reactor

A reactor is a coil of wire wound around an iron or air core that introduces inductance into an electrical circuit. Its primary function is to oppose changes in current flow. Unlike a resistor, which dissipates energy as heat, a reactor stores energy in a magnetic field and releases it back into the circuit. In industrial settings, reactors are essential for controlling current surge, smoothing voltage fluctuations, and filtering unwanted harmonics generated by nonlinear loads.

Reactors are classified by their core material and circuit position. Iron-core reactors have high inductance in a compact form and are used where space is limited; air-core reactors have lower inductance but better thermal characteristics and no saturation effects. Series reactors are placed in line with the load to limit inrush current during motor starting or to suppress harmonics. Shunt reactors connect between phases or between phase and ground to provide reactive power compensation and harmonic absorption.

Common applications and failure modes

In variable frequency drives (VFDs) and soft starters, DC link reactors smooth the DC bus voltage and reduce conducted emissions. AC line reactors upstream of rectifiers limit current distortion. Motor starting reactors gradually increase current when large induction motors are energized, preventing voltage dips that could trip contactors or brown out nearby equipment. Harmonic reactors tuned to specific frequencies (typically 5th, 7th, or 11th) work alongside capacitors to trap harmonics and prevent their propagation into the grid.

Reactor failures typically stem from insulation breakdown under sustained overvoltage, winding open circuits from thermal cycling, or core lamination separation from vibration. Overheating occurs when a reactor's thermal rating is exceeded; industrial reactors are rated in kilovolt-amperes reactive (kVAR) and must dissipate I²R losses continuously. Audible buzzing, a sign of loose laminations, indicates imminent mechanical failure.

The term 'reactor' comes from the device's reactive behavior in AC circuits: it reacts to current changes by inducing a back-voltage proportional to the rate of change (V = L × dI/dt). This distinguishes it from a transformer, which transfers power between windings, or a choke, a term often used interchangeably for reactors but typically reserved for lower-inductance filtering coils. Specification sheets list inductance in millihenries (mH) and rated current in amperes (A).