Today the VFD is perhaps the most common type of output or load for a control system. As applications become more complicated the VFD has the ability to control the acceleration of the electric motor, the direction the electric motor shaft is turning, the torque the motor provides to a load and any other motor parameter which can be sensed. These VFDs are also obtainable in smaller sized sizes that are cost-efficient and take up less space.
The arrival of advanced microprocessors has allowed the VFD works as an exceptionally versatile device that not only controls the speed of the motor, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs also provide methods of braking, power enhance during ramp-up, and a number of handles during ramp-down. The largest financial 
savings that the VFD provides is definitely that it can make sure that the electric motor doesn’t pull excessive current when it begins, therefore the overall demand aspect for the whole factory could be controlled to keep carefully the utility bill only possible. This feature only can provide payback in excess of the cost of the VFD in under one year after purchase. It is important to keep in mind that with a normal motor starter, they will draw locked-rotor amperage (LRA) when they are beginning. When the locked-rotor amperage takes place across many motors in a manufacturing plant, it pushes the electrical demand too high which often results in the plant spending a penalty for every one of the electricity consumed during the billing period. Because the penalty may become just as much as 15% to 25%, the savings on a $30,000/month electric costs can be utilized to justify the buy VFDs for virtually every electric motor in the plant even if the application form may not require working at variable speed.
This usually limited how big is the motor that could be controlled by a Variable Drive Motor frequency plus they were not commonly used. The initial VFDs used linear amplifiers to regulate all aspects of the VFD. Jumpers and dip switches were used provide ramp-up (acceleration) and ramp-down (deceleration) features by switching larger or smaller resistors into circuits with capacitors to create different slopes.
Automatic frequency control consist of an primary electric circuit converting the alternating current into a direct current, then converting it back into an alternating electric current with the required frequency. Internal energy reduction in the automatic frequency control is rated ~3.5%
Variable-frequency drives are widely used on pumps and machine device drives, compressors and in ventilations systems for large buildings. Variable-frequency motors on fans save energy by permitting the volume of atmosphere moved to match the system demand.
Reasons for employing automatic frequency control can both be linked to the features of the application form and for saving energy. For example, automatic frequency control can be used in pump applications where the flow is matched either to volume or pressure. The pump adjusts its revolutions to confirmed setpoint via a regulating loop. Adjusting the circulation or pressure to the actual demand reduces power usage.
VFD for AC motors have been the innovation which has brought the use of AC motors back to prominence. The AC-induction electric motor can have its quickness changed by changing the frequency of the voltage used to power it. This means that if the voltage put on an AC electric motor is 50 Hz (used in countries like China), the motor works at its rated speed. If the frequency can be increased above 50 Hz, the engine will run quicker than its rated speed, and if the frequency of the supply voltage is significantly less than 50 Hz, the engine will operate slower than its ranked speed. Based on the adjustable frequency drive working theory, it’s the electronic controller specifically designed to alter the frequency of voltage supplied to the induction motor.