ENERGY-EFFICIENT INDUCTION-MOTORS PERFORMANCE-CHARACTERISTICS AND LIFE-CYCLE COST COMPARISONS FOR CENTRIFUGAL LOADS

When fixed-speed motors (fed directly at power frequency) are purchase d for new installations or for replacements, the loaded shaft speed di fferences among motor options are either ignored or overestimated. The most common first-cut estimate is that the consumed shaft power will vary as the cube of the ratio of the motor rated nameplate speeds for centrifugal driven loads that have discharge control valves (on pressu re or flow control). In actuality, this is true only if the motors are loaded at approximately nameplate output. This paper discusses the tr ue ''control valve loss'' factor taking into account actual speed diff erences among motor options. A simplified equation and figure are pres ented to permit quick evaluation of motor purchase alternatives for th e lowest life-cycle cost based on efficiency and rated-load speed diff erences, NEMA standards on slip-speed variation should be made more st ringent to increase the validity of speed-difference loss evaluations. Additionally, efficiency test results and loaded speed measurements f or standard-efficiency and premium-efficiency 10- and 100-hp motors ar e presented, leading to a conclusion that standard-efficiency motors s hould be operated at no higher than rated voltage and that premium-eff iciency motors should be operated at 5%-10% above rated voltage for be st system efficiency.