As far as I know they are only used to ensure a clean power supply from the Utility to the VFD. If you are installing a drive in a plant where there are very large power consumption changes happening frequently it may be a good idea to use a line reactor to protect the drive from other equipment in the plant that may cause nuisance drive faults or even trips. Most VFD spec’s call for or recommend a line reactor be installed with the VFD but if they were that important would they not integrate them into the drive itself?? The drive manufacturers that I have dealt with still warrant VFD’s that are not protected by line reactors but they do try and falsely accuse infrequent drive trips on not having a clean power source. The biggest problems I normally have with using line reactors is the increased panel space required (or external location) and the cost which is normally about 1/4 the cost of the drive itself.
I use the following criteria when determining whether or not to install reactors:
New or existing installation? New install panel can be designed to accommodate the reactors, existing may or may not have room for them.
How critical is the application? Can you afford down time, are you running them 24/7 where they might trip at 3AM in a power surge or brown-out condition that no-one is around to witness. Is there a back up system? Do you have a spare drive on hand? Are there several drives from the same power source or something to prove it is not bad power causing trips.
Do you have the extra cash to purchase the reactors?
My personal opinion is that they are definitely not required for every application and seem to be a bit of a scapegoat and money grab item and in applications under 20HP, they are typically not required.
On a side note, if you are putting a drive on an existing application, I would be much more concerned on the motor you are applying the drive to. Is it inverter rated? What is the minimum speed the existing motor / load can handle. Self cooled motors and pumps can not always take being run for a long time at slow speeds.
Hope this helps!