The polyphase induction electric motor invented by Nikola Tesla in 1886 have been developed by 1895 practically into its present squirrel-cage form. The slip-ring induction motor was developed a short while later. Due to the simplicity and cheapness, the 3-stage squirrel-cage induction motor is currently by much the most typical type of motor in use for driving industrial plant, particularly of small power ratings (up to 20 kW). The slip-ring induction engine is not as merely constructed as the cage induction motor, but it is possible to regulate its starting efficiency and velocity by connecting external gear (commonly resistors) in to the rotor circuit via slip-rings. 3-phase induction motors can be made to any preferred power ratings, but natural running 3 Phase Induction Motor speeds are linked to the power-supply frequency.