There are several specific types of electric motor enclosures available on the market, and they can be broken down into two basic types: “totally enclosed” and “open.” I would like to take one step further and say “expensive” and “cheap.”
The Totally Enclosed Fan Cooled (TEFC) motors (photo above) are designed to operate with minimal impact from the ambient environment. Ambient temperature and the external cleanliness of the motor are the only operational concerns. In addition to routine maintenance, keep these motors clean and within the design temperature parameters and they can last 20 years or more. This is also why I refer to them as expensive. Enclosures aren’t cheap, but they can pay for themselves over the lifetime of the motor.
Open Drip Proof (ODP) motors are usually much cheaper than TEFC motors, but are highly susceptible to whatever contaminants are present in the ambient environment. Anything and everything in the air surrounding the motor is pulled through and around the interior of the motor. When we add some humidity to the mix it means a hot motor in the process of cooling off is not only pulling in air, it’s pulling in moisture too. This condensation coupled with dust and particulate from the manufacturing process attaches to the insulation. This in turn can make the motor run hotter than it should.
In order to attain full longevity on a motor, the most important thing is to keep the operating temperature of the motor within the design parameters of the insulation class. Any motor exceeding that temperature leads to motor failure. To put it bluntly, installing an ODP motor outside, or inside in a moderately dirty environment is a substantial waste of money. When they are not running, insects and critters seem to use these motors for nests. These motors last a fraction of the time that TEFC motor does. These motors are fine when installed in clean climate controlled environments.
Make sure that when you procure motors, you choose the right type of motor for the ambient environment it is designed to operate in. This might prove to be more expensive initially, but will save considerably over the life of the motor.