The polyphase induction motor invented by Nikola Tesla in 1886 had been developed by 1895 virtually into its present squirrel-cage form. The slip-ring induction motor originated a short time later. Due to the simplicity and cheapness, the 3-stage squirrel-cage induction motor is currently by far the most typical type of motor in use for driving commercial plant, particularly of small power ratings (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 efficiency and acceleration by connecting external devices (commonly resistors) into the rotor circuit via slip-rings. 3-phase induction 3 Phase Induction Motor motors could be made to any preferred 
power ratings, but organic running speeds are related to the power-supply frequency.
