Three-phase squirrel-cage induction motors comprise a considerable percentage of the electrical load in the United States. Design, operation, and maintenance of these machines is well described in some of the articles already published. This article focuses on their energy efficiency aspects.
Induction motors typically range in full load efficiency from about 87% to 94%. This efficiency is very difficult to measure accurately in the field, requiring a dynamometer and other specialized equipment. Fortunately, energy saving projects associated with electric motors do not require actual efficiency of a given motor to be established.
Rewinding a damaged induction motor is a common practice in industry, but studies have proven that rewinding an induction motor drops its efficiency by a couple percentage points. Multiple rewinds can further reduce the efficiency of the rewound motor.
While a drop in efficiency from 89% to 88% seems insignificant, a quick estimate reveals that this reduction can be costly. A standard efficiency 20 hp motor operating 8000 hours annually, for example, costs about $7000 per year to operate at an average electricity rate of 7 ¢/kWh. Once this motor fails, the least-cost option for returning it to service is typically rewinding.
The incremental cost of replacing this failed motor with an energy-efficient motor, however, is only $430. This amount assumes considers the rewound cost, and the labor necessary to perform the motor change-out, as sunk costs.
The annual energy savings associated with replacing the failed motor with an energy-efficient model, at a new efficiency of 92.9%, is approximately $510. The simple payback for the replacement, therefore, is less than one year.
Energy-efficient motor programs are applicable to any AC motor installations utilizing NEMA Design B induction motors. Since the programs are based on replacement at failure, the full savings potential is realized after three years or more.
Published efficiencies of typical rewound, standard, and energy-efficient three-phase induction motors.
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Motor Rewinding Process (VIDEO)
What is the rewind scenario if a motor fails? Read more about rewind “rules of thumb”.
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Reference: Electrical Energy Management – Bill Brown, P.E., Square D Engineering Services