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 originated a short while later. Because of its simplicity and cheapness, the 3-stage squirrel-cage induction engine is now by considerably the most typical type of motor in use for driving commercial plant, particularly of little power 3 Phase Induction Motor ratings (up to 20 kW). The slip-ring induction motor is not as merely built as the cage induction engine, nonetheless it is possible to control its starting overall performance and acceleration by connecting external equipment (commonly resistors) into the rotor circuit via slip-rings. 3-phase induction motors could be made to any preferred power ratings, but natural running speeds are linked to the power-supply frequency.