AC induction motor: the dominant motor today
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 (hp) to tens of thousands of a horsepower (hp). Induction motors 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. This kind of ac motor is so named because of the shape of the rotor bar structure. And wound-rotor models, in which coils of wire turn the rotor windings, are also available.
These are expensive but offer greater control of the motor's performance characteristics. So these ac motors are 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 premium-efficiency model. Premium-efficiency motors typically cut losses by 10 to 40 percent (increasing motor efficiency by 1 to 10 points) and carry a cost premium of 15 to 30 percent compared with a standard motor.
In retrofit situations, users also 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. This is because motor rewinds often degrade motor efficiency by 1 to 3 percent.
