Today the VFD is perhaps the most common type of result or load for a control program. As applications become more complex the VFD has the ability to control the rate of the electric motor, the direction the motor shaft is definitely turning, the torque the motor provides to a load and any other electric motor parameter which can be sensed. These VFDs are also available 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 only controls the speed of the engine, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs also provide ways of braking, power increase during ramp-up, and a number of settings during ramp-down. The biggest cost savings that the VFD provides can be that it can ensure that the engine doesn’t pull excessive current when it begins, so the overall demand element for the entire factory can be controlled to keep the utility bill as low as possible. This feature only can provide payback in excess of the price of the VFD in under one year after buy. It is important to keep in mind that with a traditional 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 frequently outcomes in the plant paying a penalty for all the electricity consumed through the billing period. Because the penalty may end up being as much as 15% to 25%, the savings on a $30,000/month electric bill can be used to justify the purchase VFDs for virtually every 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 may be controlled by a frequency plus they were not commonly used. The earliest VFDs used linear amplifiers to control 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 produce different slopes.
Are you thinking about Variable Speed Gear Motor?