ELECTRIC MOTOR STANDARDS ELECTRIC MOTORS Machinery's Handbook, 31st Edition
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Classes of NEMA Standards.— National Electrical Manufacturers Association Stan dards (NEMA), available from NEMA at 1300 North 17th Street, Suite 900, Arlington, VA 22209, www.nema.org, are of two classes: 1) NEMA Standard, which relates to a prod- uct commercially standardized and subject to repetitive manufacture, which standard has been approved by at least 90 percent of the members of the Subdivision eligible to vote thereon; and 2) Suggested Standard for Future Design, which may not have been regu- larly applied to a commercial product, but which suggests a sound engineering approach to future development and has been approved by at least two-thirds of the members of the Subdivision eligible to vote thereon. Authorized Engineering Information consists of explanatory data and other engineering information of an informative character not falling within the classification of NEMA Standard or Suggested Standard for Future Design. Mounting Dimensions and Frame Sizes for Electric Motors.— Dimensions for foot- mounted electric motors as standardized in the United States by the National Electrical Manufacturers Association (NEMA) include the spacing of bolt holes in the feet of the motor, the distance from the bottom of the feet to the centerline of the motor shaft, the size of the conduit, the length and diameter of shaft, and other dimensions likely to be required by designers or manufacturers of motor-driven equipment. The Standard provides dimen sions for face-mounted and flange-mounted motors by means of standard motor frame numbers. Standard dimensions also are given where the motor is to be mounted upon a belt-tight ening base or upon rails. The NEMA standards also prescribe lettering for dimension drawings, mounting and terminal housing locations and dimensions, symbols and terminal connections, and pro- vision for grounding of field wiring. In addition, the standards give recommended knock- out and clearance hole dimensions; tolerances on shaft extension diameters and keyseats; methods of measuring shaft run-out and eccentricity, also face runout of mounting sur- faces; and tolerances of face-mounted and flanged-mounted motors. Design Letters of Polyphase Integral-Horsepower Motors.— Designs A, B, C, and D motors are squirrel-cage motors designed to withstand full voltage starting and develop- ing locked-rotor torque and breakdown torque, drawing locked-rotor current, and having a slip as specified below: Design A: Locked-rotor torque as shown in Table 2, breakdown torque as shown in Table 3, locked-rotor current higher than the values shown in Table 1, and a slip at rated load of less than 5 percent. Motors with 10 or more poles may have a slightly greater slip. Table 1. NEMA Standard Locked-Rotor Current of 3-Phase 60-Hertz Integral-Horsepower Squirrel-Cage Induction Motors Rated at 230 Volts
Locked-rotor Current, Amps.
Locked-rotor Current, Amps.
Locked-rotor Current, Amps.
Horse power
Design Letters
Horse power
Design Letters
Horse power
Design Letters
20 25 30 40 50 64 92
B, D B, D B, D B, D B, D
127 162 232 290 365 435 580
B, C, D 50 B, C, D 60 B, C, D 75 B, C, D 100 B, C, D 125 B, C, D 150 B, C, D 200
725 870
B, C, D B, C, D B, C, D B, C, D B, C, D B, C, D
7 1 ∕ 2
1 ∕ 2 3 ∕ 4
10 15 20 25
1
1085 1450 1815 2170
1 1 ∕ 2
2 3 5
B, C, D 30 B, C, D 40
2900 B, C Note: The locked-rotor current of a motor is the steady-state current taken from the line with the rotor locked and with rated voltage and frequency applied to the motor. For motors designed for voltages other than 230 volts, the locked-rotor current is inversely proportional to the voltages. For motors larger than 200 hp, see NEMA Standard MG 1-12.34.
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