Electric Motor Frame Sizes
Learn about frame sizes, and what they can tell you about a motor.
The National Electric Motor Association, better known as NEMA, has worked to standardize frame sizes, one of many efforts in place to create unification among the electric motor industry. But in order to better understand frame size standards, it is best to first discuss some of the basics.
Motor types and sizes are dependent upon the application. In other words, a larger horsepower requires a bigger motor. Likewise, a bigger motor requires a bigger frame. The frame is the outer portion of the motor and is usually created from cast iron or rolled steel. Frame sizes are designated by either a series of numbers or numbers and letters - e.g., 42, 182T or 324U. Larger numbers mean larger frames, which, in turn, means greater horsepower!
The letters, however, are the result of years of categorizing on behalf of NEMA. Standardization is not a new concept. Standard frames built prior to 1952 are referred to as "the originals." However, in 1952, new frames were created called the "U frames," and later, in 1964, the "T frames" came to exist.
But why standardize?
Interchangeability. NEMA requires that all motors carrying one of their many frame designations must be a specific set of dimensions across manufacturers, and, in most cases, motors with the same speed, enclosure and horsepower will require the same frame size. For example, all motors requiring 182T frame size must have the following dimensions:
4 ½" from the center of the motor shaft to the bottom of the base
7 ½" from mounting hole to mounting hole at adjacent ends of the feet
2 ¾" from the mounting hole to the shaft* (NEMA, Motors & Drives)
This way, a motor from manufacturer A can easily be replaced with a motor from manufacturer B as long as both motors require the same NEMA frame size designation.
*These do not include all the dimensions for the 182T.