Electric motor frame sizes


NEMA frame size refers to the physical dimensions and mounting of the electric motor.  They are a key part of motor interchangeability; in essence, a motor from one manufacturer can be replaced with a similar motor from another company provided they are both of the same frame size.

The shaft length, shaft diameter, mounting foot dimensions, distance from the center of the shaft to the bottom center of the mounting foot are all part of the frame size standard. The physical dimensions of an electric motor have also been standardized by NEMA where “D” dimensions refers to the distance from center of shaft to bottom center of mounting foot, “U” dimensions refers to the diameter of the shaft, “N-W” dimensions refers to the length of the shaft, “F” dimensions refers to the distance from the center of the mounting foot to the center of base mounting hole in the side view and so on and so forth.

A standard Frame size designation for electric motors is either made up of either 2 or 3 digits. Typical examples include 56C, 213T, 215T, 254T, 256TS, 444T, 445T etc. The frame size number relates to the “D” dimensions of the electric motor. For a two digit frame number, the “D” dimension is frame number÷16 while for a 3 digit frame number the “D” dimension is the first two digits÷4.

Example: Frame size 56 has a “D” dimension of 56÷16 = 3.5 inches, frame 48 is 48÷16 = 3 inches while a motor with Frame size 143T has a “D” dimension of 14÷4 = 3.5 inches, frame 184T is 18 ÷ 4 = 4.5 inches.

Additionally, a two-digit frame number is mostly used on motors with less than 1 HP even though sometimes larger motors might be built on them. The term “Fractional Horsepower” is used to refer to them.


Motor frame dimension alphabets as specified by NEMA


FRAME SIZE SUFFIX
It is common to see NEMA frame sizes like 56C, 56H, 143T, 286U, 504S and so on and so forth.


Frame Suffix Alphabet
Meaning
C
Refers to a motor with flange and mounting holes on the face of the motor.
D
Refers to electric motors that are also flanged motors but in this special type mounting flange, the diameter of the flange is wider than the diameter of the motor face for bolts to pass from the back of the motor into the mating part.
H
Refers to a motor in which the “F” dimension is larger than those of similar frames. Remember that, the F dimension refers to the length from the center of the mounting base to the center of the mounting holes in the side view.
J
Refers to a motor that is essentially a standard C face mount motor with a threaded shaft
S
Indicates that the motor has a shaft shorter than those of similar frames. The shorter shaft length designed for direct coupling or for use in cramped places where standard shafts would be too long.
T
This is a motor with the standard frame dimensions i.e.  Shaft length rigid base and all. A motor with a T suffix indicates that the motor was manufactured after 1964 when NEMA introduced the present
U
This is a motor that uses Frame size standard introduced in 1952 and falls between the 1952 to 1964 era.
Y
means that the motor has a special or custom mounting configuration by the manufacturer that may not follow NEMA guidelines.
Z
means that the motor has a special or custom shaft configuration manufacturer that may not follow NEMA guidelines.


NOTE: NEMA and IEC are both  international standards organization that prepares and publishes International Standards and guide lines  for all electrical and electronic equipment but their guidelines are different. Consequently for an electric motor that follows IEC design guidelines, the frame size and frame guide above would nut be relevant and you would need a cross reference chart to exchange a NEMA standard electric motor with an IEC standard Electric motor.

In summary IEC has defined their own standards for motor frame size and some manufacturers use IEC design guidelines. Examples of IEC frame size numbers  include 160L,180L,200M,315L and so on and so forth.




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