VFD means Variable Frequency Drive, by using VFD you can use AC motors on highly controlled way. VFDs slightly adjust frequency based on given parameters and AC motors are able to achieve even extreme options for example zero speed at max torque (brake operation mode). AC motors are much less sensitive than DC ones so using VFD and AC motors after SCR and DC motors is a really joy.
Applications of VFD are becoming very common and continually get benefits from it. Basically the applications of VFD with SCIM are replacing the DC motors and drives. Induction motors are much less expensive than DC motors but they operate in fixed speeds. With the VFDs they can do the work used to be done by DC motors (variable speed application). There are many advantages of applying VFDs with induction motors.
VFD has lots of advantages, but they are not all the gold and glitter that some make out them to be, as they have their disadvantages too. They do make motors "smart", but some of their Achilles Heels have to absolutely be made aware of, understood, and mitigated, or else havoc can ensue! Harmonics are the most important issues that have to be dealt with, and can be easily remedied if proper filter/reactors are used. Another disadvantage is SCR's in general do not like is the use of capacitors, such as power factor correction capacitors used at the same time within the same grid. This will make you crazy with harmonic issues and damage equipment if not careful.
Another issue I've ever met is the Brake Chopper cooling fan, which a really tricky fault as no fault code the VFD unit is shutting down time by time because of the fault of Brake Chopper. Brake Chopper is a drive, which controls feedback energy in case of generator mode of AC motors and transfers this extra energy away for Brake Resistor where it is gonna be dissipated then. As far as I know VFD never works alone it needs some auxiliary equipment for the sake of proper operation and get the maximum benefits from it.
The major limitation is the cost of the VFDs which are becoming cheaper with further developments. I have some nice brochures about the benefits of VFDs and induction motors that I can share with you if you give me your email. I have good experience with motors design and application since I worked as design and application engineer for a large motor company for more than 10 years.