How VFDs Improve HVAC Systems


One of the most popular and successful energy management tools ever developed for building HVAC systems is the Variable Frequency Drive, or better known as VFD. It has been successfully installed on fan and pump motors in a huge range of variable load applications for more than 20 years now. Establishments and companies who use VFDs experience 35 to 50 percent energy savings compared to having conventional constant speed applications.

VFDs can be installed in almost any HVAC application present in institutional and commercial buildings.

Earlier models of drives experienced low power factor, which robs an establishment or company of electrical distribution capacity and can result in paying cost penalties that are imposed by electrical utility companies. However, the newer models that we use today operate at a nearly constant power factor over the total speed range of the motor.

Also, operational noise is lessened as latest drives operate at higher frequencies which make associated noise below audible range.

Factors that improve when using VFDs

A majority of the first generation models of building HVAC applications are created to operate pumps and fans at a constant speed. Although mechanical throttling can give a good level of control, it is not very efficient. VFDs provide efficient options for this and there are several factors operating efficiency was improved with the use of VFDs.

1. Operating at less than full load – Most HVAC systems in an establishment are sized for peak load conditions. In various applications, peak load conditions happen between 1 and 5 percent of the operating hours every year. This means that fan and pump motors use more energy than the usual 95 to 99 percent of their operating hours.

2. Oversized system designs – Designing for peak load oversizes the system for most of the operating hours. This predicament can be heightened by the practice of oversizing the system design for underestimated loads as well as future loads that could result from the changes or adjustments on how the building space is used.

3. Motor energy use is a function of speed – Induction motor is the most widely-used motor in building HVAC systems. By using induction motors, the power absorbed by the motor depends on the cube of the motor’s speed. This implies that if the motor can be slowed by 25 percent of its usual operating speed, the energy that it uses is decreased by almost 60 percent.

VFDs provide energy savings by converting 60 cycle line current to direct current, then to an output that differs in frequency and voltage depending on the load directed to the system. When the system load decreases, the VFD’s controller also reduces the motor’s operating speed in order for the flow rate in the system to meet but not surpass the load requirements.

VFD systems continuously evolve and they are getting more useful and powerful to address the changing needs and demands of modern technology and new building and energy regulations.