Today the VFD is perhaps the most common type of output or load for a control program. As applications are more complicated the VFD has the ability to control the speed of the motor, the direction the engine shaft is turning, the torque the motor provides to a load and any other engine parameter which can be sensed. These VFDs are also available in smaller sized 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 merely controls the speed of the motor, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs provide methods 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 begins, 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 cost 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 plant, 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 expenses 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 how big is the motor that may be managed by a frequency and they weren’t commonly used. The earliest 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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