Tesla’s 1888 alternating-current motor was covered by U.S. Patent No. 381,968, titled “Electro-Magnetic Motor.” The patent was granted on May 1, 1888, and described a motor operated by progressively shifting magnetic poles—an early practical expression of the rotating magnetic field.
Tesla’s design used alternating currents in separate circuits to create a moving magnetic field that turned the rotor. Because the motor had no mechanical commutator, it avoided the sparking, brush wear, and maintenance associated with many earlier direct-current motors. This made it especially important for alternating-current power systems.
Patent 381,968 was one of a group of Tesla patents issued on the same day. Related patents covered electrical transmission, distribution, and other motor arrangements, so the phrase “Tesla’s 1888 AC motor patent” can refer broadly to a patent family. Patent 381,968 is the standard single-number answer for the core electro-magnetic motor patent.
The breakthrough helped attract George Westinghouse, who licensed Tesla’s AC motor and power-system patents in 1888. The deal helped propel Tesla’s polyphase AC technology into commercial development.