NEMA MG 1-2011 Information Manual
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Condensed MG 1-2011Page 19© 2014 National Electrical Manufacturers Association10 APPLICATION DATA—AC SMALL AND MEDIUM MOTORS10.1 Service ConditionsThe following service conditions are defined as “usual”:a) An ambient temperature in the range of -15°C to 40°C or, when water cooling is used, 5°C to40°C.b) Exposure to an altitude not exceeding 3300 feet (1000 meters).c) Installation on a rigid mounting surface.d) Installation in areas or supplementary enclosures which do not seriously interfere with theventilation of the machine.e) Voltage variation within 10% of rated voltage at rated frequency (see 9.9).f) Frequency variation within 5% of rated frequency at rated voltage (see 9.9).g) Unbalance of the voltages of the alternating-current supply does not exceed 1% (see 9.10).h) V-belt drive in accordance with MG 1 Part 14.i) Flat-belt, chain, and gear drives in accordance with MG 1 Part 14.The manufacturer should be consulted if any unusual service conditions exist which may affect theconstruction or operation of the machine. [MG 1-14.3]Examples of typical applications for general purpose small and medium ac squirrel-cage induction motorsare shown in Table 11.11 APPLICATION CONSIDERATIONS FOR CONSTANT SPEED DESIGN A AND B INDUCTIONMOTORS USED ON A SINUSOIDAL BUS WITH HARMONIC CONTENT11.1 EfficiencyEfficiency will be reduced when a motor is operated on a bus with harmonic content. The harmonicspresent will increase the electrical losses which decrease efficiency. This increase in losses will also resultin an increase in motor temperature, which further reduces efficiency. [MG 1-30.1.1]11.2 Derating for Harmonic ContentHarmonic currents are introduced when the line voltages applied to a polyphase induction motor includevoltage components at frequencies other than nominal (fundamental) frequency of the supply.Consequently, the temperature rise of the motor operating at a particular load and per unit voltageharmonic factor will be greater than that for the motor operating under the same conditions with onlyvoltage at the fundamental frequency applied.When a motor is operated at its rated conditions and the voltage applied to the motor consists ofcomponents at frequencies other than the nominal frequency, the rated horsepower of the motor shouldbe multiplied by the factor shown in Figure 9 to reduce the possibility of damage to the motor. This curveis developed under the assumption that only harmonics equal to odd multiples (except those divisible bythree) of the fundamental frequency are present. It is assumed that any voltage unbalance or any evenharmonics, or both, present in the voltage are negligible. This derating curve is not intended to apply whenthe motor is operated at other than its rated frequency nor when operated from a variable voltage or avariable frequency power supply, or both. [MG 1-30.1.2]11.2.1 Harmonic Voltage Factor (HVF) DefinedThe harmonic voltage factor (HVF) is defined as follows:nV 2nn5nWhere:n = order of odd harmonic, not including those divisible by three |
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