UFC 3 -520-01
June 10, 2002
Figure B -5. Typical Motor Load Profile With Motor Operating at Half of Rated Load
Table B-1 provides a sample economic evaluation for an ASD installation for
a continuously operating HVAC system motor. This evaluation was for a hospital
application in which higher initial ASD costs were expected in order to address
harmonic distortion concerns as part of the design. Even so, a payback period of less
than 2 years was estimated. As can be seen in Table B -1, an ASD economic evaluation
is most sensitive to the following assumptions:
Total motor operating time per ye ar-- unless it is fully loaded, a continuously
energized motor will show a faster payback than an intermittently energized motor.
Estimated actual motor load/speed--for a typical centrifugal fan motor, energy
usage is proportional to the (speed)3. For example, if the motor speed can be
reduced to 90 percent of rated speed, the energy usage can be reduced to almost
70 percent of its nominal value.
Cost per kilowatt hour--the local average energy cost should be used.
ASD equipment and installation cost-- for critical locations, the added cost of
ensuring acceptable power quality can double the total initial cost.