NEMA MG 1-2011 Information Manual
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Condensed MG 1-2011Page 13© 2014 National Electrical Manufacturers Association9.9 Variations from Rated Voltage and Rated Frequency9.9.1 RunningAlternating-current motors shall operate successfully under running conditions at rated load with avariation in the voltage or the frequency up to the following:a) Plus or minus 10 percent of rated voltage, with rated frequency for induction motors.b) Plus or minus 5 percent of rated frequency, with rated voltage.c) A combined variation in voltage and frequency of 10 percent (sum of absolute values) of the ratedvalues, provided the frequency variation does not exceed plus or minus 5 percent of ratedfrequency.Performance within these voltage and frequency variations will not necessarily be in accordance with thestandards established for operation at rated voltage and frequency. [MG 1-12.44.1]9.9.2 StartingMedium motors shall start and accelerate to running speed a load which has a torque characteristic andan inertia value not exceeding that listed in 9.18, with the voltage and frequency variations specified in9.9.1.The limiting values of voltage and frequency under which a motor will successfully start and accelerate torunning speed depend on the margin between the speed-torque curve of the motor at rated voltage andfrequency and the speed-torque curve of the load under starting conditions. Since the torque developed bythe motor at any speed is approximately proportional to the square of the voltage and inverselyproportional to the square of the frequency, it is generally desirable to determine what voltage andfrequency variations will actually occur at each installation, taking into account any voltage drop resultingfrom the starting current drawn by the motor. This information and the torque requirements of the drivenmachine define the motor-speed-torque curve, at rated voltage and frequency, which is adequate for theapplication. [MG 1-12.44.2]9.10 Voltage UnbalanceAlternating-current polyphase motors shall operate successfully under running conditions at rated loadwhen the voltage unbalance at the motor terminals does not exceed 1 percent. Performance will notnecessarily be the same as when the motor is operating with a balanced voltage at the motor terminals.[MG 1-12.45]Unbalanced currents resulting from unequal line voltages applied to an induction motor produces anelevated temperature rise compared to a motor operating with balanced voltages. Should voltages beunbalanced, the motor horsepower rating should be derated in accordance with MG 1, Part 14. (SeeFigure 7.) [MG 1-14.36]9.11 Variation from Rated SpeedThe variation from the nameplate or published data speed of alternating-current, single-phase andpolyphase, medium motors shall not exceed 20 percent of the difference between synchronous speed andrated speed when measured at rated voltage, frequency, and load and with an ambient temperature of25oC. [MG 1-12.46]9.12 Variation from Nameplate Amperes—Alternating-Current Medium MotorsWhen operated at rated voltage, rated frequency, and rated horsepower output, the input in amperes shallnot vary from the nameplate value by more than 10 percent. [MG 1-12.47]9.13 Occasional Excess CurrentPolyphase motors having outputs not exceeding 500 horsepower and rated voltages not exceeding 1kVshall be capable of withstanding a current equal to 1.5 times the full load rated current for not less thantwo minutes when the motor is initially at normal operating temperature. |
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