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December 12, 2011 -
Today's tip comes from James Piper, contributing editor for Building Operating Management magazine: Improved variable frequency drive technology makes them worth considering for many HVAC applications.
The variable frequency drive (VFD) is one of the most successful energy management tools that facility managers have applied to HVAC systems. Those who have used the drives have realized energy savings ranging from 35 to 50 percent over conventional constant speed applications, resulting in a return on their investment of six months to two years.
While the number of applications suitable for early generation drives was somewhat limited based on the horsepower of the motor, today's drives can be installed in practically any HVAC application found in commercial and institutional buildings. Today, systems can be operated at higher voltages than those used by early generation systems, resulting in off the shelf systems that are available for motors up to 500 horsepower.
Early generation systems also suffered from low power factor. Low power factor robs the facility of electrical distribution capacity and can result in cost penalties imposed by electrical utility companies. Today’s systems operate at nearly constant power factor over the entire speed range of the motor.
Another problem that has been corrected by today's systems is operational noise. As the output frequency of the drives decreased in response to the decreasing load, vibrations induced in the motor laminations generated noise that was easily transmitted through the motor mounts to the building interior. Today's drives operate at higher frequencies, resulting in the associated noise being above the audible range.
Most of the latest generation systems use what is known as pulse-width modulation. In pulse-width modulation systems, the output of the VFD is a quasi-sine wave that consists of a number of narrow voltage pulses that vary in both duration and voltage.