Induction motor

An induction motor, also called an asynchronous motor, is an alternating current electric motor in which the current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding, so the rotor needs no electrical connection. Three-phase squirrel-cage induction motors are widely used as industrial drives because they are self-starting, reliable and economical, while single-phase types serve smaller loads. Induction motors were traditionally constant-speed machines but are increasingly run at variable speed using variable-frequency drives. The first commutator-free polyphase induction motors were invented independently by Galileo Ferraris and Nikola Tesla, who demonstrated working models in 1885 and 1887, and Mikhail Dolivo-Dobrovolsky invented the cage-rotor induction motor in 1889. George Westinghouse licensed Tesla's patents in 1888, and Westinghouse and General Electric later cross-licensed the squirrel-cage rotor design. The rotor always turns slightly slower than the rotating stator field, which is what induces the rotor current.