An induction or asynchronous motor is an AC motor in which current is induced in the rotor winding by the magnetic field of the stator winding, by electromagnetic induction. Therefore they do not require the sliding electric contacts, such as a commutator or slip rings, which are needed to transfer current to the rotor winding in other types of motor such as the universal motor. Rotor windings consist of short-circuited loops of conductors and are made in two types: the wound rotor and the squirrel-cage rotor.
Three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although the simple induction motor is a fixed-speed device, they are increasingly being used with variable-frequency drive (VFD) systems, which allow the speed to be varied. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel cage induction motors are very widely used in both fixed-speed and VFD applications.
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