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Types of Motors
“Induction” motors are by far the most common type found in commercial buildings. They are used in approximately 90 percent of applications, so chances are you have quite a few of them in your facility. They are reliable and inexpensive and operate on single-phase or three-phase power, although most motors larger than one horsepower are three-phase. When comparing motors, the most common characteristics to be aware of include: enclosure type, speed, efficiency and service factor. When replacing a motor, be sure to compare these specifications on the replacement motor with those recommended by the equipment manufacturer.
Enclosure type – The motor enclosure refers to the casing of the motor and is designed to match the motor to its operating conditions. The two most common types are Open Drip-Proof (ODP) and Totally Enclosed Fan-Cooled (TEFC). ODP motors are made with ventilation openings in the casing that are positioned to keep liquid or particles from falling into the motor from above. They are a very common motor for HVAC fans and pumps. TEFC motors have enclosed casings, as the name implies, to keep moisture and particles out. They are equipped with an integral fan for cooling and typically are used in dirtier or wetter environments.
Speed – Induction motors are available in a wide range of speeds although 1,800 revolutions per minute (RPM) is the most common, accounting for more than 50 percent of the motor population. Motors of 1,200 and 3,600 RPM also are popular enough to be stocked by distributors and manufacturers.
Efficiency – Standard-efficiency and energy-efficient three-phase motors are available from most manufacturers of ODP and TEFC motors in speeds of 1,200, 1,800 and 3,600 RPM. Some manufacturers also offer a line of premium efficiency motors. Higher efficiency motors are built with higher quality materials and other design features that improve their performance over standard models.
Service factor – The service factor specifies the capacity of the motor to withstand prolonged overload conditions. A service factor of 1.0 indicates that prolonged operation above full load can damage the motor. A service factor of 1.15 is typical for motors one horsepower and above, and indicates that the motor can work at 1.15 times its rated horsepower without failing.