The polyphase induction engine invented by Nikola Tesla in 1886 have been developed by 1895 virtually into its present 3 Phase Induction Motor squirrel-cage form. The slip-ring induction motor was developed a short time later. Due to the simplicity and cheapness, the 3-phase squirrel-cage induction motor is now by significantly the most typical type of motor in use for driving commercial plant, particularly of little power ratings (up to 20 kW). The slip-ring induction electric motor is not as basically constructed as the cage induction electric motor, nonetheless it is possible to regulate its starting functionality and quickness by connecting external tools (commonly resistors) in to the rotor circuit via slip-rings. 3-phase induction motors can be made to any preferred power ratings, but natural running speeds are related to the power-supply frequency.