Although electric motors have been produced for nearly a century, many changes have taken place within the industry since then. One is the physical evolution of motors themselves to be stronger, more powerful, and more compact. The other is the standardization of motor size. Today’s common frame sizes are regulated by the National Electric Manufacturers Association or NEMA. The standardization of a motor frame size chart is valuable for consumers, as it means that motors of the same speed, power output, and volume will have an identical frame size, even if they’re made by different manufacturers. Provided they have the same NEMA frame sizes, a motor made by one manufacturer can be replaced by the motor of another without harming or damaging the product. If you’re thinking of getting (or replacing) a NEMA motor, here are three things to know.
NEMA motors span three generations. The first generation dates back prior to 1952. Motors in this era are simply called “Originals.” The next generation debuted in 1952. These motors have different frame assignments than their predecessors. They are called “U frames.” The third generation began in 1964 and still exists today. This generation of current standard frames is called “T.” T frames are expected to remain the standard in motors for the foreseeable future. Although T frames were launched in 1964 and are currently in use, many motors from previous generations will need to be replaced in the future. If you have an older motor, it is a good idea to keep a reference motor frame size chart available so that you are familiar with the specific motor frame size that you need.
Re-Rating and Temperature
Over the decades, motors have changed considerably in physical composition and structure. Since motors were made, the goal has remained the same of trying to pack more horsepower into each motor frame. Improvements in insulating materials used have greatly improved the performance capabilities and longevity of motors over time. Motors in the original NEMA frame size chart ran hot at considerably low temperatures. Newer motors with better insulation are smaller and run at much higher temperatures. Today, the temperature at which a motor runs is influenced largely by its insulation, which is categorized as “Class A” or “Class B.” Class A insulation has a temperature rating of 105° C. Class B insulation has a temperature rating of 130° C. The change in engine performance and dynamics over time has also triggered an adjustment in motor frame size. The shaft and bearing size of motors today are much bigger than their original counterparts. If you’re comparing motors of two different generations, be sure to look at the original motor frame chart to understand the differences in motor frame size between the two.
Motor power is divided into two categories – “fractional” and “integral.” Fractional horsepower covers NEMA frame sizes with a two-digit designation to describe power output. The other variety, which is integral horsepower, refers to any motor that has a three-digit frame size. It’s easy to find fractional horsepower on a motor frame chart using a simple calculation. Essentially, the centerline shaft height (or the “D” dimension) over the bottom of the motor’s base is the first two frame size digits divided by four. Bear in mind that the last digit does not correlate to a standard “inch” dimension. Larger numbers translate to a larger “F” dimension, which means that the rear bolt holes are located farther away from the end bolt holes in the shaft.
Motors have changed considerably in size and capability over the years. With the NEMA classification system, replacing motors suddenly became much easier. By keeping a few pointers in mind, you’ll be able to determine common frame sizes and replace a motor much more quickly and easily.