The polyphase induction electric motor invented by Nikola Tesla in 1886 have been developed by 1895 practically into its present squirrel-cage form. The slip-ring induction motor was developed a short time later. Due to the simplicity and cheapness, the 3-stage squirrel-cage induction electric motor is now by considerably the most typical type of motor used for driving industrial plant, particularly of little power rankings (up to 20 kW). The slip-ring induction motor is not as merely built as the cage induction engine, but it is possible to control its starting efficiency 3 phase induction motor201909101626041562798and velocity by connecting external equipment (commonly 3 Phase Induction Motor resistors) into the rotor circuit via slip-rings. 3-stage induction motors can be made to any desired power ratings, but natural running speeds are related to the power-supply frequency.