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The Key to Figuring the Right Frame Size

Jul 1, 2000 12:00 PM, By Ed Cowern, P.E.

To have confidence you are ordering the right motor for your specific application, you must understand motor frame designations.

Miniaturization isn't just for electronics. And you probably know carmakers pack more ponies into smaller engines these days. The same thing has taken place with industrial motors. Compared to their predecessors, today's motors pack more horsepower into a smaller physical size. Fortunately, the National Electric Manufacturers Association (NEMA) provides us with frame size standardization, so you can make intelligent choices among the different sizes of motors.

This frame size standardization is a key part of motor interchangeability. This means the same horsepower, speed, and enclosure will normally have the same frame size from different motor manufacturers. Thus, you can replace a motor from one manufacturer with a similar motor from another, if they are both in standard frame sizes.

Thanks to standardization, we have three groups of frame sizes. The oldest is original. In 1952, manufacturers made new frame assignments: U frames. They introduced T frames in 1964.

Original frame size motors (pre-1952) still in existence will need replacement in the near future. And, yes, such motors still operate today. For example, the Panama Canal still uses two motors that have operated as long as the canal itself.

Also, the many U frame motors that followed original motors will eventually fail and require replacement. Thus, you need strong reference material on frame sizes, and some knowledge of changes that took place as a part of the so-called rerate programs.

Motors with the same three frame-size digits have the same base mounting hole spacing and shaft height.

Rerating and temperatures. Why can we rerate motor frames to get more horsepower in a frame? For the most part,this is the result of improvements made in motor design and insulating materials. Newer insulations allow a motor to run much hotter; and thus handle the extra heat generated when producing more horsepower.

The original NEMA frame sizes ran at very low temperatures, but the U frame motors have Class A insulation with a rating of 105DegrC. T frame motor designs run even hotter. Their Class B insulation has a temperature rating of 130DegrC. This increase in temperature capability made a corresponding increase in horsepower in the same size package possible.

To accommodate the larger mechanical horsepower capability, shaft and bearing sizes had to increase. Thus, the original 254 frame (5 hp at 1800 rpm) has a one and one-eighth-in. shaft. The 254U frame (7.5 hp at 1800 rpm) has a one and 3-eighths-in. shaft, and the 254T frame (15 hp at 1800 rpm) has a one and fivbe-eighths-in. shaft. Bearing diameters also increased to accommodate the larger shaft sizes and heavier loads associated with the higher horsepower.

Frame size basis. At first glance, the frame size numbering system may seem capricious, but there is some logic to it. For example, the first two digits of a three-digit frame size relate to the shaft height of a foot-mounted (rigid base) motor in quarters of an inch. You can use this value to figure shaft height (D dimension). Just divide the first two digits by four. Thus, a 405 frame (original, U frame, or T frame) has a shaft height of 40 divided by 4, or 10 in.

Although no direct inch measurement relates to it, the third digit of three-digit frame sizes is an indication of the motor body's length. The longer the motor body, the longer the distance between mounting bolt holes in the base (greater F dimension). For example, a 145T frame has a larger F dimension than does a 143T frame.

The term "fractional horsepower" covers those frame sizes having two-digit designations, as opposed to three-digit designations. The frame sizes normally associated with fractional horsepower motors are 42, 48, and 56. In this case, each frame size designates a particular shaft height, shaft diameter, and face or base mounting hole pattern.

In these motors, specific frame assignments do not relate to horsepower and speed. So, it's possible a particular horsepower and speed combination might exist in three different frame sizes. In this case, it's essential you know the frame size as well as the horsepower, speed, and enclosure type.

The two-digit frame number relates to the shaft height in sixteenths of an inch. You can figure that a 48-frame motor will have a shaft height of 48 divided by 16, or 3 in. Similarly, a 56-frame motor has a shaft height of 3.5 in.

From the 56 frame on up, motors are available in horsepowers greater than those normally associated with fractionals. For example, 56 frame motors go as high as 5 hp. For this reason, calling motors with two-digit frame sizes "fractionals" is somewhat misleading.

Selecting the right motor is a complicated process. But, if you know the frame size, you'll simplify the process significantly.

This text is an adaptation of The Cowern Papers, courtesy Baldor Electric Co., Wallingford, Conn., edited by Mark Lamendola, EC&M Technical Editor. Cowern is an Application Engineer for Baldor.

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