Today the VFD could very well be the most common kind of output or load for a control system. As applications are more complicated the VFD has the capacity to control the rate of the electric motor, the direction the engine shaft is usually turning, the torque the electric motor provides to lots and any other electric motor parameter which can be sensed. These VFDs are also obtainable in smaller sizes that are cost-effective and take up much 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 motor, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs provide ways of braking, power improve during ramp-up, and a number of handles during ramp-down. The largest savings that the VFD provides is that it can ensure that the motor doesn’t pull extreme current when it 
begins, therefore 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 less than one year after buy. It is important to keep in mind that with a traditional motor starter, they will draw locked-rotor amperage (LRA) if they are starting. When the locked-rotor amperage happens across many motors in a manufacturing plant, it pushes the electric demand too high which often outcomes in the plant paying a penalty for all of the electricity consumed during the billing period. Since the penalty may be just as much as 15% to 25%, the savings on a $30,000/month electric expenses can be utilized to justify the buy VFDs for practically every motor in the plant actually if the application may not require operating at variable speed.
This usually limited how big is the motor that may be managed by a frequency and they weren’t commonly used. The initial VFDs used 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 larger or smaller resistors into circuits with capacitors to develop different slopes.
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