A VFD is shortened for variable frequency drive it's a sort of AC drive which control the speed of the ac motors depending on frequency change. VFD is very useful in industry today, but much thought to the application must be taken. For example, if you are controlling motor speeds at or below 50% to 75% (and maintaining them in this range), you may want to consider changing motors to a different RPM. VFD produces harmonics that can have an adverse effect on the electric motor and also the utility supply power. i.e. if you have an 1800 RPM motor, but are running it 1200 RPM, it would be more cost effective to purchase a 1200 RPM motor and use a magnetic starter.
The other issue to consider is harmonic filtering. The utility does not like to see harmonics being produced from the VFD and put back onto the power source. Refer to IEEE 519 for acceptable limits and practices. Some VFDs are 24 pulse systems, so they do not require harmonic filtering, but this is another question to ask the manufacturer.
Line reactors may be something to consider, if there are surges in power from the utility company and also look at the ride through times/levels for voltage sags. Our plant is at the end of the grid and we are subject to voltage sags, so we buy VFD's that can ride through a 50% sag for 12 cycles (0.2 seconds), then a 25% sag for 18 cycles (0.3 seconds) and so on. If the VFD is not capable of riding through some sags like this, you will have nuisance trips.
VFDs can be very helpful in saving energy and controlling process, but there are a lot of things to consider before buying or upgrading.