Today the VFD is perhaps the most common type of output or load for a control system. As applications become more complicated the VFD has the ability to control the velocity 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 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 engine, but protects against overcurrent during ramp-up and ramp-down conditions. Newer VFDs provide methods of braking, power enhance during ramp-up, and a number of controls during ramp-down. The biggest cost savings that the VFD provides can be that it can ensure that the electric motor doesn’t pull excessive current when it begins, so the overall demand factor for the whole factory could be controlled to keep carefully the utility bill as low as possible. This feature alone can provide payback in excess of the price of the VFD in under one year after buy. It is important to keep in mind that with a normal motor starter, they will draw locked-rotor amperage (LRA) if they are starting. When the locked-rotor amperage occurs across many motors in a manufacturing plant, it pushes the electrical demand too high which frequently results in the plant having to pay a penalty for all the electricity consumed during the billing period. Since the penalty may become as much as 15% to 25%, the savings on a $30,000/month electric expenses can be utilized to justify the purchase VFDs for virtually 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 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 generate different slopes.

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