Today the VFD could very well be the most common type of output or load for a control system. As applications become more complicated the VFD has the capacity to control the acceleration of the engine, the direction the motor shaft is turning, the torque the engine provides to a load and any other motor parameter that can be sensed. These VFDs are also obtainable in smaller sized sizes that are cost-effective 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 also provide methods of braking, power enhance during ramp-up, and a number of controls during ramp-down. The biggest financial savings that the VFD provides is that it can ensure that the engine doesn’t pull excessive current when it begins, so the overall demand aspect for the entire factory can be managed to keep the domestic bill as low as possible. This feature only can provide payback more than the price of the VFD in less than one year after buy. It is essential to remember 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 facility, it pushes the electrical demand as well high which often results in the plant having to pay a penalty for all of the electricity consumed through the billing period. Since the penalty may become as much as 15% to 25%, the financial savings on a $30,000/month electric expenses can be utilized to justify the buy VFDs for practically every electric motor in the plant also if the application may not require operating at variable speed.
This usually limited how big is the motor that could be controlled by a frequency plus they were not commonly used. The earliest VFDs utilized linear amplifiers to control all areas of the VFD. Jumpers and dip switches were used provide 
ramp-up (acceleration) and ramp-down (deceleration) features by switching bigger or smaller sized resistors into circuits with capacitors to make different slopes.
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