ABSTRACT
During the period from 1960 to 1975, electric motors, particularly those in the 1-to 250-hp range, were designed for minimum first cost. The amount of active material, i.e., lamination steel, copper or aluminum or magnet wire, and rotor aluminum, was selected as the minimum levels required to meet the performance requirements of the motor. Efficiency was maintained at levels high enough to meet the temperature rise requirements of the particular motor. As a consequence, depending on the type of enclosure and ventilation system, a wide range in efficiencies exists for standardNEMAdesign B polyphase motors. Table 2.1 is an indication of the range of the nominal electric motor efficiencies at rated horsepower. These data are also presented in Fig. 2.1. The data are based on information published by the major electric motor manufacturers. However, the meaning or interpretation of data published prior to the NEMA adoption of the definition of nominal efficiency is not always clear. In 1977, NEMA recommended a procedure for marking the three-
phase motors with a NEMA nominal efficiency. This efficiency represents the average efficiency for a large population of motors of the same design. In addition, a minimum efficiency was established for each level of nominal efficiency.
