Calculating ASD savings…without fudging fan laws
by Carolyn Roos, E.E.

Fan and pump motors are a major energy user in industry and commercial buildings. In many cases, substantial energy savings can be achieved by using adjustable speed drives to vary the output of fans and pumps in response to actual needs. For example, fan speed can be adjusted to vary the quantity of ventilation air to spaces with changing occupancies.

ASDs are not appropriate for all motor applications, however. In deciding if an ASD is the right choice, it is important to accurately calculate potential energy savings. Part of such an analysis involves applying a set of equations known as fan and pump laws.

Experience has shown that energy savings are often over-predicted when applying fan and pump laws. Wary of the potential for over-prediction, engineers often reduce the exponent 3 in the fan law relating power P to fan speed N:

An exponent as low as 2.0 is sometimes used, but values between 2.5 and 2.7 are more common. However, this adjustment cannot be relied upon for accurate estimates. The best practice is to carefully consider part-load efficiencies of the ASD, the motor and the fan or pump itself, as well as drive losses.

Performance changes of the fan or pump often represent the biggest loss in efficiency, yet frequently are not adequately considered. This can best be determined from the manufacturer’s performance curve for the particular unit.

Motor efficiency also may fall off rapidly at loads less than 40 to 50 percent of the rated full load. ASD efficiencies can be as low as 11 percent for small and very lightly loaded motors, and even at full load may range from 89 to 97 percent, depending on motor size. Drive losses should also be considered and may vary from 2 to 5 percent for direct drives to as high as 10 percent for belt drives on small motors.

Of equal importance in deciding whether an ASD is an appropriate choice is an evaluation of the required loads. If the loads do not vary substantially and the pump or fan is simply oversized, it is often more cost effective to replace the pump or fan with a properly sized unit rather than to install an ASD.

In summary, the best practice in calculating savings of ASDs is to apply fan and pump laws without modifying any exponents and to carefully account for all losses in efficiency at part-loads.

For a more complete treatment of fan laws, refer to the Air Movement and Control Association’s Standard 99-0021-01, The Fan Laws, available for $15.

Efficiencies for integral horsepower NEMA Design B motors at full and part-load can be obtained from the Industrial Technologies Program MotorMaster+ 4.0 software. MotorMaster+ 4.0 is one of many useful software tools on the U.S. Department of Energy’s Decision Tools for Industry CD. Contact the ITP Clearinghouse at 800-862-2086 to obtain this CD or to request additional information on motor and driven-equipment energy efficiency opportunities.

The U.S. Department of Energy’s Energy Matters newsletter is another good resource for energy efficiency and BestPractices news, technical tips, and case studies for industrial end users. Also, look for announcement of the release of the Motor Tip Sheet, Use Adjustable Speed Drive Part-Load Efficiency When Determining Energy Savings, to be posted in 2005.

(Carolyn Roos is an energy engineer with the Washington State University Extension Energy Program.)