The polyphase induction engine 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 electric motor is currently by much the most typical type of motor in use for driving industrial plant, particularly of little power ratings (up to 20 kW). The slip-ring induction electric motor is not as just built as the cage induction motor, nonetheless it is possible to regulate its starting efficiency and speed by connecting external 3 Phase Induction Motor apparatus (commonly resistors) into the rotor circuit via slip-rings. 3-stage induction motors can be made to any desired power ratings, but natural running speeds are linked to the power-supply frequency.