Today the VFD could very well be the most common kind of output or load for a control program. As applications are more complex the VFD has the capacity to 
control the speed of the motor, the direction the motor shaft is usually turning, the torque the engine provides to lots and any other motor parameter that can be sensed. These VFDs are also available in smaller sized sizes that are cost-efficient and take up much 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 engine, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs provide methods of braking, power increase during ramp-up, and a number of handles during ramp-down. The biggest financial savings that the VFD provides can be that it can ensure that the engine doesn’t pull extreme current when it starts, therefore the overall demand factor for the whole factory could be controlled to keep the domestic bill as low as possible. This feature alone can provide payback in excess of 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) if they are beginning. When the locked-rotor amperage occurs across many motors in a manufacturing facility, it pushes the electrical demand too high which frequently results in the plant having to pay a penalty for all the electricity consumed through the billing period. Since the penalty may become just as much as 15% to 25%, the financial savings on a $30,000/month electric expenses can be used to justify the purchase VFDs for virtually 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 controlled by a frequency plus they were not commonly used. The initial 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 sized resistors into circuits with capacitors to produce different slopes.
Automatic frequency control consist of an primary electrical circuit converting the alternating electric current into a immediate current, then converting it back into an alternating electric current with the required frequency. Internal Variable Speed Gear Motor energy reduction in the automatic frequency control is ranked ~3.5%
Variable-frequency drives are widely used on pumps and machine tool drives, compressors and in ventilations systems for huge buildings. Variable-frequency motors on enthusiasts save energy by permitting the volume of surroundings moved to complement the system demand.
Reasons for employing automatic frequency control may 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 usually matched either to quantity 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 already been the innovation which has brought the utilization of AC motors back to prominence. The AC-induction electric motor can have its speed transformed by changing the frequency of the 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 speed. If the frequency is increased above 50 Hz, the electric motor will run faster than its rated quickness, and if the frequency of the supply voltage is usually significantly less than 50 Hz, the electric motor will run slower than its rated speed. According to the adjustable frequency drive working basic principle, it is the electronic controller particularly designed to alter the frequency of voltage provided to the induction engine.