What is the function of VFD?

VFD is a deveice used to control the speed of an AC motor by controlling the frequency of the AC input. It consist mainly three parts, they are Rectifier,DC bus and Inverter. 

operate and protect and control Ac Motors 

Control the motor speed and torque as well

The VFD converts a basic fixed-frequency, fixed voltage sine-wave power (line power) to a variable- frequency, variable-voltage output used to control speed of induction motors. A basic block diagram of this process is depicted in Fig. 1. Primary function of a VFD in aquatic applications is to provide energy savings.

Control the speed of the driving Machines(Motor and Generator) automatically due the frequency and voltage.

To control the speed of the AC motor we use VFD and it control the speed by changing frequency and voltage.

VFD Variable Frequency Drives a devices use to control the speed of AC motors by varying the frequency and configure the VFD base on the name plate of the AC motor. 

VFD is not the proper term. Variable Frequency Drives are best described as Adjustable Frequency Drives (AFD) as they are intended to hold the speed of an AC motor as set by the control program or operator.

Using vfd ,,,process control

Smooth operation..

Acceleration and deceleration control..

Allow accurate position..

Saving energy.

TO CONTROL THE MOTOR SPEED AND TORQUE AND USED ALSO IN CLOSED PROCESS TO FULL CONTROL

Thanks Brother for you input (Correct)