Variable Frequency Drives convert AC to DC and then from the DC creates an output voltage at an adjustable frequency to run the load motor at whatever speed you want from 0 to 60 Hz. The voltage generally varies from 0 to full voltage as the speed goes from 0 to 60 Hz. They can also go above 60 Hz up to 400 Hz, but the torque falls off as the speed increases.
A VFD is a device that changes the frequency that an AC motor sees and thus the speed. Frequency determines the speed of AC motors and changing it changes the speed:
RPM = 2 x 60 x f / P
f = system frequency (60 Hz in US, 50 Hz in Europe)
P = no. of poles wound in the motor
60 is a conversion from sec. to minutes
2 is for the N and S of each pole
A VFD is an electronic device that allows an off the shelf NEMA T frame motor to be operated in one or more of the four quadrants. It can impose a signal on an induction motor for the purpose of controlling, its speed, torque, rate of acceleration-deceleration. The purpose of a control system is to balance the supply of energy with the demand for that energy. A VFD provides the ability to work like that. In addition to saving energy, a VFD also has been proven to reduce maintenance cost and improve product quality.
You can select VFDs based on Voltage and Power, for example 480 V ac and 50 hp. Some VFD families are designed for working on constant torque loads such as conveyors and other mechanical movement. Other VFDs are designed for working on variable torque loads such as centrifugal pumps and fans. The main difference between the two is the available starting torque and the amount of overload capability.
VFD has advantages in saving power for situations where you need to control a variable flow rate such as pumps and fans. Instead of cycling motors on and off, you can smoothly adjust the speed, avoiding the need for dampers or other power dissipating devices.
One more good thing about the VFD is that it is possible to design the motor and the VFD to meet application requirements. Example, a motor to drive a winding paper machine. This machine requires the motor to work as constant torque up to base speed (rated volts and frequency) and above base speed up to top speed as constant HP (constant volts). So, a 480V 60Hz 4P induction motor, as example, can be designed per the requirements of the VFD, let's say, constant torque up to 480V/30Hz (900 rpm) and above that frequency to up to 1800 rpm 480V/60Hz constant HP. There are many occasions when this is done, especially in paper mill and steel mill applications.