During the past 100 years industrial electrical motors have evolved to contain more horsepower in a smaller space. Standardization of frame sizes now allows interchangability of motors. This makes it easy to replace a motor from one manufacturer with a motor from another company.
Standardization has created the first grouping of frame sizes called "original." In 1952, new frame assignments were made called "U frames." "T frames" were introduced in 1964. These T frames are the present-day standard.
NEMA frame specifications are organized by codes, which include a series of letters and numbers. NEMA publishes a comprehensive chart of codes, which details different specifications for a wide range of frames. In addition to the standard numbering system, some variations appear to explain uses. These are always designated as letters.
Each code designation offers information on motor frame specifications. Basic data includes diameter of the motor, the length of the motor's base, and key width and thickness. Alphabetic variations expound on the basics. For example the designation "H" is used on some 56 frame motors. This conveys the information that the base is suitable in either 56, 143 T or 145 T mounting dimensions.
A " J" designation is also used with 56 frames to convey that the motor is made for "jet pump" service. The letter "S" indicates the motor has a "short shaft."
A "T" at the end of the frame size indicates that the motor is of the 1964 and later T frame vintage. A "U" at the end of the frame size indicates that the motor is from the U frame era (1952 to 1964).
NEMA 's technical standards insure that motors meet or exceed industry and governmental standards established by NEMA and other global associations providing standardization.
Although T frames were adopted nearly 50 years ago, many U frame motors, which came before, are still operational. Many of the original frame size motors manufactured before 1952 are now approaching 60 years of service and will soon need to be replaced.