Star and delta connections, phase and line voltages and currents

In three-phase circuits, two types of connections of the generator windings are used — in star and delta (Fig. 1).

When star-connected, all ends of the phase windings are connected at a single node called the neutral or zero point, and is usually denoted by the letter O.

When connected in a delta, the generator windings are connected so that the beginning of one is connected to the end of the other. The EMF in the coils in this case is denoted EBA, ECB, EAC, respectively... If the generator is not connected to the load, then currents do not pass through its coils, because the sum of the EMF is zero.

Rice. 1 Generator winding connections — star and delta

Star connection of resistors: a — arrangement of resistors along the rays of a star, b — parallel arrangement of resistors

Connection of resistors with a triangle: a — arrangement of resistors on the sides, b — parallel arrangement of resistors

Load resistances are also included in the star and delta as shown in Fig. 2. Phase resistance Za, Zb, Z° C, Zab, Zpr. n.f., Connected in a delta or star are called charging phases.

Rice. 2 Star and delta loads

There are five types of connection of generators to the load: star is star with neutral wire, star is star without neutral wire, delta is delta, star is delta and delta is star (Fig. 3).

The connecting wires between the beginning of the load phases and the beginning of the generator phases are called line wires... As a rule, the beginning of the phases of the generators are indicated by capital letters, and loads - by capital letters. The wire connecting the zero points of the generator and the load is called the zero or neutral wire.

It is customary to choose the direction of the currents in linear wires from the generator to the load, and in zero - from the load to the generator. In fig. 3 Uab (AB), Ubc (BC), Uca (CA), Ia, Ib, Ic — line voltages and currents. Ua (A), Ub (B), Uc (C), Iab, Ibc, Ica — phase voltages and currents.

Line voltage (voltage between line conductors) This is the difference between the corresponding phase voltages Uab — Ua — Uc, Ubc = Ub — Uc, Uca = Uc — Ua

Line currents in the accepted directions of currents (Fig. 3) are determined by Kirchhoff's first lawIa = Iab — Ica, Ib = Ibc — Iab, Ic = Ica — Ibc

Thus, the generator phase voltages are the voltages applied to the generator windings. UAO, UCO, UBO, and the load phase voltages are the voltages across the respective resistances UaO1, UbO1, UcO1. Phase currents — these are currents flowing in the phases of the generator or the load. it should be noted that the phase and line voltages in delta are equal, as are the phase and line currents in star.

The combination of the corresponding phase of the generator, connecting wire and load phase is called the phase of a three-phase circuit.

Rice. 3 Phase and line voltages and currents in star-delta connections

See on this topic: Calculation of phase and line values ​​of three-phase current