The polyphase induction motor invented by Nikola Tesla in 1886 have been developed by 1895 virtually into its present squirrel-cage form. The slip-ring induction motor originated a short while later. Due to its simplicity and cheapness, the 3-phase squirrel-cage induction engine is currently by significantly the most common type of motor used for driving commercial plant, particularly of small power 3 Phase Induction Motor ratings (up to 20 kW). The slip-ring induction motor is not as basically built as the cage induction motor, but it is possible to control its starting efficiency and velocity by connecting external apparatus (commonly resistors) into the rotor circuit via slip-rings. 3-stage induction motors could be made to any desired power ratings, but natural running speeds are related to the power-supply frequency.