Today the VFD is perhaps the most common kind of output or load for a control program. As applications are more complicated the VFD has the capacity to control the rate of the electric motor, the direction the electric motor shaft is turning, the torque the electric motor provides to a load and any other motor parameter that 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 exceptionally versatile device that not only controls the speed of the electric motor, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs also provide methods of braking, power improve during ramp-up, and a number of controls during ramp-down. The largest financial savings that the VFD provides is that it can ensure that the electric motor doesn’t pull extreme current when it begins, so the overall demand aspect for the entire factory could be controlled to keep carefully the domestic bill only possible. This feature only can provide payback more than 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) if they are beginning. When the locked-rotor amperage happens 
across many motors in a manufacturing facility, it pushes the electrical demand too high which frequently outcomes in the plant paying a penalty for all the electricity consumed during the billing period. Because the penalty may end up being just as much as 15% to 25%, the savings on a $30,000/month electric expenses can be utilized to justify the purchase VFDs for practically every engine in the plant actually if the application form may not require working at variable speed.
This usually limited the size of the motor that may be controlled by a frequency and they weren’t commonly used. The earliest VFDs utilized 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 make different slopes.
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