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The polyphase induction motor invented by Nikola Tesla in 1886 have been developed by 1895 virtually into its present 3 Phase Induction Motor 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 electric motor is now by considerably the most common type of motor used for driving industrial plant, particularly of little power ratings (up to 20 kW). The slip-ring induction engine is not as simply constructed as the cage induction motor, but it is possible to regulate its starting functionality and quickness by connecting external apparatus (commonly resistors) in to the rotor circuit via slip-rings. 3-phase induction motors could be made to any preferred power ratings, but natural running speeds are related to the power-supply frequency.