by Joe Evans

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.

Nameplate Information

NEMA MG-1 requires that certain information be included on the nameplate of all single- and three-phase motors. Typical nameplate information includes horsepower, volts, amps, Hz, phase, rpm, insulation class, enclosure, frame size, efficiency, service factor, power factor, duty, ambient, code and design. Most are straightforward but, there are several that require further explanation.


Efficiency (Eff) defines how well a motor converts electrical energy into mechanical energy. The motor's full load efficiency is shown on the nameplate, and it is often less than the actual maximum efficiency. Maximum motor efficiency typically occurs between 70 percent and 95 percent of full load, and most NEMA motors can be operated as low as 60 percent of full load without any significant loss in efficiency. This allows the flexibility of upgrading to the next higher horsepower when loading is at or very near the full load capacity of the lower horsepower motor.

Service Factor

Service factor (SF) is an often misunderstood piece of information. SF is a multiplier that indicates the actual horsepower that the motor can deliver over and above the nameplate horsepower. For example, if a 10 horsepower motor has an SF of 1.15, it is designed to deliver 11.5 horsepower without overloading.

SF is intended to handle small, intermittent overloads, occasional increases in ambient temperature and periods when the actual voltage is lower than the nameplate voltage. For example, a typical 10 horsepower, 230-volt motor draws approximately 24 amps at full load. If the voltage is reduced to 200 volts, the current increases to 28 amps, which is the normal current draw of an 11.5 horsepower motor.

Therefore, a 230-volt motor with a 1.15 SF can accommodate such a short-term voltage drop. However, it should not be operated on a true 200/208-volt circuit since there would be no remaining SF available to accommodate any additional drop in voltage.

Power Factor

Power factor (PF) is the ratio of active power in watts to the apparent power in volts/amps at full load. A motor can be designed for high efficiency or high PF but not both. Since efficiency cannot be enhanced in the field, motors are designed for high efficiency and the trade off is lower PF. Fortunately, PF can be easily increased in the field by adding an appropriate capacitor to the circuit.


Duty defines the length of time that the motor can operate while meeting its other nameplate ratings. Most industrial motors are rated continuous (or cont.) while certain special application motors will show intermittent run times in minutes.

Ambient Temperature

Ambient temperature is the maximum allowable temperature of the air surrounding a motor when it is operated continuously at full load. A typical ambient rating is 40 degrees C (104 degrees F). The actual operating temperature is the sum of the ambient temperature and the internal temperature rise at full load.

For example, Class B insulation is rated at 130 degrees C and is designed to handle an internal temperature rise of 90 degrees C (assumes a 1.15 SF) when operating in a 40 degrees C ambient environment.

Code Letters

Code letters (A – V) provide the range of current required (locked rotor current—LRC) during across-the-line starting for a particular motor. The value indicated by the code letter is in units of KVA/horsepower. A typical industrial motor will require five to seven times full load current during starting. Motors 15 horsepower and above require a lower KVA/horsepower when starting than do lower horsepower designs.

The simple equation below will provide approximate LRC results at 460 volts (For 230 volts, change the constant to 2.5 and for 200 volts, change it to 2.9):

460 Volts LRC = 
Code Letter Value x horsepower x 1.25

Design Letters

Design letters (A-D) indicate the shape of the torque curve produced by a particular motor. Design B is the standard for industrial duty motors and provides excellent performance in most industrial applications. Design C provides a higher starting torque while Design D is a high slip motor that provides the highest starting torque. Design A is a special purpose motor that offers the highest pullout torque.

NEMA & IEC frame dimension charts can be found at

Next month's column will review the conditions that affect the life of an AC motor.

Pumps & Systems, June 2011