Line Reactors are a simple and low cost means of increasing the source impedance for an individual load (such as a variable frequency drive, or an adjustable speed DC motor drive). They are connected in series with a harmonic producing load, such as an adjustable speed drive. By increasing source impedance, the magnitude of harmonic distortion can be reduced for the load on which the reactor is added. However, the lowest values of Total Harmonic Distortion (current) normally attained using line reactors are about 25% to 45% THD-i depending on the total input circuit impedance.
Line reactors are always recommended for drive systems because they reduce harmonics right at the input to the drive, so they will help to reduce the cost of any upstream harmonic filtering that must take place. In the case of Thyristor (SCR) rectifiers, the line reactor will reduce the voltage notching typically associated with SCRs. Additionally, line reactors will protect voltage source inverters (typical VFDs) from transient over voltages which may otherwise cause either nuisance (OV) tripping of the VFD or damage to the input rectifiers.