Engine efficiency remains a top priority for maintenance and engineering managers. But discussions about efficiency inevitably include difficult financial decisions. In order to be considered energy efficient, the performance of an engine shall match or exceed the nominal load efficiency values provided by the National Electrical Manufacturers Association (NEMA) in publication MG 1. Full rated nominal efficiency values are provided for each power (hp), enclosure type and speed combination. Low-energy engines use less electricity, run colder, and often outlast NEMA B engines of the same size.
To effectively evaluate the benefits of high efficiency electric motors, we must define "efficiency". For an electric motor, efficiency is the ratio of the mechanical power delivered by the motor (output) to the electrical power supplied to the motor (input).
Efficiency = (Mechanical Power Output / Electric Power Input) x 100%
Therefore, an engine that is 85 percent efficient converts 85 percent of the input of electrical energy into mechanical energy. The remaining 15 percent of electrical energy dissipates as heat, evidenced by an increase in engine temperature. Energy-efficient electric motors use improved engine design and high-quality materials to reduce engine losses, thereby improving engine efficiency. The improved design results in less heat dissipation and reduced noise output.
Most electric motors manufactured before 1975 were designed and built to meet minimum performance levels in exchange for a low purchase price. Efficiency was only maintained at levels high enough to meet the particular temperature rise restrictions of the engine. In 1977, the (NEMA) recommended a procedure for labeling standard three-phase motors with average nominal efficiency. These efficiencies represent an industry average for a large number of engines of the same design. Table 1 compares the current standard full load rating efficiencies for standard and energy efficiency motors of various sizes. Note that these efficiencies are averages for 3-phase Design B motors. (B-type motors represent 90 percent of all general-purpose induction motors.) Refer to the NEMA MG-1-1.16 specification publication for induction motors ratings.) Engines of other types A, C or D) have slightly different efficiencies, whereas single phase motors have substantially lower efficiencies. Energy-efficient motors are only marketed with NEMA B speed torque characteristics.
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