The Best Applications for Variable Frequency Drives (VFDs)

Advances

The most commonly used motor in building HVAC applications is the three-phase, induction motor, although some smaller applications may use a single-phase induction motor.
VFDs can be applied to both.
While VFD controllers can be used with a range of applications, the ones that will produce the most significant benefits are those that require variable speed operation.

For example, the flow rate produced by pumps serving building HVAC systems can be matched to the building load by using a VFD to vary the flow rate.

Similarly, in systems that require a constant pressure be maintained regardless of the flow rate, such as in domestic hot and cold water systems, a VFD controlled by a pressure setpoint can maintain the pressure over most demand levels.

The majority of commercial and institutional HVAC systems use variable volume fan systems to distribute conditioned air. Most are controlled by a system of variable inlet vanes in the fan system and variable air volume boxes.

 

As the load on the system decreases, the variable air volume boxes close down, increasing the static pressure in the system. The fan’s controller senses this increase and closes down its inlet vanes. While using this type of control system will reduce system fan energy requirements, it is not as efficient or as accurate as a VFD-based system.

Another candidate for VFD use is a variable refrigerant flow systems. Variable refrigerant flow systems connect one or more compressors to a common refrigerant supply system that feeds multiple evaporators. By piping refrigerant instead of using air ducts, the distribution energy requirements are greatly reduced.
Because the load on the compressor is constantly changing based on the demand from the evaporators, a VFD can be used to control the operating speed of the compressor to match the load, reducing energy requirements under part-load conditions.

Additional VFD Applications

While the primary benefit of both of these VFD applications is energy savings, VFDs are well suited for use in other applications where energy conservation is of secondary importance. For example, VFDs can provide precise speed or torque control in some commercial applications.
Some specialized applications use dual fans or pumps. VFDs, with their precise speed control, can ensure that the two units are operated at the desired speed and do not end up fighting each other or having one unit carry more than its design load level.

Advances in technology have increased the number of loads that can be driven by the units. Today, units are available with voltage and current ratings that can match the majority of three-phase induction motors found in buildings. With 500 horsepower units or higher available, facility executives have installed them on large capacity centrifugal chillers where very large energy savings can be achieved.

One of the most significant changes that has taken place recently is that with the widespread acceptance of the units and the recognition of the energy and maintenance benefits, manufacturers are including VFD controls as part of their system in a number of applications. For example, manufacturers of centrifugal chillers offer VFD controls as an option on a number of their units.
Similarly, manufacturers of domestic water booster pump systems also offer the controls as part of their system, providing users with better control strategies while reducing energy and maintenance costs.

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