NEMA stands for National Electric Manufacturer's Association. This trade association sets standards and produces publications for electrical equipment in the United States, including motors. Motors have two types of casings: ODP and TEFC.
ODP
ODP stands for "open drip proof." This means that the motor casing has open vents that allow for cooling. These vents are angled downward to prevent water from dripping directly onto the motor; however, they are still susceptible to wind-driven rain and splashing. Additionally, they are not protected from dust or other airborne contaminants, or vermin.
TEFC
TEFC stands for "totally enclosed fan cooled." This means that the motor casing is completely sealed with a fan blowing over it. The heat moves within the casing through convection and dissipates through the surface with assistance from the fan. These motors are protected from the outside environment.
Price Difference
TEFC motors are generally more expensive. This is because they contain an extra piece of equipment -- the fan -- and they serve a specialized purpose. Also, TEFC motors have a thinner, larger casing to allow for better heat dissipation, which adds to the cost.
Availability
Most manufacturers stock both ODP and TEFC motors. However, they are more likely to carry ODP. If you're purchasing a piece of equipment that already has a motor, such as a fan, it is probably equipped with an ODP motor.
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