Theory and Construction. Induction motors are common in most industrial applications. They all have primaries, or stator windings, connected to a power source, with a secondary winding, or rotor, which drives the mechanical load.
The most common type of induction motor is the squirrel-cage motor, so named because its rotor construction entails bars of copper or aluminum conductors, resembling an animal’s exercise wheel.
Wound-rotor induction motors entail rotors constructed of polyphase windings, which are connected through slip rings to a controlled external resistance. These are sometimes used when high breakdown torque or variable speed is required.
Induction motors can be either single speed, where the motor operates at a single, fairly constant speed, or multispeed, where the motor can be operated at two or more definite speeds.
Induction motors are likewise classified as either general purpose or special purpose. General purpose motors carry the NEMA’s design B ratings, feature standard operating characteristics and mechanical construction, and are used for applications such as pumps and fans, under normal service conditions. Special-purpose motors are designed for specific torque characteristics, such as design C or design D motors.
Speed-Torque Characteristics. Motor torque, the turning force delivered by the motor shaft, is defined at four points and is usually expressed as a percentage of running torques:
- Breakaway, starting, or locked-rotor torque.
- Minimum, or pull-up torque.
- Breakdown, pull-out, or peak torque.
- Full load, or running torque