National Electrical Manufacturers Association (NEMA) is a trade
association in the US who sets up technical standards. NEMA
standardized on electric motor sizes based upon each specific
horsepower, speed, and enclosure type to allow for easier
replacement interchange between manufacturers. These size
assignments are referred to as "Frame". In the tables above, you
will see the 3 different times that these standardized Frame
sizes were revised to take advantage of motor technology and
materials improvements which basically allowed for a reduction
in physical sizes of the motors for the same horsepower, speed,
and enclosure. Keep in mind, that when replacing an older "U
Frame" (1952 standard) or "Pre-U Frame" (Original standard) with
the current "T Frame" size standard (1964 standard), we can
provide you with Transition Bases which will place the smaller
current frames into the same basic mounting position as the
older frames. These Transition Bases are relatively inexpensive.
However you will often have to rep