STANDARD MOTOR EFFICIENCY
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 standard NEMA design 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
TABLE 2.1 Full-Load Efficiencies of NEMA Design B Standard Three-Phase Induction Motors
|hp||Nominal efficiency range||Average nominal efficiency|
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.
The minimum efficiency is the lowest level of efficiency to be expected when a motor is marked with the nominal efficiency in accordance with the NEMA standard. This method of identifying the motor efficiency takes into account variations in materials, manufacturing processes, and test results in motor-to-motor efficiency variations for a given motor design. The nominal efficiency represents a value that should be used to compute the energy
FIGURE 2.1 Nominal efficiency range of standard open NEMA design B 1800-rpm polyphase induction motors.
consumption of a motor or group of motors. Table 2.1 shows a wide range in efficiency for individual motors and, consequently, a range in the electric motor losses and electric power input. For example, a standard 10-hp electric motor may have an efficiency range of 8188%.
Therefore, for the same output the input can range from 8477 to 9210 W, or an increase in energy consumption and power costs of 8%, to operate the less efficient motor.