AC line reactor for a variable-frequency drive — why it is often not an optional extra

The guide on harmonics and THD covers the mains pollution caused by non-linear loads, including variable-frequency drives. This article covers a concrete component that limits that pollution at the source: the AC line reactor, placed between the mains and the input rectifier of a variable-frequency drive.

The problem: a rectifier does not draw sinusoidal current

The input stage of a variable-frequency drive is typically a diode rectifier that converts the mains voltage to a DC voltage on the DC bus, after which a large electrolytic capacitor stabilises that DC voltage. Without any impedance between the mains and that rectifier, the capacitor only charges around the peaks of the mains sine wave, in short, high current pulses — a strongly non-sinusoidal, harmonic-rich current waveform that the drive draws from the mains (see also the guide on K-factor and measuring harmonics).

What the line reactor does

A line reactor is a simple choke placed in series with the drive's mains connection (an impedance of typically a few percent, usually expressed as %impedance relative to the rated current). This impedance:

significantly reduces the harmonic pollution (THDi) the drive feeds back into the mains, by flattening the current pulses and spreading them over a wider part of the sine period;
limits the inrush current into the DC bus capacitor when the drive is switched on — without a line reactor, this inrush can be a multiple of the rated current, adding extra stress to cabling, fuses and the rectifier diodes themselves;
damps mains-side voltage transients (for example from switching operations elsewhere in the installation) before they reach the drive's sensitive input electronics, thereby also indirectly protecting against premature failure of the rectifier diodes and the DC bus capacitor itself.

Note: a line reactor is not the same as a motor reactor or a sine-wave filter on the output side of the drive (see the guide on motor cable length, reflection and sine-wave filters): those address the effects of the rapidly switching output voltage on the motor cable and motor winding — an entirely different physical problem from the mains-side harmonic pollution and inrush that a line reactor addresses.

When this becomes relevant

Multiple variable-frequency drives on the same circuit or transformer: the combined harmonic pollution of several drives without line reactors can exceed the permitted THDi limits at the point of connection, which a grid operator may act on. IEC 61800-3 sets requirements for the electromagnetic emission of variable-frequency drives, and a line reactor is a common measure to meet those requirements.
A relatively stiff mains supply (low mains impedance): the stiffer the mains (i.e. the lower its own impedance), the higher the peak current an unreactored rectifier draws — a line reactor deliberately adds impedance that the mains itself does not provide.
An environment with frequent switching transients (for example near large capacitor banks or circuit breakers that switch regularly), where the line reactor also protects the drive itself.

Practical relevance

When specifying a new variable-frequency drive installation, or in case of repeated, unexplained failure of rectifier diodes or the DC bus capacitor of an existing drive, it is worth checking whether a line reactor has been fitted and, if not, consulting the manufacturer's specification for the recommended %impedance for the given mains stiffness and drive power. Many drive manufacturers make a line reactor a mandatory warranty condition for certain power ratings and mains configurations.

Common mistakes

Treating a line reactor as an optional accessory rather than a functional component that limits both mains pollution and inrush stress — with multiple drives on the same supply, its absence can lead to cumulative THDi problems.
Confusing a line reactor with a motor reactor or sine-wave filter — these sit on different sides of the drive and solve different problems.
Selecting too low a %impedance for a particularly stiff mains supply, so that the intended damping of harmonics and inrush has insufficient effect.
Attributing repeated diode or capacitor failure on the input side of a drive to a manufacturing defect, without considering the absence of a line reactor as a possible underlying cause.