Current Limiting Reactor Symbol
Definition: The Current Limiting Reactor symbol represents a series air-core or iron-core inductor — drawn as a series of loops or arcs (ANSI zigzag/coil) or as a rectangle (IEC) with two terminals (In and Out) — inserted in a power circuit to limit fault current, reduce motor starting inrush, or suppress harmonics and transients, as specified under IEC 60076-6 (reactors) and IEEE C57.16 (requirements for series capacitor equipment and reactors); the component is classified as an inductor with inductance measured in henries (H) and its limiting effect measured in per-unit impedance or ohms at power frequency.
Also known as: line reactor, series reactor, current limiting inductor, choke coil, AC reactor, inrush limiting inductor, fault current limiter.
What the Current Limiting Reactor symbol means
The current limiting reactor symbol denotes a passive inductive component inserted in series with a power circuit to oppose rapid changes in current. Because an inductor's voltage-current relationship is V = L di/dt, it inherently resists sudden current increases — the hallmark of motor starting inrush, capacitor bank switching transients, and short-circuit fault currents. The reactor's inductive reactance (XL = 2πfL, in ohms) limits the rate of current rise and the peak current magnitude.
Current limiting reactors appear in medium-voltage and low-voltage power systems between the supply bus and motors, VFDs, rectifiers, capacitor banks, or busbar tie points. They reduce the prospective short-circuit current at downstream equipment, lower harmonic distortion from non-linear loads, and protect sensitive equipment from voltage transients. In motor applications a reactor in series with the motor terminals reduces starting current and mechanical shock, with some voltage reduction at the motor during starting.
How to identify the Current Limiting Reactor symbol
The current limiting reactor symbol is drawn as a series of curved arcs or loops (representing the coil winding) along the circuit path — identical to the standard inductor/coil symbol in ANSI/IEEE 315. In IEC 60617 the reactor is shown as a rectangle (consistent with the IEC inductor convention) labelled 'L' or 'REACTOR'. An iron-core reactor includes two parallel lines (the core) beneath or beside the coil symbol. The two terminals are labelled In (line/source side) and Out (load side). In power system one-line diagrams the reactor is often shown as a box or circle with the letter 'L' or 'XL' and a reactance value in ohms or percent.
Function in a circuit
A current limiting reactor limits peak and steady-state fault current by inserting inductive impedance in series with the circuit. At power frequency (50 Hz or 60 Hz), the reactor presents an impedance XL = 2πfL that drops voltage proportional to the current flowing through it. Under normal load, this voltage drop is small (typically 2%–5% of system voltage). During a fault or motor starting event, the large di/dt causes the reactor to develop a proportionally large opposing voltage, limiting the current to a lower peak value and extending the current rise time. Air-core reactors are used where linear inductance is critical (fault current limiters); iron-core reactors are used for lower-frequency harmonic filtering and high-inductance applications.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60076-6 (Power transformers — Part 6: Reactors) defines the design, testing, and rating of series reactors including current-limiting reactors. IEC 60617-02 specifies the schematic symbol. Reactor impedance is commonly specified in percent (%) on the rated VA base, consistent with IEC transformer convention. |
|---|---|
| ANSI/IEEE 315 | IEEE C57.16 (Requirements, Terminology, and Test Code for Dry-Type Air-Core Series-Connected Reactors) specifies design and test requirements for series reactors in North American practice. ANSI Y32.2 / IEEE 315-1975 defines the coil symbol. Reactor impedance is specified in ohms or in percent on the rated MVA base. |
| Key difference | IEC uses the rectangle symbol for inductors/reactors (consistent with its rectangle-for-passive-components convention) while ANSI/IEEE uses the coil/arc symbol. Both add two parallel lines for an iron-core reactor. In power engineering one-line diagrams, the reactor is almost always shown as a simple box with a label in both IEC and ANSI practice. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Inductance: 0.1 mH to 50 mH (typical low-voltage ranges); 1 mH to 1 H (medium-voltage). Impedance (% on rated kVA): 2%–10% for motor-starter reactors; 3%–8% for VFD line reactors; up to 14% for fault current limiting. Current rating: from 10 A (small VFD reactors) to thousands of amperes (bus-tie reactors). Voltage rating: 230 V to 36 kV (LV and MV applications). Core type: air-core (linear, no saturation) for fault limiters; iron-core (gapped) for harmonic filters. Power loss: typically 0.1%–0.5% of rated power at full load.
Where the Current Limiting Reactor symbol is used
- VFD (Variable Frequency Drive) line reactor — a 3%–5% impedance reactor installed on the AC input of a VFD reduces harmonic current distortion (THDi) from the drive's rectifier, protecting the supply transformer and neighbouring equipment from harmonic voltages.
- Motor starter series reactor — inserted in series with a three-phase motor to limit starting inrush current to 2–3 times full-load current (instead of 6–8× without limiting), reducing mechanical shock and voltage dip on the supply bus.
- Bus-tie reactor — installed between two bus sections in a medium-voltage switchboard to limit the fault current that would flow through the bus coupler during a fault on one bus section, allowing smaller and lower-rated switchgear on each section.
- Capacitor bank reactor — a series reactor (detuning reactor, typically 7% impedance) is installed in series with a power-factor correction capacitor bank to prevent resonance with the supply impedance and to limit the capacitor switch-on inrush current.
- Generator-to-bus reactor — series reactors between a generator and the main busbar limit the short-circuit contribution from the generator, reducing fault level at the bus and the required breaking capacity of downstream breakers.
- DC bus choke in rectifier systems — a DC-side series reactor (choke) in a thyristor rectifier or diode bridge smooths the DC output current, reduces harmonics fed back to the AC supply, and protects the DC bus capacitors from current spikes.
Example
In a 400 V industrial plant wiring diagram, a 4% line reactor (LR1, inductance approximately 0.3 mH at 50 Hz, rated 100 A) is shown in series between the main distribution board and the input terminals of a 37 kW VFD. The reactor symbol — three coil loops in the ANSI style, or a labelled rectangle in IEC style — is drawn on all three phases of the three-phase supply. A note beside the symbol reads '4% ACL, 100 A, 400 V, 50 Hz', documenting the reactor specification for installation and maintenance personnel.
Key facts
- The Current Limiting Reactor symbol is a coil/arc symbol (ANSI/IEEE) or rectangle (IEC) inserted in series in a circuit; it represents an inductor that limits the rate of current rise and peak fault current using its inductive reactance XL = 2πfL measured in ohms (Ω).
- A current limiting reactor is a purely passive component — it stores energy in a magnetic field and returns it; it does not consume power except for resistive winding losses (typically < 0.5% of rated power).
- Pins on this symbol: In (x=0 y=10) — line/source side; Out (x=60 y=10) — load side. The reactor is always wired in series in the circuit.
- Reactor impedance in power engineering is often specified in percent (%) on a rated kVA base: a 5% reactor means the voltage drop across it equals 5% of the system voltage at rated current, corresponding to an inductive reactance of (0.05 × V²) / kVA ohms.
- Air-core reactors are used for fault current limiting because their inductance is constant regardless of current magnitude (no iron saturation); iron-core (gapped) reactors are used for harmonic filtering where cost and size are priorities.
- Adding a line reactor to the input of a VFD reduces the total harmonic current distortion (THDi) from typically 80–100% (without reactor) to 35–45%, significantly reducing the harmonic impact on the supply bus and extending drive and motor life.
- IEC 60076-6 defines the reactor impulse withstand voltage, overload capability (typically 1.5× rated current for 30 seconds), and temperature rise limits; IEEE C57.16 covers the equivalent North American requirements.
- Current limiting reactors are distinguished from passive harmonic filters: a reactor alone reduces harmonics but does not eliminate them; a harmonic filter combines the reactor with capacitors and resistors tuned to specific harmonic frequencies (5th, 7th, etc.).
Frequently asked questions
What does the current limiting reactor symbol look like in a schematic?
In ANSI/IEEE 315 style the current limiting reactor symbol is a series of curved arcs or loops (a coil) drawn along the circuit line — identical to the standard inductor symbol. In IEC 60617 style it is a rectangle labelled 'L' or 'REACTOR'. Both represent a series inductor with two terminals labelled In (source side) and Out (load side). Iron-core variants show two parallel lines below or beside the coil or rectangle.
What does a current limiting reactor do in a circuit?
A current limiting reactor limits the rate of current rise (di/dt) and the peak current magnitude in a circuit by inserting inductive impedance in series. During normal operation the voltage drop is small (2%–5%). During a fault or motor start, the large di/dt causes the reactor to develop a large opposing voltage that limits current to a lower, safer peak value, protecting switchgear, motors, and downstream equipment.
What is the difference between a line reactor and a current limiting reactor?
Line reactor and current limiting reactor are two common names for the same device — a series inductor inserted in a power circuit. 'Line reactor' is the typical term used for reactors on VFD input circuits; 'current limiting reactor' is used in power systems for bus-tie and feeder protection applications. Both operate on the same inductive impedance principle.
What is reactor impedance in percent and why is it used?
Reactor impedance in percent (%) expresses the voltage drop across the reactor at rated current as a fraction of the system rated voltage, multiplied by 100. For example, a 5% reactor on a 400 V, 100 A circuit drops 5% × 400 V = 20 V at 100 A. Percent impedance is used in power engineering because it is independent of voltage level, making it easy to compare reactors across different voltage systems and to calculate fault current reduction.
Does a current limiting reactor reduce harmonics from a VFD?
Yes. A line reactor installed on the AC input of a VFD reduces the total harmonic current distortion (THDi) from typically 80–100% (bare VFD) to 35–45% by increasing the supply impedance seen by the VFD's rectifier and smoothing the pulsed input current waveform. An output reactor on the VFD's motor output reduces the voltage rise rate (dv/dt) to protect motor winding insulation.
What standard governs current limiting reactors?
IEC 60076-6 (Power transformers — Part 6: Reactors) is the primary international standard for series reactors including current-limiting types. IEEE C57.16 covers dry-type air-core series reactors for North American applications. IEC 60617-02 and ANSI Y32.2 / IEEE 315-1975 define the schematic symbols.
What is the difference between an air-core and iron-core current limiting reactor?
An air-core reactor has no magnetic material in its core; its inductance is constant and independent of current magnitude (linear), making it ideal for fault current limiters where the inductance must remain constant even during large fault currents. An iron-core (gapped) reactor uses a ferromagnetic core with an air gap to prevent saturation; it achieves higher inductance in a smaller package but can saturate at very high overcurrents, reducing its limiting effect.
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