Rotating magnetic field


Rotating magnetic field

A rotating magnetic field is a magnetic field which changes direction at (ideally) a constant angular rate. This is a key principle in the operation of the 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 US patent|0381968 for his work. Also in 1888, Ferraris published his research in a paper to the "Royal Academy of Sciences" in Turin.

Description

A symmetric rotating magnetic field can be produced with as little as three coils. The 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, the magnetic field is taken to be the linear function of the coil's current.

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

The result of adding three 120-degrees phased sine waves on the axis of the motor is a single rotating vector. The rotor has a constant magnetic field. The N pole of the rotor will move toward the S pole of the magnetic field of the stator, and vice versa. This magneto-mechanical attraction creates a force which will drive rotor to follow the rotating magnetic field in a synchronous manner.

A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was utilized in early alternating current electric motors. A rotating magnetic field can be constructed using two orthogonal coils with a 90 degree 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 the standardization of the conductor size. In order to overcome this, three-phase systems are used where the three currents are equal in magnitude and have a 120 degree 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 the rotating field utilized in electric motors is one of the main reasons why three phase systems dominate in the world electric power supply systems.

Rotating magnetic fields are also used in induction motors. Because magnets degrade with time, induction motors use short-circuited rotors (instead of a magnet) which follow the rotating magnetic field of a multicoiled stator. In these motors, the short circuited turns of the rotor develop eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force. These types of motors are not usually synchronous, but instead necessarily involve a degree of 'slip' in order that the current may be produced due to the relative movement of the field and the rotor.

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

ee also

* 1882 in science
* Alternator
* Dynamo theory
* Tesla's Egg of Columbus
* Magnetic stirrer
* Shaded-pole motor
* Squirrel cage rotor
* Synchronous motor
* Timeline of motor and engine technology
* War of Currents

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, " [http://pubweb.bnl.gov/users/frenkel/www/PHYS-LETT/labz-frenkel.pdf 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.)

Patents

*, Tesla, "Electromagnetic motor".
*, Ress, "Particle accelerator".

External articles

* Naval Electrical Engineering Training Series, Module 05 - Introduction to Generators and Motors, Chapter 4 Alternating Current Motors, [http://www.tpub.com/content/neets/14177/css/14177_87.htm Rotating magnetic fields] (ed. different copy of the NEETS book is available, [http://www.tpub.com/neets/book5/18a.htm Rotating magnetic fields] )
* [http://www.eng.ox.ac.uk/~epgmdm/A2/img89.htm Rotating Magnetic Field] , eng.ox.ac.uk
* Tesla's Autobiography, III. My Later Endeavors; [http://www.teslaplay.com/autosection3.htm The Discovery of the Rotating Magnetic Field]
* [http://web.mit.edu/Invent/iow/tesla.html Nikola Tesla and the electro-magnetic motor] , Inventor of the Week Archive.
* [http://www.geocities.com/SiliconValley/Lakes/3015/history004.html Galileo Ferraris: the rotating magnetic field]
* [http://www.lmphotonics.com/single_phase_m.htm Single Phase Induction Motors]
* H.Y. Guo, A.L. Hoffman, D. Lotz, S.J. Tobin, W.A. Reass, L.S. Schrank and G.A. Wurden, [http://wsx.lanl.gov/Publications/RSI-RMF-Tobin-paper.pdf The Rotating Magnetic Field Oscillator System for Current Drive in the Translation, Confinement and Sustainment Experiment] , March 22, 2001.
* Putko, V. F., and V. S. Sobolev, [http://adsabs.harvard.edu/abs/1981gnpn.conf..113P Effect of a rotating magnetic field on the characteristics of a direct-current plasma generator] .

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