The VFD Answer Man, Volume 4: The NEMA Motor Frame Puzzle

December 20, 2015 |By: Terry M. Smith

Let's discuss NEMA frames first. NEMA frames are grouped into families in which all the mounting dimensions are the same except the 2F dimension. Dimensions other than the mounting dimensions can be different. This is most easily seen in the "C" dimension, which is the distance from the tip of the shaft to the back of the motor. Since it's not a mounting dimension, there are no standards from one brand to another. Today's frames are "T" frames. From 1952-1964, the "U" frames were standard. The "original" frames were built prior to 1952.

Now, notice I put shaft height first. There's a good reason for that. If you take the first two digits of a NEMA āUā or "T" frame and divide it by four, you will get the shaft height. For example, on a 184T frame, that would be 18 divided by 4, or 4.5 inches. Conversely, if you measure the shaft height of a NEMA U or T frame motor and multiply by 4, you'll have the first two digits of the frame size.

Let's say you have a motor where you can't read parts of the nameplate. If you measure the shaft height as 4.5" and multiply by 4, you know you have a 182U or T or 184U or T frame motor. You have just eliminated all the other possible frames with one quick measurement. Now, all you have to do is check the 2F dimension to see which of the two frames you have. If you know any two out of the horsepower, frame size, and RPM, you can usually figure out the third. So, in our motor with the 4.5" shaft height, if we know its speed is 1750 RPM, it's typically a 5 horsepower motor. This is no guarantee because sometimes OEMs don't follow the rules when having a manufacturer build their motors. Also, in the larger frames, there can be frame size differences between an open (ODP) motor and an enclosed (TEFC) motor. If you find a frame size where the last letter is a Z, that indicates a special frame, generally only available from the OEM that had the motor built for them.

Take a look at the Nema Motor Frame Chart below: