In electrical engineering, reactance is the imaginary element of impedance, and it is caused by the presence of inductors or capacitors in the circuit, which cause a phase difference between voltage and current.

In practice, the term reactance is used as a synonym for inductive impedance; i.e. an inductive electrical component (inductor) produced using a coil which has either an air core or is wound around a magnetic core.

Reactors are employed in various fields, from electronics to electrical engineering, for a wide range of uses. The most common uses in industrial applications are: for current and voltage harmonic filtering systems, rectified current smoothing, adapter circuits, resonant circuits, limitation of inrush current, electric motor start-up systems.

E.F.C. designs and builds reactors based on current, voltage, and frequency of use requirements.