ELECTRIC MOTOR FRAME SIZE STANDARD SPECIFICATIONS

ELECTRIC MOTOR FRAME SIZE STANDARD SPECIFICATIONS
How is electric motor frame size being specified?

Motor frame dimensions have been standardized with a uniform frame size numbering system. This system was developed by NEMA and specific frame sizes have been assigned to standard motor ratings based on enclosure, horsepower and speed.

The current standardized frames for integral horsepower induction motors ranges from 143T to 445T. These standards cover most motors in the range of one through two hundred horsepower. Typical example of where you can locate the frame is shown in Fig 1.2.D – Frame No.

The numbers used to designate frame sizes have specific meanings based on the physical size of the motor. Some digits are related to the motor shaft height and the remaining digit or digits relate to the length of the motor.

The rerate, or frame size reduction programs were brought about by advancements in motor technology relating mainly to higher temperature ratings of insulating materials, improved magnetic steels and improved bearings. At the present time, NEMA frame assignments do no exist for motors larger than 445T and each manufacturer may have different frame designations for these motors.

One additional suffix that may be used on standard motors in frames 286T and larger is an “S” inserted after the “T”. This “S” stands for short shaft.

In addition to having a short shaft, the motor will have a small diameter shaft (“U” dimension) and the bearing in the drive shaft end of the motor will be somewhat smaller than the equivalent long shaft motor. Short shaft motors are intended for use only on direct coupled centrifugal pumps and other direct coupled loads where there will not be a side pull (overhung load) exerted on the shaft by “V” belts.

The following explanations of the various fame suffixes used on NEMA frame motors have been compiled for the benefit of EASA members. The suffixes for NEMA frame motors are the letters that immediately follow the frame numbers. Notice that more than one suffix may be used on any given motor.

Note: “D” dimension (shall height) of a motor or generator in these frame sizes equals 1 /4 the value of the first two digits in the frame number. Example: 284 frame: 28/4 = 7, D = 7"

A — Industrial direct-current machine.

B — Carbonator pump motors, (See NEMA MG 1-2006, 18.270 – 18.281)

C — Type C face mounting on drive end.

CM — Face mounting dimensions are different from those for the frame designation having the suffix letter “C” (The letters “CH” are considered as one suffix and should not be separated.)

D — Type D flange mounting on drive end.

E — Shaft extension dimensions for elevator motors in frames larger than 326T frames.

FC — Face mounting on opposite drive end.

FD — Flange mounting on opposite drive end.

G — Gasoline pump motors. (See NEMA MG 1-2006, 18.91.)

H — Indicates a small machine having an “F” dimension larger than that of the same frame without the suffix

letter “H”. (See NEMA MG 1-2006, 4.4.1 and 4.5.1.) HP or HPH — Type P flange-mounted, vertical sotid-shaft motors having dimensions in accordance with NEMA MG 1- 2006, 18.252. (The letters “HP” and “HPH” are considered as one suffix and should not be separated)

J — Jet pump motors. (See NEMA MG 1-2006, 18.132.)

JM — Face-mounted, close-coupled pump motor having antifriction bearings and dimensions in accordance with Table 1 of MG 1-2006, 18.250. (The letters “JM” are considered as one suffix and should not be separated.)

JP — Type C face-mounted, close-coupled pump motor having antifriction bearings and dimensions in accordance with Table 2 of MG 1 -2006, 18.250. (The letters “MP” are considered as one suffix and should not be separated.)

K — Sump pump motors. (See NEMA MG 1-2006, 18.78.) LP or LPH — Type P flange-mounted, vertical solid-shaft motors having dimensions in accordance with MG 1-2008, 18- 251. (The letters “LP” and “LPH” are considered as one suffix and should not be separated.)

M — Oil burner motors. (See NEMA MG 1-2006, 18.106.)

N — Oil burner motors. (See NEMA MG 1-2006, 18.108.)

P or PH— Type P flange-mounted, vertical hollow-shaft motors having dimensions in accordance with NEMA MG 1- 2006, 18.238.

R — Drive end tapered shaft extension having dimensions in accordance with NEMA MG 1-2008, 4.4.2.

S — Standard short shaft for direct connection.

T — Included as part of a frame designation for which standard dimensions have been established.

U — Previously used as part of a frame designation for which standard dimensions had been established.

V — Vertical mounting only.

VP — Type P flange-mounted, vertical solid-shaft motors having dimensions in accordance with NEMA MG 1- 2008, 18.237. (The letters “VP” are considered as one suffix and should not be separated.)

X — Wound-rotor crane motors with double shaft extension. (See NEMA MG 1-2006, 18229 and 18.230.)

Y — Special mounting dimensions, (Dimensional diagram must be obtained from manufacturer.)

Z — All mounting dimensions are standard except the shaft extension(s). Also used to designate machines with double shaft extension.

Note: Manufacturers may use any letter preceding the frame number, but such a letter will have no reference to standard mounting dimensions. Suffix letters shall be added to the frame number in the following sequences:

Suffix Letter Sequence
A, H……………………………………………………… 1
G, J, M, N, T, U, HP, HPH, JM, JP, LP, LPH, &VP .......... 2
R, S ..................................................................................... 3
C, D, P, PH ………………………………………………. 4
FC, FD…………………………………………………… 5
V…………………………………………………………. 6
E, X, Y, Z ............................................................................ 7

Example: “T” frame motor with a “C” face mounted vertically with a nonstandard shaft extension; (Sequences 2.4.8 and 7) 184TCVZ. Note: This material is reproduced by permission of the National Electrical Manufacturers Association from NEMA Standards, MG 1-2006, 4.2.2. It was originally published as EASA Tech Note No. 7 (September 1985) and reviewed and updated as necessary in November 2007.

source:2010 IIEE Technical Manuals