NEMA MG 1-2011 Information Manual
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Condensed MG 1-2011Page 18© 2014 National Electrical Manufacturers Associationspecified as that of a lower class of insulation system, the temperature for resistance correction shall bethat of the lowest insulation class. [MG 1-12.58.1]9.21.2 Efficiency of Polyphase Squirrel-Cage Medium Motors with Continuous RatingsThe full-load efficiency of Design A and B single-speed polyphase squirrel-cage medium motors in therange of 1 through 400 horsepower for frames assigned in accordance with Table 15 or 16, above 400horsepower up to and including 500 horsepower, and equivalent Design C ratings shall be identified onthe nameplate by a nominal efficiency selected from the Nominal Efficiency column in Table 50 whichshall be not greater than the average efficiency of a large population of motors of the same design.The efficiency shall be identified on the nameplate by the caption “NEMA Nominal Efficiency” or “NEMANom. Eff.”The full-load efficiency, when operating at rated voltage and frequency, shall be not less than theminimum value indicated in Column B of Table 50 associated with the nominal value in Column A.Variations in materials, manufacturing processes, and tests result in motor-to-motor efficiency variationsfor a given motor design; the full-load efficiency for a large population of motors of a single design is not aunique efficiency but rather a band of efficiency. Therefore, Table 50 has been established to indicate alogical series of nominal motor efficiencies and the minimum associated with each nominal. The nominalefficiency represents a value which should be used to compute the energy consumption of a motor orgroup of motors. [MG 1-12.58.2]9.21.3 Efficiency Levels of Energy Efficient Polyphase Squirrel-Cage Induction MotorsThe nominal full-load efficiency of polyphase squirrel-cage induction motors rated 600 volts or lessdetermined in accordance with 9.21.1, identified on the nameplate in accordance with 9.21.2, and having acorresponding minimum efficiency in accordance with Column B of Table 50 shall equal or exceed thevalues listed in Table 51 for the motor to be classified as “energy efficient.” [MG 1-12.59]9.21.4 Efficiency Levels of Premium Efficiency Electric Motors9.21.4.1 60 Hz Motors Rated 600 Volts or Less (Random Wound)The nominal full-load efficiency of random wound Premium Efficiency electric motors rated 600 volts orless determined in accordance with 9.21.1, identified on the nameplate in accordance with 9.21.2, andhaving a minimum efficiency in accordance with Column B of Table 50 shall equal or exceed the valueslisted in Table 52. [MG 1-12.60.1]9.21.4.2 60 Hz Motors Rated Medium Voltage, 5000 Volts or Less (Form Wound)The nominal full-load efficiency of form wound Premium Efficiency electric motors rated at a mediumvoltage of 5000 volts or less determined in accordance with 9.21.1, identified on the nameplate inaccordance with 9.21.2, and having a minimum efficiency in accordance with Column B of Table 50 shallequal or exceed the values listed in Table 53. [MG 1-12.60.2]9.21.4.3 50 Hz Motors Rated 600 Volts or Less (Random Wound)The nominal full-load efficiency of random wound Premium Efficiency electric motors rated 600 volts orless determined in accordance with 9.21.1, identified on the nameplate in accordance with 9.21.2, andhaving a minimum efficiency in accordance with Column B of Table 50 shall equal or exceed the valueslisted in Table 54. [MG 1-12.60.3]9.21.5 Effects of Load on Motor EfficiencyThe efficiency of polyphase induction motors varies from zero at no load to a maximum value near ratedload and then decreases as load increases further. The efficiency versus load curves in Figure 8illustrates the typical profile of efficiency variation for various motor ratings. Actual values of motorefficiencies at various load levels can be obtained from the motor manufacturer. [MG 1-14.47] |
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