Fleeter1 made some valid points, however, here is when I would recommend the use of a line reactor, particularly on new installations:
1. As fleeter1 mentioned, if the input line power is prone to have disturbances such as surges, spikes, transients, etc.
2. When the supply line power is very stiff (greater than 10 times the kVA rating of the connected VFD) – to minimize damage to the drive under supply transformer faults.
3. When harmonic distortion is a concern – an input line reactor reduces harmonics that the VFD generates back onto the line.
3% impedance line reactors can be used to reduce power line transient voltages caused by capacitor switching, line notching, DC Bus over-voltage tripping and inverter over-current and over-voltage conditions. They improve input power factor and reduce cross-talk between drives.
5% impedance line reactors have the same benefits plus provide maximum harmonic reduction without added capacitance. The harmonic signals generated by the VFD could produce distortion levels that may not be acceptable for high frequency or noise sensitive equipment.
My two cents!