Today the VFD could very well be the most 
common type of output or load for a control system. As applications become more complex the VFD has the capacity to control the rate of the engine, the direction the electric motor shaft is certainly turning, the torque the engine provides to a load and any other engine parameter that can be sensed. These VFDs are also obtainable in smaller sizes that are cost-effective and take up less space.
The arrival of advanced microprocessors has allowed the VFD works as an extremely versatile device that not merely controls the speed of the electric motor, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs also provide methods of braking, power boost during ramp-up, and a variety of regulates during ramp-down. The biggest financial savings that the VFD provides is usually that it can make sure that the electric motor doesn’t pull extreme current when it begins, so the overall demand factor for the whole factory could be controlled to keep carefully the utility bill as low as possible. This feature only can provide payback more than the price 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 starting. When the locked-rotor amperage happens across many motors in a manufacturing facility, it pushes the electrical demand too high which often results in the plant having to pay a penalty for all of the electricity consumed during the billing period. Since the penalty may become just as much as 15% to 25%, the cost savings on a $30,000/month electric costs can be used to justify the buy VFDs for practically every engine in the plant actually if the application form may not require working at variable speed.
This usually limited the size of the motor that could be managed by a frequency and they were not commonly used. The earliest VFDs utilized linear amplifiers to regulate all areas of the VFD. Jumpers and dip switches were utilized provide ramp-up (acceleration) and ramp-down (deceleration) features by switching larger or smaller resistors into circuits with capacitors to generate different slopes.
Automatic frequency control contain an primary electrical circuit converting the alternating current into a direct current, then converting it back to an alternating current with the required frequency. Internal energy reduction in the automated frequency control is rated ~3.5%
Variable-frequency drives are widely used on pumps and machine device drives, compressors and in ventilations systems for huge buildings. Variable-frequency motors on enthusiasts save energy by allowing the volume of air moved to match the system demand.
Reasons for employing automatic frequency control can both be related to the functionality of the application and for conserving energy. For example, automatic frequency control is used in pump applications where the flow is usually matched either to volume or pressure. The pump adjusts its revolutions to a given setpoint with a regulating loop. Adjusting the flow or pressure to the real demand reduces power intake.
VFD for AC motors have been the innovation that has brought the utilization of AC motors back into prominence. The AC-induction motor can have its rate transformed by changing the frequency of the Variable Drive Motor voltage used to power it. This means that if the voltage applied to an AC motor is 50 Hz (found in countries like China), the motor functions at its rated quickness. If the frequency is definitely improved above 50 Hz, the engine will run quicker than its rated velocity, and if the frequency of the supply voltage is definitely less than 50 Hz, the motor will run slower than its rated speed. According to the adjustable frequency drive working theory, it is the electronic controller particularly designed to alter the frequency of voltage supplied to the induction engine.