The polyphase induction motor invented by Nikola Tesla in 1886 had been developed by 1895 virtually into its present squirrel-cage form. The slip-ring induction motor was developed a short time later. Because of its simplicity and cheapness, the 3-phase squirrel-cage induction motor is now by far the most common type of motor in use for driving industrial plant, particularly of small power ratings (up to 20 kW). The slip-ring induction motor is not as simply constructed as the cage induction motor, but it is possible to control its starting performance and speed by connecting external equipment (commonly resistors) into the rotor circuit via slip-rings. 3-phase induction motors can be made to any desired power ratings, but natural running speeds are related to the power-supply frequency.