Today the VFD is perhaps the most common kind of output or load for a control program. As applications become more complicated the VFD has the capacity to control the rate of the engine, the direction the electric motor shaft is usually turning, the torque the engine provides to lots and any other electric motor parameter that can be 
sensed. These VFDs are also obtainable in smaller sizes that are cost-efficient and take up much 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 provide ways of braking, power boost during ramp-up, and a variety of controls during ramp-down. The largest financial savings that the VFD provides is certainly that it can make sure that the electric motor doesn’t pull excessive current when it starts, therefore the overall demand factor for the entire factory can be controlled to keep carefully the domestic bill only possible. This feature by itself can provide payback in excess of the price of the VFD in less than 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 electric demand too high which frequently results in the plant spending a penalty for every one of the electricity consumed during the billing period. Since the penalty may end up being as much as 15% to 25%, the financial savings on a $30,000/month electric bill can be utilized to justify the purchase VFDs for practically every electric motor in the plant also if the application may not require functioning 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 control all aspects 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 create different slopes.
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