power electronics by bhimbra
U can start picking up by triggering triacs, scrs.Controlling the triggering angle of an AC voltage with those.
Add microcontroller to that circuit and implement the same.
Then get into motors.
VFD is a vast area and the technology is booming.
There are lots and lots of techniques and things implemented in VFD.
VFD's are used where the motor speed,torque etc are to controlled ina most efficient way.
Say for eg:- spinning mills,washing machines,a/c,other automated machines etc..
start witha discrete VFD drive and then go with the integrated ones.
Look into the sites of international rectifier,infineon,sanyo semiconductors etc.They have quite lot of application notes on VFD's
gudluk