Star connection

The figure shows a diagram of the connection of the phases of a star generator. The symbol for this circuit Y... The ends K of the three phases are connected to a common point called zero. If only three wires are taken from the generator A, B, C, then such a system is called three-phase three-wire. If the fourth, zero or «zero» wire N (O) is also tapped, then the system is called three-phase four-wire. The zero point of the generator and therefore the neutral wire is reliably grounded.

Current in the neutral wire will only appear when the three phases are loaded unequally. The current flowing through the neutral conductor is equal to the algebraic sum of the currents in the three phases:

In absolute value, in is always less than the current in each of the phases if the load is connected to all phases. Therefore, the cross-section of the neutral conductor is taken to be less than the cross-section of the phase conductors.

Rice. 1. Scheme for connecting the windings of the generator to a star.

Only if the load is connected between one of the phases and the neutral conductor, and the load is not connected to the other phases, the current in the loaded phase is equal to the current in the neutral conductor.

The voltage between any of the phases and the neutral conductor is called phase voltage and is denoted by Ue... It is equal to the voltage between the beginning of each phase and its end (Fig. 2).

The voltage between the phase conductors is called linear voltages denoted by Ul... It is equal to the geometric difference between the two phase voltages (Fig. 2), that is, the linear voltages between phases A and B, B and C, C and A

Rice. 2. Line and phase voltage vectors.

The absolute value of the line voltage can be determined from the triangle of vectors AOB. The base of this triangle AB is equal to the line tension:

Or

Thus, two voltages are obtained in a three-phase four-wire system: Ue — phase and Ul — linear… The line voltage is greater than the phase voltage, by 1.73 times. Line current Il but equal to the magnitude and direction of the current in the phase coil Ie.

The following voltages are assumed for low voltage networks (Table 1).

Table 1 Standard voltages in consumer networks

As can be seen from Table 1, the supply voltage (generator or secondary side of the transformer) is always taken 5% more than the nominal line voltage, taking into account the fact that about 5% of the voltage will be lost in the line. This is done to supply consumers with electricity at rated voltage and to ensure their satisfactory operation.

In agriculture, the most widespread is a three-phase four-wire system 380/220 V, that is, a system with network voltage Ul = 380 V and phase Uph = 220 V.Three phases with a voltage of 380 V between them are used to power electric motors and three-phase heating devices, and the voltage between the phase and the neutral wire of 220 V is used to power light sources and household electrical appliances.