3 tips on Variable Frequency Drives
1. Variable Frequency Drive Technology Has Improved For HVAC Applications
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.
2. A VFD Offers Energy Savings, Other Benefits from Part-Load HVAC Operation
Today's tip comes from James Piper, contributing editor for Building Operating Management: Variable frequency drives, or VFDs, offer multiple benefits for HVAC systems.
For more than 20 years, VFDs have successfully been installed on fan and pump motors in wide range of variable load applications. The most significant benefit of the use of a VFD is energy savings. By matching system capacity to the actual load throughout the entire year, major savings in system motor energy use is achieved.
Another benefit of the units is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This inrush current can be three to ten times the full-load operating current for the motor, generating both heat and stress in the motor's windings and other components. For motors that start and stop frequently, the heat and other stresses produced contribute to early motor failures.
In contrast, when a motor connected to a VFD is started, the VFD applies a very low frequency and low voltage to the motor. Both are gradually ramped up at a controlled rate to normal operating conditions. With no significant inrush current, heating and stresses are practically eliminated, extending motor life.
VFDs also provide more precise levels of control of applications. For example, high rise buildings use a booster pump system on the domestic water supply to maintain adequate water pressure at all levels within the building. Conventional pump controls in this type of application can maintain the pressure within a certain range, but a VFD based system can maintain more precise control over a wider range of flow rates, while reducing energy requirements and pump wear.
3. BAS Can Help Facility Managers Take Advantage of Smart Grid
The electric grid is in line for a nationwide makeover, and building automation systems could help facility managers take advantage of the development.
Today, the electric grid is essentially a one-way street, directing power from the utility to the facility. The so-called "smart grid" will allow both power and data to flow in both directions, with smart meters not only gathering data about the facility's electric use, but also relaying information from the utility to the facility.
The flow of data promises to be increasingly important to facility managers. A big goal of the smart-grid movement is to trim utility peak loads. That will both cut costs and improve the reliability of the grid by reducing the risk of blackouts and brownouts.
One way to achieve that goal is through time-of-use pricing, which brings higher rates at times of peak demand. Another strategy is to send out signals to facilities to cut back on loads as part of a demand response program. A building automation or energy management system can help facility executives take advantage of time-of-use pricing by automatically making adjustments to building systems to reduce peak energy use. For example, the building automation system could automatically direct variable frequency drives on fans to slow in sequence, thereby trimming demand without a noticeable effect on occupants.
The transition to the smart grid will take years. But for facility managers, now is the time to start thinking about how the building automation system will help take advantage of opportunities presented by the smart grid.
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