The polyphase induction electric motor invented by Nikola Tesla in 1886 had 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 motor is currently by far the most typical type of motor in use for driving industrial plant, particularly of little power ratings (up to 20 kW). The slip-ring induction engine is not as basically built as the cage induction engine, nonetheless it is possible to regulate its starting functionality and quickness by connecting external apparatus (commonly resistors) into the rotor circuit via slip-rings. 3-phase induction motors could be made to any preferred power ratings, but organic 3 Phase Induction Motor running speeds are related to the power-supply frequency.