Understanding Three Phase AC Motors Part 3: NEMA Frame Sizes and Enclosures
There are several types of enclosures that can be specified when choosing a motor. It is particularly important to choose the right enclosure type, especially when installing a motor in hazardous locations or in environmentally harsh conditions.
A totally enclosed non-ventilated enclosure (TENV) limits airflow through the motor enclosure. This type of enclosure utilizes a seal where the shaft passes through the housing to prevent dust and other contaminants from entering the motor. Note that this enclosure is not airtight.
A totally enclosed fan cooled enclosure (TEFC) is similar to the TENV motor. The main difference is that the TEFC motor is constructed with an externally mounted fan which blows air over the motor’s exterior body. Since the fan is external, and no air is flowing through the enclosure, this type of enclosure is permissible in hazardous locations or environmentally harsh conditions.
An open drip proof enclosure (ODP) permits cooling air to flow through the motor enclosure. Vents allow air flow, which is paired with an internal fan attached to the rotor. The vents are designed such that liquids cannot enter the enclosure from above.
NEMA Frame Sizes and Dimensions
In order to easily specify common motor dimensions such as shaft length and mounting base length, NEMA has defined standard dimension letters to indicate the dimensions of common components. This is useful in replacing existing motors without having to redesign mounting installations. For example, the dimension letter “E” helps indicate the length between the mounting feet of a motor. Refer to the drawing below for further details.
Motor Side Profile NEMA Frame Dimensions
Motor Shaft NEMA Frame Dimensions
Motor Drive End Profile NEMA Frame Dimensions
There are two categories of frame sizes: fractional horsepower and integral horsepower.
Integral horsepower motors are designated by frame sizes 143 and beyond.
A “T” in the motor frame size designation for an integral horsepower motor indicates that the motor is built to current NEMA frame standards. Motors that have a U in their motor frame size designation, are built to older NEMA standards
To help identify key frame dimensions, a frame size designation code is used. The first two digits of the frame size designation determine shaft height, which is the distance from the center of the shaft to the mounting surface. Shaft height can be determined by dividing the first two digits of the code by 4. A motor with frame size code 210LP will have a shaft height of 21/4 inches, or 5 ¼ inches.
The third digit is used to determine the distance between the feet mounting bolt holes. A motor with frame size code 210LP would be used for a vertically mounted motor – there are no feet for a vertically mounted motor. However, a motor with frame size code 213LP would have a distance of 3 inches between the feet mounting bolt holes.
To determine other dimensions for a given frame size code, one can read NEMA MG-1 or refer to the many online, readily available NEMA frame size charts.
Floor mounted motors are indicated with the prefix F, while wall mounted motors are indicated with the prefix W and ceiling mounted motors are indicated with the prefix C. An F-1 motor indicates the location of the conduit box of a floor mounted motor to be on the left of the motor, as viewed from the shaft end. An F-2 motor indicates the conduit box to be on the right as viewed from the shaft end.