Editing Rotating magnetic field (section)

==Description==
The rotating magnetic field is the key principle in the operation of [[Induction motor|induction machines]]. The induction motor consists of a [[stator]] and [[Rotor (electric)|rotor]]. In the stator a group of fixed windings are so arranged that a two phase current, for example, produces a magnetic field which rotates at an [[angular velocity]] determined by the [[frequency]] of the [[alternating current]]. The rotor or [[Armature (electrical)|armature]] consists of coils wound in slots, which are short circuited and in which the changing [[flux]] generated by the field poles induce a current. The flux generated by the armature current reacts upon the field poles and the armature is set in rotation in a definite direction.<ref name=":0" />
[[File:Rotating magnetic field.png|center|thumb|640x640px|'''Rotating fields.''' As the direction of the current through the windings changes, the polarity of the windings changes as well. Since there are two windings acting in conjunction with each other, the polarity of the main field will depend upon the polarity of each winding. The arrow or vector below each diagram indicates the direction of the magnetic field in each case.<ref>{{Cite book|last=United States|first=Bureau of Naval Personnel|url=https://archive.org/details/AdvWorkInAircraftElectricity|title=Advanced Work In Aircraft Electricity|publisher=U.S. Govt. Print. Off.|year=1945|isbn=|location=Washington|pages=149–150}}</ref>]]
A [[symmetric]] rotating magnetic field can be produced with as few as two polar wound [[Electromagnetic coil|coils]] driven at 90-degree [[Phase (waves)|phasing]]. However, three sets of coils are nearly always used, because it is compatible with a [[3 phase power|symmetric three-phase AC sine current system]]. The three coils are driven with each set 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.

The result of adding three 120-degree phased [[sine wave]]s on the axis of the motor is a single rotating vector that always remains constant in magnitude.<ref name="rmf">[http://www.electricaleasy.com/2014/02/production-of-rotating-magnetic-field.html Production of rotating magnetic field], | electricaleasy.com</ref> The rotor has a constant magnetic field. The north pole of the rotor will move toward the south pole of the magnetic field of the stator, and vice versa. This [[magnetomechanical effects|magnetomechanical]] attraction creates a force that will drive the rotor to follow the rotating magnetic field in a [[Synchronization (alternating current)|synchronous]] manner.

[[File:Rotating-3-phase-magnetic-field.svg|alt=|thumb|640x640px|Rotating three-phase magnetic field, as indicated by the rotating black arrow|center]]
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 alternating 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 in which 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 the world's 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 current]]s 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.