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

When fixed-speed motors (fed directly at power frequency) are purchased for new installations or for replacements, the loaded shaft speed differences 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 pressure or now control). In actuality, this is true only if the motors are loaded at approximately 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 the lowest life-cycle cost based on efficiency and rated-load speed differences. NEMA standards on slip-speed variation should be made more stringent to increase the validity of speed-difference loss evaluations. Additionally, efficiency test results and loaded speed measurements for standard-efficiency and premium-efficiency 10 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 premium-efficiency motors should he operated at 5%-10% above rated voltage for best system efficiency