The polyphase induction engine invented by Nikola Tesla in 1886 have been developed by 1895 3 Phase Induction Motor practically into its present squirrel-cage form. The slip-ring induction motor originated a short while later. Because of its simplicity and cheapness, the 3-phase squirrel-cage induction engine is now by much the most common type of motor used for driving commercial plant, particularly of little power rankings (up to 20 kW). The slip-ring induction motor is not as just constructed as the cage induction electric motor, nonetheless it is possible to regulate its starting performance and acceleration by connecting external devices (commonly resistors) in to the rotor circuit via slip-rings. 3-stage induction motors could be made to any preferred power ratings, but natural running speeds are related to the power-supply frequency.