Motor Enclosure Types | Open and Totally Enclosed Motors

The article discusses motor enclosure types as outlined by the NEMA standards, focusing on open and totally enclosed motors. Open motors feature ventilating openings for cooling, while totally enclosed motors restrict air exchange. Various subtypes within each category cater to specific environmental conditions, with distinctions like drip-proof, splash-proof, and explosion-proof designs.

The NEMA has developed standards for the dimensions of motor enclosures of various sizes. The size (height, length, shaft diameter, etc.) is indicated by a frame number. There is considerable variation in the frame number for a given-horsepower motor. This variation is the result of using different insulating and core materials. As better materials have been developed, a given frame has been able to house a motor with a larger horsepower rating. For example, a 56 frame has been for many years a common size for fhp motors, but it is now possible to get multi-horsepower motors in this size of the frame. In general, with all other factors ­being equal, the larger the frame number, the larger the horsepower rating. For example, a 2-hp motor is commonly in a 145-T frame, whereas a 10-hp motor is in a 213-T frame. The 145-T frame is approximately 7 in. in diameter and 8 in. long excluding the shaft. Comparable dimensions for the 213-T frame are 10 and 11 in.

Depending on the style of enclosure, motors can be classified as either open motors or ­totally enclosed motors. There are many different enclosure types within each of these broad ­categories.

Open Motors

Open motors (see Figure 1) have enclosures with ventilating openings that allow surrounding air to be forced through the enclosure to cool the motor windings. Various types of open motors put restrictions on the ventilating openings. These restrictions are associated with the type of environment in which the motor is intended to operate. For example, a drip-proof motor is an open motor in which the openings are designed so that particles or drops striking the motor enclosure at an angle no greater than 15° from the vertical will not impair the operation of the motor. Splash-proof motors are designed so that matter splashing within specified angles will not harm the motor. Lint-free motors have smooth, streamlined openings so that lint in the air will not build up and clog the openings. Guarded motors have the openings screened or grilled so that objects greater than a specified size and shape cannot enter the closure and make contact with electric or moving parts of the motor. Technical specifications of these and other types of open motors are published by the NEMA.

The shaft end of a drip-proof open motor.

Figure 1. The shaft end of a drip-proof open motor.

Totally Enclosed Motors

Totally enclosed motors (see Figure 2) are motors that do not allow free exchange between surrounding air and air within the enclosures. Although they have no openings, they are not sealed or airtight. Totally enclosed nonventilated (TENV) motors have no external fans to force air over the external surface of the enclosure. Totally enclosed fan-cooled (TEFC) motors have an external fan, attached to the rotor shaft, which forces air circulation around the motor. An explosion-proof motor is totally enclosed and designed so that an explosion of a gas inside the motor will not cause a gas around the motor to explode also. Other types of totally enclosed motors include waterproof motors which can be “hosed down” and dust-ignition-proof motors which prevent significant amounts of explosive dust from entering the enclosure and which will not ignite explosive dust around or on the enclosure.

A totally enclosed, fan-cooled motor.

Figure 2. A totally enclosed, fan-cooled motor.

Many totally enclosed motors have fins on the exterior of the enclosure to aid in transferring heat from the motor windings to the external air. Larger totally enclosed motors may use circulating water or air as well as heat exchangers to help cool the motor.

Key Takeaways of Motor Enclosure Types

Understanding motor enclosure types is crucial for selecting the appropriate motor for specific applications and environments. Whether it’s an open motor with ventilating openings for cooling or a totally enclosed motor designed to prevent external elements from entering, choosing the right enclosure ensures optimal performance and longevity of the motor. The various subtypes within each category cater to different environmental conditions, highlighting the importance of matching the motor enclosure type with the operating environment to ensure safety, reliability, and efficiency.

Motor Enclosure Types Review Questions

  1. How is the physical size of a motor specified?
  2. What does the abbreviation “TEFC” stand for?
  3. List two categories used to classify motor enclosures.
  4. True or false. Explosion-proof motors are designed to prevent gas and vapor from exploding within the motor enclosure.


  1. by frame number
  2. totally enclosed fan-cooled
  3. open and totally enclosed.
  4. F