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. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. 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.

The rotating magnetic field is mainly utilized in electric rotating machinery e.g. electric generators or induction motors and therefore electromechanical applications. However it can also be used in purely electrical applications as e.g. induction regulators.

Description

A symmetric rotating magnetic field can be produced with as few 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.

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.

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 the rotor to follow the 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 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 the 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.

See also

• Dynamo theory
• Halbach array a magnetic field that rotates spatially
• 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 (1970). "Nikola Tesla and the induction motor". Phys Educ 5: 280–7. doi:10.1088/0031-9120/5/5/306. 
• Owen, E.L. (October 1988). "The induction motor's historical past". IEEE Potentials 7 (3): 27–30. doi:10.1109/45.9969. 
• 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. (1987). "Science and Engineering Theory in the Invention and Development of the Induction Motor, 1880–1900". Technology and Culture. 
• Cēbers, A. (December 13, 2002). "Dynamics of an elongated magnetic droplet in a rotating field". Phys Rev E 66 (6): 061402. doi:10.1103/PhysRevE.66.061402. 
• Cēbers, A., and I. Javaitis (2004). "Dynamics of a flexible magnetic chain in a rotating magnetic field". Phys Rev E 69: 021404. doi:10.1103/PhysRevE.69.021404. 
• Cēbers, A., and M. Ozols (2006). "Dynamics of an active magnetic particle in a rotating magnetic field". Phys Rev E 73: 021505. doi:10.1103/PhysRevE.73.021505. 
• Tao Song, et al. (June 2004). "Rotating permanent magnetic fields exposure system for in vitro study". IEEE Transactions on Applied Superconductivity 14 (2): 1643–6. doi:10.1109/TASC.2004.831024. 
• 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.)

Patents

• U.S. Patent 0,381,968, Tesla, "Electromagnetic motor".
• U.S. Patent 3,935,503, Ress, "Particle accelerator".

External articles

• Naval Electrical Engineering Training Series, Module 05 - Introduction to Generators and Motors, Chapter 4 Alternating Current Motors, Rotating magnetic fields (ed. different copy of the NEETS book is available, Rotating magnetic fields)
• Rotating Magnetic Field, eng.ox.ac.uk
• Tesla's Autobiography, III. My Later Endeavors; The Discovery of the Rotating Magnetic Field
• Nikola Tesla and the electro-magnetic motor, Inventor of the Week Archive.
• Galileo Ferraris: the rotating magnetic field
• 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, 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, Effect of a rotating magnetic field on the characteristics of a direct-current plasma generator.

External links

• Rotating magnetic field: interactive lecture