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