The description of the rotating magnetic field

Feature of multiphase systems is the opportunity to create in mechanically motionless device (for example, in the stator of the elektyorichesky machine) the rotating magnetic field. Being placed in such field, any electroconductive body or the magnet tests the torque. This phenomenon is the basis for operation of asynchronous and synchronous electric motors.

Асинхронный двигатель

Asynchronous engine.

Along the axis of the coil, streamline the pulsing magnetic field exists alternating current. Really, if to accept the direction of current for some instant such as it is shown in coil section in fig. 1a (the cross - current is directed from the observer, the point - to the observer), then according to the rule the magnetic flux and magnetic induction will be directed along the coil axis in the direction designated by the sign "plus". Let this moment vremenit1 be the share of the half-cycle of the sinusoidal current when current has positive values (fig. 1b).

Схема сечения катушки

Figure 1. Scheme of section of the coil.

Let's say at last, that the magnetic indukyotion changes in proportion to current (it can take place only in the linear chain). Then with the further growth of current induction of magnetic field will accrue, will reach the maximum, then will begin to fall down, remaining directed as well as in momentt1, and only after transition of current through zero the direction of the magnityony field (induction) will change.

Thus, in the reviewed example two processes are imposed at each other: change of magyonitny induction in vreyomen (under the sinusoidal law B = W of sin? t) and in space.

Now we will address three-phase system. Let's take three coils with three currents forming three-phase system and we will place them in space at an angle 120 ° rather each other (fig. 2a - in). The positive directions of axes of three coils are designated + 1, + 2 and + 3.

Схема трехфазной системы

Scheme of three-phase system.

Along the axis of each coil the pulsing magnetic field is formed, however all three fields will be imposed at each other, and in the nuclear core of coils there will be the uniform resulting magnetic field which is characterized by the veyoktor of total magnetic induction.

In fig. 2v three instants of t1, t2, t3 for which vectors of the magnetic indukyotion of each phase and the resultant of Century are constructed are considered consistently. At the time of t1 current in the kayotushka And (and magnetic induction) is positive and maximum, and currents in coils B and C are identical, negative and make the half from current in the t1 coil. The resultant of magnetic induction is directed on the axis of that coil in which current is maximum: in this case on the coil A axis. At the time of t2 current in the coil A decreased:

Current is equal in the coil C to it, but is negative, current is equal in the coil B to zero; the resultant of magnetic induction "turned" on the corner 30 ° aside, corresponding to alternation of phases (clockwise). In momentt3 currents are identical, positive and equal in coils A and B to the half of amplitude value, and current in the coil C is negative and maximum.  The resultant of magnetic induction is placed in the negative direction of the axis of the coil C. During the sinusoidal current the vector of the resulting magnetic induction will make full turn on 360 °, therefore, it will rotate with angular speed, alternating-current corresponding frequency.

Magnetic field which magnetic displacement vector rotates in space is called the rotating magnetic field.

The rotating magnetic field  which magnetic displacement vector does not change in size and rotates with constant angular speed, is called circular.

If the geometrical or electromagnetic symmetry in the three-phase electrical machine (amplitudes of currents of separate phases are not identical, there is no current in one of phases, the winding of one of phases is included incorrectly, etc.) is broken, that rotating magnetic field becomes elliptic, i.e. the vector of the resulting magnetic induction changes in size and rotates with variable angular speed. The best conditions for operation of electrical machines are created by the kruyogovy rotating magnetic field.

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