Three-phase Winding

The six secondary phases are obtained by shorting the centre points of each of the three-phase windings of a 3p transformer secondary. 

Table 6.1. Letters used to identify three-phase windings...

The three-phase windings of an a.c. generator may be star connected or delta connected as shown in Fig. 2.54. The important relationship between phase and line currents and voltages is also shown. The square root of 3 (Vs) is simply a constant for three-phase circuits, and has a value of 1.732. The delta connection is used for electrical power transmission because only three conductors are required. Delta connection is also used to connect the windings of most three-phase motors because the phase windings are perfectly balanced and, therefore, do not require a neutral connection. 

Continuous positive torque can be applied to a superconducting rotor using a three-phase winding, as indicated in Fig. 5. In the position shown, phase A exerts a force normal to each flat surface on the rotor as indicated by the forces F. During the instant represented by this rotor position no current flows through phases B or C. Current pulses, following in sequence from A to B to C to A as indicated, maintain rotation. 

It is observed that the insulation resistance of one phase with the other two phases grounded is approximately twice that of the entire winding. Therefore, when the three phases are tested separately. the observed alue of the resistance of each phase is divided by two to obtain the actual insulation resistance. 

These are single-, double- or three-phase wound-type trans-formers with windings on both primary and secondary sides (Figures 15..3(a) and (b)). 

When the primary of a three-phase two-winding transformer, having its secondary wound for a three-phase open delta, is connected across an unbalanced supply system, a residual voltage across the open delta will appear. This is the principle on which this transformer is based (Figure 15.4(a)). As discussed in Section 21.2.2, and illustrated in Figure 21.7, the phasor sum of all the three line to ground voltages in a three-phase balanced system is zero, i.e. 

When this balance is disturbed, due to either an unbalance in the loads or due to a ground fault, a residual or zero phase sequence voltage in the neutral circuit will appear. When one of the phases in the secondary of a three-phase transformer is open circuited and a three-phase supply is applied to its primary windings, there will appear... 

In this case, all the three phases would be balanced and the residual voltage, Ve, will be zero. The three voltage phasors in the open della windings will be as illustrated in Figure 15.5(a). The phasor sum of these phasors is zero. Therefore W,. = 0. 

Figure 15.35 Single and three-phase VTs with one and two windings in the secondary...

Z( = zero phase sequence or residual impedance. This is measured between the three-phase terminals of a star winding shorted together and the neutral (Figure 20.9(b)) and is calculated by... 

Three phase two winding transformer with ON load tap changer (GT)... 

The common connection of all three armature windings in the Y connection allows a fourth, or neutral, conductor to be used. This neutral point is often grounded in transmission and distribution circuits. Such a circuit is termed a three-phase, four-wire circuit. 

Three-phase, four-wire, A-connected systems in which the midpoint of one phase winding is used as a circuit conductor... 

On the top of the chamber there is a cylindrical extension C which is cooled in order to effect condensation of mercury vapour. Six iron anodes A (there may be twelve or even eighteen of them) are suspended from ring R which covers the top of the chamber. Current for the rectifier is supplied by a three-phase transformer the secondary windings of which have their middles connected in a neutral point in this way the secondary windings have six ends connected to six anodes A by choking coils D in series. 

Synchronous AC machines have a stator similar to the asynchronous machines, which has usually a three-phase stator winding. Whereas there are two types of rotor structures round cylindrical rotor and salient pole rotor as illustrated schematically in Fig. 5.6. The rotor field is generated by permanent magnets or a DC current winding, as reported in the same figure. In the first case they are called permanent-magnet synchronous AC machines. 

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