What device is installed on power lines feeding VFDs to lessen harmonics?

• A. Line reactors
• B. Fuses
• C. Capacitors
• D. Transformers

Answer: (A)

Harmonics on a VFD supply come from the input rectifier, which draws current in pulses and creates high-frequency components on the line. A line reactor is an in-series inductive device that adds impedance to the supply path. This impedance slows the current changes, smooths the pulsed input waveform, and damps switching transients, which reduces both the peak harmonic content and the overall distortion seen on the line. In short, it filters the high-frequency components the VFD generates and protects the line from sharp current spikes, improving power quality.

Fuses protect against fault conditions but don’t address harmonic currents. Capacitors are used for power factor correction and can even worsen resonance or introduce or amplify harmonics if placed incorrectly in front of a VFD. Transformers can isolate or step voltage and may affect voltage quality, but they don’t inherently suppress the harmonics in the same straightforward way a line reactor does. So the device designed to lessen harmonics on lines feeding VFDs is the line reactor.