The Valuable Frequency Drive (VFD) is one of the most innovative energy management gadgets ever developed in the building HVAC systems. For more than 20 years now, VFDs have been installed on pump motors and fans in a wide range of valuable load applications. When VFDs are installed, energy savings can range anything between 35 and 50% compared to the conventional constant speed applications. This results in a return on investment of between 6 months and 2 years.
Initially, VFDs could only be installed on a limited number of applications based on the horsepower of the motors. Today, VFDs can be installed in almost any HVAC application both in commercial and domestic settings. The systems can now be operated at higher voltages compared to the earlier generations thereby resulting in systems with motor capacities of up to 500 horsepower.
As VFDs continue to evolve, numerous benefits will be available and homeowners will realize most of these benefits in both the operational aspect of the HVAC systems and the overall comfort levels.
Understanding VFDs
Most conventional HVAC applications are designed to operate pumps and fans at constant speeds. However, building loads are never constant hence the need for a variable system that can adjust based on the load demands. In a conventional system, mechanical throttling is required in order to lower water or airflow in the system. However, the drive motor continues to operate at full speed using the amount of energy irrespective of the cooling or heating load on the system. Inasmuch as mechanical throttling can give you a good level of control, it is not as efficient as the effect of VFDs.
Factors that Improve the Efficiency with VFDs
Operating at less than full load – Building systems are usually sized for peak load conditions. Typically, peak load conditions in applications occur anywhere between 1 and 5% of the annual operating hours. What this means is that fan motors and pumps are using more energy that is required 95 to 99% of the operating hours.
Oversized System Designs – When you design for peak load conditions, chances are you will oversize the system for most operating hours. This condition is further worsened by the practice of intentionally oversizing the system design to cater for unexpected and underestimated loads as well as future loads which may result from changes in the usage of building space.
Motor Energy Depends on Speed – The motor commonly used in building HVACs is known as the induction motor. This type of motor draws power which varies with its speed. This means if the motor is slowed by about 25% of its operating speed, the energy used will be reduced by almost 60%.
Therefore, installation of a VFD in any HVAC application helps to address the system inefficiencies brought about less than full load operations as well as oversized system designs. They also save energy by varying the speed of the motor.
