Drawing from U.S. Patent 381968, illustrating principle of Tesla's alternating current motor.


An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.

Less commonly, linear AC motors operate on similar principles as rotating motors but have their stationary and moving parts arranged in a straight line configuration, producing linear motion instead of rotation.

Operation principles[]

AC motors operate with two rotating (or moving) magnetic fields on the rotor and stator respectively. Pulling or pushing the poles of the two magnetic fields along, the speed of the stator rotating magnetic field (Ws) and the speed of the rotor rotating magnetic field (Wr), which is relative to the speed of the mechanical shaft (Wm), must maintain synchronism for average torque production by satisfying the synchronous speed relation (i.e., ±Ws ±Wr = Wm).[1] Otherwise, asynchronously rotating magnetic fields would produce pulsating or non-average torque.

The two main types of AC motors are classified as induction or synchronous. The induction motor (or asynchronous motor) always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the rotor AC winding. As a result, the induction motor cannot produce torque about synchronous speed where induction (or slip) is irrelevant or ceases to exist. In contrast, the synchronous motor does not rely on slip-induction for operation and uses either permanent magnets, salient poles, or an independently excited rotor winding. The synchronous motor produces its rated torque at exactly synchronous speed. The brushless wound-rotor doubly-fed synchronous motor system has an independently excited rotor winding that does not rely on the principles of slip-induction of current. The brushless wound-rotor doubly-fed motor is a synchronous motor that can function exactly at the supply frequency or sub to super multiple of the supply frequency.

Other types of motors include eddy current motors, and also AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection.



Drawing from U.S. Patent 381968, illustrating principle of Tesla's alternating current motor.

Alternating current technology was rooted in Michael Faraday’s and Joseph Henry’s 1830-31 discovery that a changing magnetic field can induce an electric current in a circuit. Faraday is usually given credit for this discovery since he published his findings first.[1]

In 1832, French instrument maker Hippolyte Pixii generated a crude form of alternating current when he designed and built the first alternator. It consisted of a revolving horseshoe magnet passing over two wound wire coils.[2]

Because of AC's advantages in long distance high voltage transmission, there were many inventors in the United States and Europe during the late 19th century trying to develop workable AC motors.[3] The first person to conceive of a rotating magnetic field was Walter Baily, who gave a workable demonstration of his battery-operated polyphase motor aided by a commutator on June 28, 1879 to the Physical Society of London.[4] Describing an apparatus nearly identical to Baily’s, French electrical engineer Marcel Deprez published a paper in 1880 that identified the rotating magnetic field principle and that of a two-phase AC system of currents to produce it.[5] Never practically demonstrated, the design was flawed, as one of the two currents was “furnished by the machine itself.”[4] In 1886, English engineer Elihu Thomson built an AC motor by expanding upon the induction-repulsion principle and his wattmeter.[6] In 1887, American inventor Charles Schenk Bradley was the first to patent a two-phase AC power transmission with four wires.

"Commutatorless" alternating current induction motors seem to have been independently invented by Galileo Ferraris and Nikola Tesla. Ferraris demonstrated a working model of his single-phase induction motor in 1885, and Tesla built his working two-phase induction motor in 1887 and demonstrated it at the American Institute of Electrical Engineers in 1888[7][8][9] (although Tesla claimed that he conceived the rotating magnetic field in 1882).[10] In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation;[11] Tesla, in the same year, was granted a United States patent for his own motor.[12] Working from Ferraris's experiments, Mikhail Dolivo-Dobrovolsky introduced the first three-phase induction motor in 1890, a much more capable design that became the prototype used in Europe and the U.S.[13][14][15] He also invented the first three-phase generator and transformer and combined them into the first complete AC three-phase system in 1891.[16] The three-phase motor design was also worked on by the Swiss engineer Charles Eugene Lancelot Brown,[13] and other three-phase AC systems were developed by German technician Friedrich August Haselwander and Swedish engineer Jonas Wenström.[17]


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  8. The Electrical Engineer. (1888). London: Biggs & Co. Pg., 239. [cf., "[...] new application of the alternating current in the production of rotary motion was made known almost simultaneously by two experimenters, Nikola Tesla and Galileo Ferraris, and the subject has attracted general attention from the fact that no commutator or connection of any kind with the armature was required."]
  9. Galileo Ferraris, "Electromagnetic rotation with an alternating current," Electrican, Vol 36 [1885]. pg 360-75.
  10. Prodigal Genius: The Life of Nikola Tesla. Pg 115
  11. "Two-Phase Induction Motor" (2011), The Case Files: Nikola Tesla, The Franklin Institute.
  12. Lance Day, Biographical Dictionary of the History of Technology, page 1204
  13. 13.0 13.1 Arnold Heertje, Mark Perlman Evolving Technology and Market Structure: Studies in Schumpeterian Economics, page 138
  14. Victor Giurgiutiu, Sergey Edward , Micromechatronics: Modeling, Analysis, and Design with MATLAB, Second Edition, page 141
  15. M. W. Hubbell, The Fundamentals of Nuclear Power Generation: Questions & Answers - Page 27
  16., The History of the Transformer, Edison Tech Center 2010
  17. Template:Cite journal


See also[]

DC motor