User:Kevin Breitenstein/MagneticField

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A rotating magnetic field is a magnetic field which periodically changes direction. This is a key principle to the operation of alternating-current motor. In 1882, Nikola Tesla identified the concept of the rotating magnetic field. In 1885, Galileo Ferraris independently researched the concept. In 1888, Tesla gained U.S. Patent 0,381,968 for his work. Also in 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

1 Description
2 See also
3 Further reading
4 Patents
5 External articles

[edit] Description

Symmetric rotating magnetic field can be produced with as little as three coils. Three coils will have to be driven by a symmetric 3-phase AC sine current system, thus each phase will be shifted 120 degrees in phase from the others. For the purpose of this example, magnetic field is taken to be the linear function of coil's current.

Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis. Magnetic fields add as vectors.

Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of resulting rotating magnetic field.

Result of adding three 120-degrees phased sine waves on the axis of the motor is a single rotating vector. Rotor (having a constant magnetic field driven by DC current or a permanent magnet) will attempt to take such position that N pole of the rotor is adjusted to S pole of the stator's magnetic field, and vice versa. This magneto-mechanical force will drive rotor to follow rotating magnetic field in a synchronous manner.

U.S. Patent 381968: Mode and plan of operating electric motors by progressive shifting; Field Magnet; Armature; Electrical conversion; Economical; Transmission of energy; Simple construction; Easier construction; Rotating magnetic field principles.

A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was utilised in early alternating current electric motors. A rotating magnetic field can be constructed using two orthogonal coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case.The ability of the three phase system to create a rotating field utilized in electric motors is one of the main reasons why three phase systems dominated in the world electric power supply systems. Because magnets degrade with time, synchronous motors and induction motors use short-circuited rotors (instead of a magnet) following rotating magnetic field of multicoiled stator. (Short circuited turns of rotor develop eddy currents in rotating field of stator which (currents) in turn move the rotor by Lorentz force).

NOTE: Rotating magnetic field can actually be produced by two coils, with phases shifted about 90 degrees, but such field would not be symmetric due to difference between magnetic susceptibility of ferromagnetic materials of pole and air. In case two phases of sine current are only available, four poles are commonly used.

[edit] See also

[edit] Further reading

C Mackechnie Jarvis, "Nikola Tesla and the induction motor". 1970, Phys. Educ. 5 280-287 doi:10.1088/0031-9120/5/5/306
Owen, E.L., "The induction motor's historical past". IEEE Potentials, Oct 1988, Volume 7, Issue 3, pg. 27-30, ISSN 0278-6648
Beckhard, Arthur J., "Electrical genius Nikola Tesla". New York, Messner, 1959. LCCN 59007009 /L/AC/r85 (ed. 192 p.; 22 cm.; biography with notes on the inventions of the rotating magnetic field motors for alternating current.)
Kline, R., "Science and Engineering Theory in the Invention and Development of the Induction Motor, 1880-1900". Technology and Culture, 1987.
Cēbers, A., "Dynamics of an elongated magnetic droplet in a rotating field". Phys. Rev. E 66, 061402, Issue 6, December 13, 2002.
Cēbers, A., and I. Javaitis, "Dynamics of a flexible magnetic chain in a rotating magnetic field". Phys. Rev. E 69, 021404 2004.
Cēbers, A., and M. Ozols, "Dynamics of an active magnetic particle in a rotating magnetic field". Phys. Rev. E 73, 021505, 2006.
Tao Song, et. al., "Rotating permanent magnetic fields exposure system for in vitro study". IEEE Transactions on Applied Superconductivity, June 2004, Volume 14, Issue 2, pg 1643-1646. ISSN 1051-8223
Labzovskii, L.N. , A.O. Mitrushchenkov, and A.I. Frenkel, "Parity Nonconserving Current in Conductors of Electricity". 6 July 1987. (ed., Shows that the continuous current arises under the influence of the rotating magnetic field.)