One of the most successful energy management tools ever used in HVAC (Heating, Ventilating and Air Conditioning) systems of buildings is the Variable Frequency Drive (VFD).
Over the past 20 years, VFD has been successfully installed on fan and pump motors in a variety of variable load applications. The saved energy is up to 35% to 50% compared with using traditional constant-speed applications. And the investment return period is 6 months to 2 years.
3D model of HVAC system in an apartment
The early appliances using VFDs were limited because of the power limit of motors, but today VFDs can be installed in any HVAC appliances in commercial and residential buildings. New VFDs can be operated at higher voltages than those of previous generations, providing ready and stable, and variable power supply for motors up to 500 horsepower.
Previously, VFD also suffered from low power, which can disrupt distribution capacitor facilities and may cause power company to impose cost penalties. Now, VFDs can operate with an almost constant power over the entire speed range of the motor.
Another problem of operational noise has been corrected too.
As the output frequency of the drive decreases in response to the load, the noise of the vibration caused by the lamination of the motor can easily be transmitted through the motor to the interior space of the building. But now the drive is functioning at higher frequencies, making the operational noise above audible range of human being.
That's just an example. And VFDs are evolving continuously.
From various advantages of VFDs to an increasing number of available appliances, VFDs are proved to be even more useful and powerful.
The most important advantage of using VFD is energy saving.
By matching the system capacity with the actual load in the whole year, the goal of saving the energy consumption of the HVAC system motor is achieved.
Another benefit is reduced wear of the motor. When an induction motor is started, it consumes a higher current than when it normally operates.
This current surge can be three to ten times the full operating current of the motor, creating heat and stress in the motor windings and other components. The frequent starts and stops will lead to early failure of the motor.
In contrast, if the motor is connected to a VFD, it will be applied extremely low frequency and low voltage.
Both the two devices are gradually going normal at a controlled rate, thus extending motor's service life.
The VFD also provides a more precise control of the appliance. For example, high-rise buildings use booster pump systems for domestic water to maintain adequate water pressure at all levels of the building.
This makes conventional pump control keep pressure within a range, but the VFD can maintain more precise control over a wider range of flow rates while reducing energy requirements and pump wear.
The three-phase induction motor is most commonly used in building hvac engineer applications, some smaller applications may use a single-phase induction motor though.
Both the two can be installed with VFDs.
While the VFD controller can be used in lots of fields, it's applications requiring variable speed that get the most benefit. For example, use the VFD to vary the flow rate. The pump used to build the HVAC system can produce a flow rate that matches the building load.
Similarly, in systems where constant pressure is required regardless of flow rate, such as in domestic hot and cold water systems, a VFD controlled by a pressure set point can maintain pressure at most required levels.
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