VFD is by far the most effective type of speed control, especially when used with centrifugal pumps and fans. When VFD is applied to a centrifugal pump or fan, the load characteristics are subject to the "affinity law". These laws show that speed is proportional to current. Torque is proportional to the square of the speed change, and horsepower is proportional to the cube of the speed change.
When plotting the horsepower curve, it can be seen that as the speed decreases, the horsepower required at a given speed drops very rapidly. Horsepower requirements for other types of speed control will also be reduced at reduced speeds, but not lower than VFDs.
The difference in horsepower requirements between a VFD and any other type of shift control at a given speed is that the VFD provides greater energy savings. This is one of the basic advantages of VFD.
The inherent characteristic of AC VFD is that it functions like a buck starter. In other words, it will limit the amount of inrush current. A motor that is started across the line will pull about 600% of the inrush current. The VFD limits the inrush current to a maximum of 150%. The power company supplies surge current to the motor when it starts up. This is called demand cost. Limiting the amount of inrush current from large motors can save money by reducing demand costs.
2.Improve process control
One goal of most plants is to automate their processes. A key aspect of automation is better communication between plant instruments. AC VFD is a solid state device that is ideal for process control networks. These VFD receive process control signal inputs for start/stop, speed control and output signals. The signal is then transmitted to the DCS system, the PLC system or returned to the personal computer. Other types of shift control are typically limited (if not completely) to these interface capabilities.
It is easy to bypass the VFD when critical applications require an alternate control. The VFD bypass in parallel with the VFD allows the bypass mode to be completed in a matter of seconds. Other types of shift control are physically coupled to the motor and load. Whenever a speed control device fails, it must be decoupled, repaired, and reinstalled. VFDs are bypassed in a matter of seconds, while other types of speed control and their applications can be down for hours or even weeks.
In theory, a single VFD has the ability to control multiple motors. Other types of shift control do not provide this possibility. The smaller physical size and lower initial cost of a VFD system is an additional advantage.
5.Reduce system maintenance
The use of VFDs significantly reduces the maintenance of the entire system. This applies in particular to mechanical design systems such as eddy current clutches and variable pitch bundle applications. VFD helps reduce wear on belts, pulleys, gearboxes and couplings. All of these often cause serious maintenance problems due to wear caused by very corrosive or other harsh materials.
VFD does not turn the motor on and off, which is common in other processes. The VFD simply slows down the motor and load to operate in strict accordance with specifications. By eliminating the loop process, the VFD can eliminate the cumbersome surges and torque ripples that exist throughout the operating system.