NEMA T-Frame Motors and Why They Are Not All the Same
May 28, 2020
If you have a specific NEMA-T frame motor, you can be assured that, regardless of the manufacturer, that specific T frame (for example, a 324T) will have the same shaft length, shaft diameter, and bolt hole mounting dimensions. However, what many people don’t realize is that the physical size of specific T frame motors can vary greatly. That means you could buy what you are assuming is a simple drop-in replacement that doesn’t quite fit in the space allotted for it.
NEMA Class Designations
NEMA, which stands for National Electrical Manufacturers Association, revolutionized electric motor manufacturing by using a motor’s starting-torque and accelerating load to categorize it into specific classes. NEMA standard motors make it much easier for you to find the right one for your particular application.
The four standard classes they developed are NEMA A, NEMA B, NEMA C, and NEMA D (and very large motors classified as Above-NEMA). Each classification has certain applications they are well-adapted for (e.g., NEMA C is for situations that require high starting torque while NEMA A motors lend themselves to applications that require a high speed at full load).
NEMA T-Frame Sizes
The combination of NEMA designations and frame sizes can tell you quite a bit about a motor. The letter T in a frame size tells you frame number that has been standardized by NEMA so that horsepower, speed, and certain dimensions will be the same for all motors with that frame size. These dimensions include the shaft diameter, shaft height, shaft length, mounting-base dimensions and bolt-hole sizes. You can also take the frame size and refer to tables of NEMA frame assignments to find out other critical details including power and speed.
The first two digits in the frame size designation give you information about shaft height (the distance from the shaft center to the mounting surface) in inches. For example, the shaft height is the first two digits divided by 4, so if you have a 324T frame motor then the shaft height is 32/4 = 8 inches. The last digit is a NEMA code that allows you to look up the distance between the center lines of the mounting bolt holes.
What NEMA T-Frame Sizes Don’t Tell You!
As we noted earlier, those standardized dimensions don’t include the physical size. While three different motors with the same NEMA T frame size may have the same shaft length and diameter, the overall size of the motor can vary greatly. That means that a motor from manufacturer A would not necessarily fit into the same space as a motor from manufacturers B and C.
If you look up a frame dimension table for a T frame motor, you will see columns for the distance from the base to the center of the shaft, the distance between bolt holes, and critical dimensions related to the shaft itself. Based on these values, you can be sure that a replacement motor can be mounted to the same base as the original and have the same shaft setup — but the enclosure dimensions can be very different.
This causes a problem when you’re in the market for a replacement NEMA motor with a specific T frame size. You may buy a motor that, based on the dimensions given, should install and work fine, but there may not be enough space for the enclosure to fit. Before you invest in a motor, check the manufacturer’s dimensions for the enclosure to make sure it’s actually compatible with your setup. You don’t want to be counting on a surplus motor that you don’t have room to install.
When you are in the market for a NEMA T-frame motor, don’t just assume that the motor enclosure dimensions are identical for all the same T designation motors. While horsepower, speed, shaft dimensions, and mounting hole dimensions may be the same, the overall height, width, and length can vary from manufacturer to manufacturer. Do your research before you purchase a NEMA T-frame motor and you can avoid costly, time-consuming mistakes.
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