Characteristics of the three-phase current
Now production and distribution of electrical energy is generally carried out by trekhyofazny current.
Three identical on the frequency and amplitude of alternating current, shifted relatively each other for 1/3 periods (120 °), form three-phase system.
There are two ways of connection of windings of elektyorichesky machines and receivers in three-phase system: wye and delta connection.
Three phases of the power source can be connected to three loadings six wires. Such system of the chain is called untied. Now it is not applied.
At connection of three-phase system on the star circuit the ends of all windings of phases of the source connect in the generic point. The same connection is proizvoyodit in loading. Then all three return conductors soyediyonyat in one and connect to generic points of the source and loading. On this wire the sum of currents of all three phases proceeds. But if in all phases odinayokovy currents proceed, then their sum will be equal to zero as they are shifted rather each other on 120 °. Therefore current in the general wire will not proceed. This wire is called neutral, or zero. Other wires connecting generator windings to the receiver nazyyovatsya by linear.
Loading at which currents are equal in all phases in size and have identical phase shifts in relation to phase EMFs, is called symmetric. At the soyedineyoniya in the star with the symmetrical load the zero wire is absent as in it is mute there is no need. Such system is called three-wire. In other cases the system with the zero wire — chetyrekhyoprovodny is applied.
Tension between the end and the beginning phase the obmoyotok in three-phase system is called phase, and the nayopryazheniye between line conductors - linear. The currents proceeding in windings of phases of the source or the nayogruzka are called as phase currents, and in linear proyovoda - linear. Between phase and linear veyolichina at connection in the star there is the sleduyuyoshchy communication (at the symmetrical load):
At delta connection phase windings of the isyotochnik are connected consistently so that the beginning of one winding connected to the end of sleyoduyushchy. Generic points of each couple of phase windings of the isyotochnik and generic points of each couple of branches of the receiver connect wires which are called lineynyyom. It is easy to be convinced that delta connection in three-phase system also turns out from the three-phase untied chain by association with each other of the proyovod drawn by the row.
At the symmetrical load of the system connected in the triangle, line currents it is more phase by 3 times, and phase tension is equal linear, i.e.Top