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