The AC induction motor is the dominant motor technology in use today, representing more than 90 percent of installed motor capacity. Induction motors are available in single-phase and polyphase configurations, in sizes ranging from fractions of a horsepower to tens of thousands of horsepower. They may run at fixed speeds—most commonly 900, 1,200, 1,800, or 3,600 rpm—or be equipped with an adjustable-speed drive. The most commonly used AC motors by far have a squirrel-cage configuration, so named because of the shape of the rotor bar structure. Wound-rotor models, in which coils of wire turn the rotor, are also available. Although they are expensive, they offer greater control of the motor’s performance characteristics and are therefore most often used for special torque and acceleration applications and for adjustable-speed applications.
The major choice facing motor specifiers is whether or not to select a motor that complies with an efficiency specification developed by the National Electrical Manufacturer's Association (NEMA)—known as NEMA Premium. To meet NEMA Premium specifications, a motor must exceed the minimum efficiency mandated by law (through the 1992 Energy Policy Act in the U.S. and the Canadian Standards Association's 1995 Standard C-747) by between 0.4 and 3.0 percentage points, depending upon the size and type of motor. More information about NEMA Premium specifications is available here. Note that the NEMA Premium specification will become the minimum efficiency standard for all general-purpose motors between 1 and 200 horsepower sold in the U.S. after December 19, 2010. It has also been proposed as a new standard for motors sold in Canada beginning January 1, 2011. Premium-efficiency motors often carry a price premium as well, but depending on motor size and type, some premium-efficiency models are available at little or no price premium (Figure 1).
Figure 1: Normalized price premium versus efficiency gain, 10- and 100-horsepower TEFC motors
Price and efficiency often bear little or no relationship, as can be seen in these charts for 10- and 100-hp totally enclosed fan-cooled (TEFC) motors. In each chart, diamonds indicate general-purpose, 2-pole motors with identical frames and no special features. Diamonds enclosed in squares indicate motors that also share the same insulation class and service factor.
In retrofit situations, users have the choice of repairing failed motors or replacing them. It is becoming common practice among energy-conscious companies to replace all failed, moderate-duty induction motors up to about 125 horsepower (hp) with new premium-efficiency models rather than repairing and rewinding the failed motor. If not done carefully, rewinding can decrease motor efficiency by up to 2 percent.