﻿ Three-phase systems

# General information about three-phase systems

The multiphase system of electric circuits represents set of electric circuits in which work sinusoidal э. the of page of identical frequency shifted relatively each other on the phase and created by one power source. The system corresponding to this definition from three chains is called three-phase.

Principle of work of three-phase system.

## Three-phase system э. of page.

Fig. 1 Scheme of the three-phase generator.

Three-phase symmetric system э. of page turns out by means of the three-phase generator in which имеют­ся three
the independent windings shifted relatively each other in space on 120 °. Schematically it is shown in fig. 1  in relation to the generator with one couple of poles on the stator and windings on the rotor. However it is necessary to notice that in real generators the alternating-current winding is not mobile (it is located on the stator), and magnetic poles rotate (are located on the rotor). Such design of the generator is better, and the principle of its work does not change.

If the number of rounds in windings is identical, then at rotation of the rotor in all windings are directed э. of page of identical size. Initial phases of these э. of page are shifted rather each other on 120 ° according to spatial arrangement of windings.

Three-phase symmetric system э. of page — set of three э. of the page having the identical frequency and amplitude dephased relatively each other on corners 120 °.

Fig. 2 Graphics and vectogram of symmetric three-phase system э. of page.

Sign of asymmetry of three-phase system э. of page is inequality of amplitudes or inequality of phase angles between kazhyody couple э. of page.

Considering initial (prit 0) the provision of windings represented in fig. 1, and rotation of the rotor — counterclockwise, the uravneyoniya э. of page can be written down in the following look:

еА = Em sin? t

eB = Em sin (? t – 120o)

eC = Em sin (? t – 240o)

To these equations there correspond schedules э. of page and the vectogram represented in fig. 2

Complexes of these operating values э. of page:

EA = E,  EB = EC-/120 °,  EC = Ee-/240 °,

If to take for the initial vector э. of page of EA, э. of page of EB lags behind EA, and aa. of page of EC lags behind EB. Therefore, maximum values э. of page in phases reach in such order: at first in the phase A, then in the phase B and further in the phase C.

Vectors э. of page rotate counterclockwise, and by the fixed axis + /' they will pass in the following order: EA-> EB-> EC. Such order of alternation is called direct phase sequence. At the return rotation of the rotor of the generator the return  phase sequence turns out. Further by consideration of three-phase systems the straight line последовательнось phases which is considered normal is accepted.