by Joe Evans, Ph.D.
December 17, 2011

The frame sizes (physical dimensions) of AC motors have changed substantially through the years. Originally, they were considerably larger than those in use today. This increased size was the result of inefficiency and the need to dissipate heat.

There was not much standardization, and a particular motor might be built on several different frames. This made replacement more difficult since the dimensions, including shaft height and the placement of the base mounting holes, could also change.

As new materials and advanced design techniques became available, the frame size necessary to produce a particular horsepower was reduced and, eventually, size standardization became the norm.

In 1952, the National Electric Manufacturers Association (NEMA) introduced a new frame size standardization called the "U" frame. The U frame size was designed for Class A insulation, which has a temperature rating of 105 degrees C. Twelve years later, the standardization was revamped and the "T" frame motor appeared. It was designed for higher temperature, Class B insulation, which has a rating of 130 degrees C.

The T frame remains the standard today, but the U frame is still in use, especially in the automotive industry. Surprisingly, I recently visited a pump station in Seaside, Ore., that still operates two 100-horsepower original frame (manufactured before 1952) motors.

The result of these two standardizations was a systematic decrease in motor frame size. For example, prior to 1952, a 10-horsepower, 1,800-rpm motor was built on a 324 frame. After 1952, that same motor used a 256U frame, and in 1964, it was reduced to a 213T frame. Today, even higher temperature insulation classes (Class F, 155 degrees C and Class H, 180 degrees C) allow smaller frame sizes to accommodate even higher horsepowers.

The three digits that make up the frame size are directly related to the dimensions of the motor built on that particular frame. The first two digits of the frame size when divided by four will result in the height of the shaft centerline above the bottom of the mounting foot. For a 445T frame shaft height would be 11 inches (44/4 = 11).

Although there is no inch reference, the third digit is indicative of the distance from the motor's vertical centerline to the front and rear foot mounting holes. It is also indicative of the overall motor length. A link to a frame size dimensional chart for U and T frame NEMA motors and IEC motors is included at the end of this article.

In addition to the standard, three digit nomenclature, an alphabetical suffix is added to designate any modifications to the standard T frame design. For example, a suffix of "C" or "D" designates a C face or D mounting flange while "JM" or "JP" designates a close coupled pump motor that is designed for mechanical seals or packing. "S" specifies a short shaft that is designed for direct coupling and should not be used in belt dive applications. "Y" specifies a custom, nonstandard mounting configuration while "Z" specifies a custom, nonstandard shaft.

Motor Enclosures

Motors have two basic types of enclosures—open and enclosed. Open enclosures allow for the free flow of air through the motor internals while those that are enclosed greatly restrict or prohibit the entry of outside air. The basic designs used in applications are described below.

Open Drip Proof

The open drip proof (ODP) enclosure is intended for installation in clean and dry environments and tends to be the standard in the HVAC industry and other clean, indoor applications. An internal fan circulates ambient air through the enclosure and provides a highly efficient cooling process.

Totally Enclosed Fan Cooled

The totally enclosed fan cooled (TEFC) enclosure is designed for outdoor installation and dirty or dusty indoor applications. Special TEFC designs are also used in processing plants in which periodic wash down is required. Unlike the ODP enclosure, it uses an external fan to force ambient air over the motor's exterior surface. Cooling is not as efficient as that of the ODP enclosure and service factor (SF) is sometimes limited to unity (1.0).

Totally Enclosed Air Over

The totally Enclosed Air Over (TEAO) enclosure is designed for damp or wet environments in which the driven machine provides the air flow required for cooling. A common application is cooling tower fans. TEAO motors often have multiple horsepower ratings, and the usable horsepower depends upon the velocity and temperature of the air flowing over the motor.

Totally Enclosed Non-Ventilated

The totally enclosed non-ventilated (TENV) enclosure is designed for dusty environments and is usually limited to 5 horsepower and less. It uses the motor surface area to transfer heat to the surrounding air without the aid of an external fan.

Hazardous Locations

Hazardous location motors are a totally enclosed design that are intended for use in potentially dangerous areas. The Class I, Explosion Proof (XP) enclosure is a special type that is designed for use in locations in which potentially explosive liquids and gases may be present. Class II enclosures are used in locations that are subject to combustible dusts, such as coal and grains. The area where the rotor shaft exits the enclosure is designed to contain any sparks that could occur inside the motor enclosure.