Here is brief explanation of Line Reactors that I borrowed from the Lenze web site, it's better than my explanation would be:
"An input or line reactor helps protect a variable frequency drive (VFD) from input power line disturbances that couldcause nuisance tripping or damage to the drive. An input (line) reactor also reduces the harmonics that the VFD generates back onto the line. Line reactors are sized based on the HP and voltage ratings of the drive in use. Input (line) reactors should be used in these circumstances:
- The input line power is prone to have disturbances such as surges, spikes, transients, etc.
- The supply line power is very stiff (greater than 10 times the kVA rating of the connected VFD).
- Where harmonic distortion is a concern. (IEEE-519 Harmonic Control in Electrical Power Systems)"
My further thoughts - While line reactors can reduce the noise reflected back into your shop/house electrical system, they are primarily used to lessen the inrush current to the drive on startup. When first powered on the drive can "appear" to be a short circuit to the supplying electrical system while it's capacitors charge, this can result in a large in-rush spike that "can" damage a drive. In my experience line reactors on larger motors are good protection in an industrial environment where the electrical system has the potential to momentarily supply hundreds or thousands of amps to the input of the drive. Industrial electrical systems can be very "dirty" as well so the reactor smooths out the fluctuation on the supply.
It's less of an issue with smaller VFD's that we might use in a home shop (5hp or less) because our available current potential is less. It's limited by our incoming utility transformer size. All that being said, there is no reason not to install a line reactor if you can source one for a reasonable price. It can't hurt anything and it may help as well.
There are also "load" reactors as well. These go between the VFD and motor and again from the Lenze site:
"An output (load) reactor, on the other hand, is used to protect the motor if the wiring distance between the VFD and motor is very long. The drive generates a high frequency PWM three-phase output and noise spikes are generated on the leading edge of these signals. These noise spikes get amplified due to the long cable lengths and the additional capacitance of the cable. The resulting voltage can exceed the motor’s peak voltage rating where insulation breakdown occurs. The general rule of thumb is that an output reactor should be used if the motor wiring is over 100 feet, but this value varies depending on the motor. If the motor meets the NEMA MG-1 Part 31 standard, it is possible to have as much as 300 ft of cabling without a reactor. If it does not meet the standard, the maximum cable length should be 100 ft. Also, if the distance is between 300 and 500 ft, a load reactor must be installed"
As you can see we home shop guys don't need load reactors as our wiring distances are typically very short.