Energy-efficient induction motors-performance characteristics and life-cycle cost comparison for centrifugal loads

When fixed-speed motors (fed directly at power frequency) are purchased for new installations or for replacements, an aspect either ignored or overestimated is the loaded shaft speed differences among motor options. 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 pressure or flow control). This is true only if the motors are loaded at approximately their nameplate output. This paper discusses the true "control valve loss" factor, taking into account actual speed differences among motor options. A simplified equation and figure are presented to permit quick evaluation of motor purchase alternatives for lowest life-cycle cost based on efficiency and rated-load speed differences. NEMA standards on slip-speed variation should be made more stringent to make speed-difference loss evaluations more valid. Additionally, efficiency test results and loaded speed measurements for standard efficiency and premium efficiency 10 hp and 100 hp motors are presented, leading to a conclusion that standard efficiency motors should be operated at no higher than rated voltage and that lower rated premium efficiency motors should be operated at 5% to 10% above rated voltage for best system efficiency.