NEMA has assigned a series of standardized numbers and letters to describe the various dimensions and mounting types of motor frames. Because of the uniform NEMA frame size standards when changing out a motor, selecting the same frame size designation regardless of manufacturer, ensures that the mounting mechanism and hole positions will match. The frame size designation provides information about the motor frame size, the mounting type, and the dimensions according to the NEMA standard. Frame size designations are not intended to directly indicate electrical characteristics. However, in a general way, as the frame size designation number increases, so does the physical size and horsepower of the motor. There are many motors of the same horsepower and size that are built with different frame sizes so they can be mounted in various manners. Frame size designations may contain a prefix of letters, the frame number, and a suffix of letters. "EF56C" is an example.
There are three separate NEMA frame size standards you may encounter. Prior to 1954, a standard existed known as the original. In 1952, NEMA adopted a new re-rated standard to reflect advances in materials and manufacturing and motor frames designated under this standard are called "U" Frame motors. In 1964, the present NEMA frame standard was adopted and today NEMA rated frames are known as "T" Frame motors.
Standard frame number designations are expressed in either two or three numbers. Some examples of motor frame numbers are 48, 56, 145, and 215.
If the nameplate displays a two digit frame number, then the motor is less than one horsepower; and the frame has its D dimension (distance from the center of the drive shaft to the center bottom of the mount) expressed in sixteenths of an inch. For example, an EF56C frame motor is less than one horsepower and has a D dimension of 56. Fifty-six sixteenths translates to 3.5 inches.
Motors with frame numbers of three digits, indicate a motor of one horsepower or greater. To calculate the D dimension divide the first two digits by four. The D dimension of a EF145C frame motor is 14 divided by 4 or 3.5 inches.
As the D dimension increases the physical diameter of the motor becomes larger. The third digit in the three digit frame number is related to the length of the motor. It is an indication of the distance between the mounting holes parallel to the base.
Letters or numbers appearing in front of the NEMA Frame Designation are those of the manufacturer. They have no specific significance under NEMA's Frame Designation standard. The importance and meaning varies between manufacturers. In the previous examples, the "E F" in the frame designation "E F 5 6 C" denotes something unique only to this manufacturer.
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