The polyphase induction motor invented by Nikola Tesla in 1886 have been developed by 1895 practically into its present squirrel-cage form. The slip-ring induction motor originated a short time later. Due to the simplicity and cheapness, the 3-stage squirrel-cage induction electric motor is 3 Phase Induction Motor currently by far the most typical type of motor in use for driving commercial plant, particularly of small power ratings (up to 20 kW). The slip-ring induction motor is not as merely constructed as the cage induction motor, nonetheless it is possible to control its starting overall performance and speed by connecting external equipment (commonly resistors) in to the rotor circuit via slip-rings. 3-stage induction motors can be made to any desired power ratings, but organic running speeds are related to the power-supply frequency.