substantially attenuating higher-order harmonic currents generated by the non-linear switching loads it protects.
In power-system single-line diagrams and VFD installation schematics, the line reactor symbol marks the point at which harmonic filtering and fault-current limiting are applied. The In pin connects to the upstream supply or converter output and the Out pin connects to the load or downstream converter stage. The presence of the line reactor symbol communicates to the installer that both the conductor ratings and the reactor's impedance value (expressed as a percentage reactance, %Z) must be selected for the specific drive or load current.
How to identify the Line Reactor symbol
The line reactor symbol is drawn as a series inductor between the In and Out terminals. In IEC-style schematics (IEC 60617-04) the inductor is represented as a rectangle with terminal lines; an iron core (indicated by one or two parallel lines beside or inside the rectangle) signifies a core-type reactor. In ANSI/IEEE 315 schematics the inductor is shown as a series of semicircular loops (coil symbol) between the input and output terminals, with a straight line above or below the coils representing the magnetic core. In three-phase systems three such symbols are drawn in parallel on the three line conductors, sometimes combined into a single three-phase reactor block labelled '3-phase line reactor' or '3L'.
Function and usage boundary
A line reactor functions as a series impedance in the power circuit. For a three-phase VFD installation, the reactor is typically rated at 3–5% impedance (%Z) of the drive's kVA rating. This impedance: (1) reduces the peak and RMS harmonic current injected back into the supply by the VFD's rectifier front end by 40–50%; (2) limits the rate of current rise (di/dt) during input rectifier commutation, reducing notching on the supply voltage waveform; (3) provides fault-current limiting — during a short circuit downstream, the reactor limits peak fault current and gives protective devices time to operate; (4) attenuates switching-frequency noise from the drive's PWM output on output reactors, protecting the motor insulation from voltage spikes.
Standards: IEC vs ANSI
| IEC 60617 | IEC 61000-3-2 and IEC 61000-3-12 govern harmonic current emission limits for equipment connected to public LV networks; line reactors are a common mitigation measure used to bring non-linear loads into compliance. IEC 60076-6 covers reactor specifications. IEC 60617-04 defines the inductor schematic symbol used for line reactor representation in IEC-style schematics. |
|---|---|
| ANSI/IEEE 315 | IEEE 519-2014 specifies harmonic voltage and current limits at the point of common coupling (PCC) in North American power systems; line reactors are listed as a standard mitigation method. ANSI Y32.2 / IEEE 315-1975 defines the inductor coil symbol used for line reactors in North American schematics. |
| Key difference | The IEC symbol for an inductor (and hence a line reactor) is a filled rectangle; the ANSI/IEEE symbol is a series of semicircular arcs (coil loops). Both may show an iron-core indicator (a straight line parallel to the coil axis). The functional meaning is identical. |
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Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| in | In |
| out | Out |
Where the Line Reactor symbol is used
- VFD (variable frequency drive) AC input lines: 3–5% line reactors on each phase reduce harmonic distortion injected into the supply and protect the drive's rectifier bridge from supply-voltage notches
- VFD output lines: output reactors attenuate PWM carrier-frequency voltage spikes on the motor cable, reducing dV/dt stress on motor winding insulation, particularly for long cable runs
- Active front end (AFE) converters and regenerative drives: line reactors are mandatory on both AC supply and DC bus lines to achieve required LCL filter performance
- Soft-starter AC input: line reactors reduce inrush current surge and voltage dips during motor starting
- Harmonic mitigation for non-linear loads: UPS systems, rectifiers, and switch-mode power supplies use line reactors to meet IEC 61000-3-2 and IEEE 519-2014 harmonic limits
- Fault current limiting in industrial distribution: reactors in series with bus tie or feeder switches limit fault current to values within the interrupt rating of downstream circuit breakers
Key facts
- A line reactor is a series inductor inserted in an AC power line to reduce harmonic distortion, limit fault current, and suppress electromagnetic interference; it is most commonly used on VFD input and output lines rated at 3–5% impedance.
- The line reactor symbol in IEC 60617-04 schematics is a rectangle (the IEC inductor symbol) with an optional iron-core indicator; in ANSI/IEEE 315 schematics it is a series of semicircular coil loops with an optional core line.
- A 3–5% line reactor on a VFD input reduces total harmonic distortion (THD) of the input current from 80–120% (bare drive) to 35–45%, helping meet IEC 61000-3-2 and IEEE 519-2014 harmonic limits.
- Output line reactors on VFDs reduce dV/dt voltage spikes on the motor cable caused by PWM switching; without output reactors, long motor cables can cause voltage doubling at the motor terminals due to transmission-line reflection, damaging motor winding insulation.
- The reference designator for a line reactor (inductor) in schematics is L followed by a number: L1, L2, L3 for three-phase reactors; the designator follows both IEC and IEEE 315 inductor-naming conventions.
- Line reactor inductance is expressed as percentage impedance (%Z) at the nominal voltage and frequency: a 5% reactor drops 5% of the nominal voltage when carrying rated current at rated frequency.
- IEC 61000-3-2 limits harmonic current emissions from equipment consuming ≤ 16 A per phase on LV public networks; line reactors are a recognised and widely used compliance solution for class A and C equipment.
Frequently asked questions
What does the line reactor symbol look like in a schematic?
The line reactor symbol is a series inductor drawn between the In and Out terminals. In IEC 60617-04 schematics it is a small rectangle with a core line (iron core) and terminal stubs. In ANSI/IEEE 315 schematics it is a series of semicircular loops (coil) with a straight core line above or below. In three-phase circuits three such symbols appear on each phase conductor, or a single labelled three-phase block is used.
What does a line reactor do in a VFD installation?
A line reactor on a VFD input reduces the harmonic current the drive injects back into the supply network, limits inrush and fault current to the drive's rectifier, and attenuates voltage notches on the supply waveform caused by rectifier commutation. A line reactor on a VFD output reduces the dV/dt of the PWM voltage pulses sent to the motor, protecting long motor cables and motor winding insulation from voltage overshoot and reflected-wave damage.
What is the difference between a line reactor and a harmonic filter?
A line reactor is a passive series inductor that attenuates all harmonics above the fundamental by presenting increasing impedance at higher frequencies. A harmonic filter is a tuned LC circuit that specifically targets one or more harmonic frequencies (typically 5th, 7th, 11th) with a notch filter, providing deeper attenuation at those harmonics but requiring careful tuning. Line reactors are simpler and more robust; harmonic filters achieve better total harmonic distortion (THD) reduction for the same cost when specific harmonics dominate.
What standard governs line reactor harmonic requirements?
IEC 61000-3-2 and IEC 61000-3-12 set harmonic current emission limits for LV equipment in international (European) installations. IEEE 519-2014 sets harmonic voltage and current distortion limits at the point of common coupling in North American power systems. Line reactors are a widely accepted mitigation measure for achieving compliance with both standards.
What percentage impedance should a line reactor be rated for a VFD?
A 3% or 5% line reactor is standard for most VFD applications. A 3% reactor offers a balance between harmonic reduction and voltage drop; a 5% reactor provides greater harmonic attenuation but imposes a 5% fundamental-frequency voltage drop across the reactor at full load. Reactors below 1.5% provide minimal benefit; above 5% the voltage drop becomes significant for the drive's input rectifier voltage regulation.
What is the reference designator for a line reactor?
The reference designator for a line reactor follows the inductor convention: L followed by a sequential number, e.g. L1, L2, L3. For a three-phase reactor on three phases the designators L1R, L2R, L3R (reactor) or a single block label 3L1 may be used, depending on the drawing convention of the engineering office.
Does a line reactor affect motor speed or torque?
A correctly rated line reactor causes a small voltage drop (3–5% at full load) but does not directly affect motor speed or torque in a VFD system, because the VFD's DC bus and output inverter compensate for the input voltage reduction within the drive's rated input voltage tolerance. However, if the supply voltage is already near the drive's minimum input threshold, adding a 5% reactor may push the DC bus voltage below the minimum, reducing maximum available motor voltage and torque at high speed.
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Physical breadboard, PCB, and panel mapping
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