The polyphase induction electric motor invented by Nikola Tesla in 1886 had been developed by 1895 virtually into its present 3 Phase Induction Motor squirrel-cage form. The slip-ring induction motor originated a short while later. Because of its simplicity and cheapness, the 3-stage squirrel-cage induction motor is currently by much the most typical type of motor in use for driving industrial plant, particularly of small power rankings (up to 20 kW). The slip-ring induction motor is not as simply built as the cage induction motor, but it is possible to control its starting performance and swiftness by connecting external devices (commonly resistors) into the rotor circuit via slip-rings. 3-phase induction motors could be made to any desired power ratings, but organic running speeds are related to the power-supply frequency.