electric motors

Frame Sizes

In the United States, the National Electric Manufacturers Association (NEMA) establishes standard dimensions for electric motors. The rating plate of a motor will give a “frame size,” consisting of a number which may be followed by a letter. In purchasing a replacement motor, it is only necessary to get the same frame size to be sure of a physical fit (horsepower, voltage, type of enclosure, bearings, etc. are another matter). In the home, the most commonly encountered frame sizes are 48 and 56.

U is the diameter of the shaft. NW is the length of the shaft. D is the distance from the vertical base on which the motor sits to the center of the shaft. E is the horizontal distance from a mounting hole to the center of the shaft.F is half the distance between mounting holes. BA is the horizontal distance between mounting hole nearest the shaft end and the end of motor.

All dimensions in inches.
Size D 2E 2F BA Mounting NW U
48 2⅝ 11/32 slot ½
56 4⅞ 3 11/32 slot 1⅞ ½

The NEMA standards also describe the way the motor shaft is modified so that pulleys and gears can be securely fastened to it.

drawing of two motor shafts, one with a flat and one with a slot

Size Key on shaft
48 29/64″ flat
56 3/16″ wide, 3/16″ deep, 1 3/8″ long slot

Largest electric motors

8,000 horsepower electric locomotive.

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