Practical rotating AC induction motors were independently
invented by Galileo Ferraris and Nikola Tesla a working motor model having
been demonstrated by the former in 1885 and by the latter in 1887. In
1888, the Royal Academy of Science of Turin published Ferraris's
research detailing the foundations of motor operation while however
concluding that "the apparatus based on that principle could not be of any
commercial importance as motor."
In 1888, Tesla presented his paper
A New System for Alternating Current Motors and Transformers to
the AIEE that described three patented two-phase four-stator-pole
motor types: one with a four-pole rotor forming a non-self-starting
reluctance motor, another with a wound rotor forming a self-starting
induction motor, and the third a true synchronous motor with separately
excited DC supply to rotor winding.
One of the patents Tesla filed in
1887, however, also described a shorted-winding-rotor induction motor.
George Westinghouse promptly bought Tesla's
patents, employed Tesla to develop them, and assigned C. F. Scott to help
Tesla, Tesla left for
other pursuits in 1889.
The constant speed AC induction motor was found
not to be suitable for street cars but Westinghouse engineers
successfully adapted it to power a mining operation in Telluride, Colorado
in 1891. Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the three-phase cage-rotor induction motor in
1889 and the three-limb transformer in 1890.
This type of motor is now
used for the vast majority of commercial applications. However, he claimed
that Tesla's motor was not practical because of two-phase pulsations,
which prompted him to persist in his three-phase work. Although
Westinghouse achieved its first practical induction motor in 1892 and
developed a line of polyphase 60 hertz induction motors in 1893, these
early Westinghouse motors were two-phase motors with wound rotors until B.
G. Lamme developed a rotating bar winding rotor.
The General Electric Company began developing
three-phase induction motors in 1891. By 1896, General Electric and
Westinghouse signed a cross-licensing agreement for the bar-winding-rotor
design, later called the squirrel-cage rotor. Induction motor improvements
flowing from these inventions and innovations were such that a 100
horsepower (HP) induction motor currently has the same mounting dimensions
as a 7.5 HP motor in 1897.