The polyphase induction engine invented by Nikola Tesla in 1886 had been developed by 1895 3 Phase Induction Motor practically into its present squirrel-cage form. The slip-ring induction motor was developed a short time later. Because of its simplicity and cheapness, the 3-stage squirrel-cage induction motor is now by much the most common type of motor used for driving industrial plant, particularly of little power ratings (up to 20 kW). The slip-ring induction motor is not as basically constructed as the cage induction engine, nonetheless it is possible to control its starting overall performance and swiftness by connecting external apparatus (commonly resistors) into the rotor circuit via slip-rings. 3-stage induction motors can be made to any desired power ratings, but organic running speeds are linked to the power-supply frequency.
3 Phase Induction Motor