Motor Enclosures

March 5, 2025

Motor Minute™ Technical Tip – Motor Enclosures

Hi, Chris from Regal Rexnord here with your Motor Minute technical tip. Helping you become an HVAC motor pro one minute at a time.

Today’s Motor Minute topic is about motor enclosures.

The enclosure is the case (shell or main frame and end frames or end shields) around the motor.

The motor enclosure basically serves two functions:

• Allows the motor to operate at the proper temperature, meaning how the motor dissipates heat to operate under load without overheating.
• Protects the internal components (windings, bearings and other mechanical parts) from the environment it will exist in. This includes but is not limited to liquids including moisture, dust and debris.

There are many different enclosure types. The enclosure type may be listed on the motor rating plate. The most widely used motor enclosures in HVAC applications are:

• Totally Enclosed Air Over (TEAO)
• Open Air Over (OAO)
• Open Drip Proof (ODP)
• Totally Enclosed Fan Cooled (TEFC)

Totally Enclosed Air Over (TEAO)

The motor shell and end shields are built without any openings. This motor is designed to be located in the air stream it is moving, as it is air over cooled. The most common HVAC application for these motors is the outdoor condensing fan motor.

Universal multi-position or all angle mount outdoor condensing fan motors may be built with drain plugs in the end shields. Simply put, always remove the drain plug on the side of the motor facing down.

Open Air Over (OAO)

The motor shell and/or end shields are built with openings or vents to increase heat dissipation. This motor is designed to be located in the air stream it is moving, as it is air over cooled. This type of motor is commonly used in a controlled environment (relatively clean, dry locations). The most common HVAC application for this motor is the indoor blower motor.

Maintaining equipment with quality air filters is important to keep the motor vents from becoming blocked with dirt that will reduce heat dissipation. Restricted air filters may also reduce the volume of air cooling the motor. Both of these issues could lead to reduced motor life.

Some open air over motors are designed specifically for outdoor condensing fan motor applications in commercial HVAC systems. However, these motors are typically designed to be mounted in the vertical shaft-up position and are built with a rain shield or slinger on the shaft.

This is a good time to point out that understanding the mounting position specification of the motor being selected is just as important as the enclosure.

These motors may also be found in condensing fan applications where the motor is protected from the outdoor environment by the HVAC unit.

Open Drip Proof (ODP)

This design allows air to circulate through the motor for heat dissipation and protects the motor from droplets of liquid falling within a 15⁰ angle of vertical. An internal fan blade provides self cooling by circulating air through the vents. Some rating plates will list the enclosure as simply drip proof (DP). This motor can be used for indoor or outdoor applications.

Totally Enclosed Fan Cooled (TEFC)

The motor shell and end shields are built without openings or vents for heat dissipation, preventing the free exchange of air between the inside and outside of the motor. However, this does not make the motor completely airtight. While it’s not specified, this motor is durable enough for environments of light moisture, dust and debris.

This motor is built with an external fan blade attached to the opposite drive end of the motor, inside a fan shroud, that blows air across the outside of the motor for heat dissipation.

Selecting the correct enclosure will ensure the motor is properly protected and cooled in the environment it will exist in.

While the motor being replaced is likely to list the enclosure type on the rating plate, occasionally it is not. And even if it is, it’s a good practice to evaluate the environment, application and mounting position the motor will work in, to select the correct enclosure based on the knowledge learned here.

The motor being replaced may have failed because the wrong enclosure was selected, or the motor was mounted in an orientation it was not designed for.

And that’s a wrap on this segment of Motor Minute technical tips. Remember that we provide motor training and product information in multiple formats including videos, literature, podcasts, articles, of course Motor Minute technical tips and classroom education both online and face to face. All of this industry leading training is available at no charge to HVAC professionals at regalmmu.com. Thank you for taking the time to build your motor knowledge.