A
rotating magnetic field is a
magnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
which changes direction at (ideally) a constant angular rate. This is a key principle in the operation of the
alternating-current motorAn AC motor is an electric motor driven by an alternating current.It 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 that is given a torque by the...
.
Nikola TeslaNikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
claimed in his autobiography that he identified the concept of the rotating magnetic field in
1882The year 1882 in science and technology involved some significant events, listed below.-Biology:* Robert Koch isolates the Tuberculosis bacillus.* Élie Metchnikoff discovers phagocytosis.-Chemistry:...
. In 1885,
Galileo FerrarisGalileo Ferraris was an Italian physicist and electrical engineer, noted mostly for the studies and independent discovery of the rotating magnetic field, a basic working principle of the induction motor...
independently researched the concept. In 1888, Tesla gained for his work. Also in 1888, Ferraris published his research in a paper to the
Royal Academy of Sciences in
TurinTurin is a city and major business and cultural centre in northern Italy, capital of the Piedmont region, located mainly on the left bank of the Po River and surrounded by the Alpine arch. The population of the city proper is 909,193 while the population of the urban area is estimated by Eurostat...
.
The
rotating magnetic field is mainly utilized in
electric rotating machineryElectric rotating machinery includes:* Electric motor* Electrical generator* Motor-generator* Rotary transformer...
e.g. electric generators or
induction motorAn induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
s and therefore electromechanical applications. However it can also be used in purely electrical applications as e.g.
induction regulatorAn Induction regulator is a device, based on an induction motor, which can provide a continuous variable output voltage. In the past, it was utilized to control the voltage of electric networks. Nowadays, it has been replaced in this function by the tap transformer. Its usage is now mostly...
s.
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 functionIn mathematics, the term linear function can refer to either of two different but related concepts:* a first-degree polynomial function of one variable;* a map between two vector spaces that preserves vector addition and scalar multiplication....
of the coil's current.
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-Electricity generation:Electricity generation requires the connection of large numbers of alternators in parallel and additional alternators must be switched in when demand rises....
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 motorAn electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s. 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 motorAn induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
s. Because magnets degrade with time, induction motors use short-circuited
rotorRotor may refer to:*A rotating part of a mechanical device, for example in an electric motor, generator, alternator or pump.In engineering:...
s (instead of a magnet) which follow the rotating magnetic field of a multicoiled
statorThe stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
. In these motors, the short circuited turns of the rotor develop
eddy currentEddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
s in the rotating field of the stator which in turn move the rotor by
Lorentz forceIn physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
. 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 susceptibilityIn electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
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
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
- Halbach array
A Halbach array is a special arrangement of permanent magnets that augments the magnetic field on one side of the array while cancelling the field to near zero on the other side...
a magnetic field that rotates spatially
- Tesla's Egg of Columbus
Nikola Tesla, at the 1893 World's Columbian Exposition, demonstrated a device he constructed known as the "Egg of Columbus." It was used to demonstrate and explain the principles of the rotating magnetic field model and the induction motor. Tesla's Egg of Columbus performed the feat of Columbus...
- Magnetic stirrer
A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir bar immersed in a liquid to spin very quickly, thus stirring it. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath...
- Shaded-pole motor
A shaded-pole motor is a type of AC single-phase induction motor. It is basically a small squirrel cage motor in which the auxiliary winding is composed of a copper ring surrounding a portion of each pole. This auxiliary winding is called a shading coil. Currents in this coil delay the phase of ...
- Squirrel cage rotor
A squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor with a squirrel cage rotor is termed a squirrel cage motor.- Structure :In overall shape, it is a cylinder mounted on a shaft...
- Synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....
- Timeline of motor and engine technology
Timeline of motor and engine technology* – Hero of Alexandria describes the first documented steam-powered device, the aeolipile.* 1698 – Thomas Savery builds a steam-powered water pump for pumping water out of mines....
- War of Currents
In the "War of Currents" era in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison's promotion of direct current for electric power distribution over alternating current advocated by several European companies and Westinghouse Electric based out of Pittsburgh,...
Further reading
- 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.)
- 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.)
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.