This is a good article. Follow the link for variable voltage variable frequency drive pdf information.
VFDs are used in applications ranging from small appliances to large compressors. VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software. Motors that are designed for fixed-speed operation are often used. AC line input to AC inverter output. AC voltage output using the inverter’s active switching elements. Controller advances have exploited dramatic increases in the voltage and current ratings and switching frequency of solid-state power devices over the past six decades.
VFDs as an inverter switching device. Hz control is sub-optimal in high-performance applications involving low speed or demanding, dynamic speed regulation, positioning, and reversing load requirements. Hz and above rated nameplate speed. Wound-rotor synchronous motors and induction motors have much wider speed range. Wound-rotor synchronous motors can be run at even higher speeds. The mechanical strength of the rotor limits the maximum speed of the motor. VFD, motor, and driven equipment.