Three Phase to Six Phase Conversion

In certain applications like thyristors and rectifiers six phase supply is required. Therefore it becomes necessary to convert three phase a.c. supply into six phase. By using three identical single phase transformers suitably interconnected this can be achieved. The primary winding is connected in delta whereas its secondary winding is split up into two halves. Thus conversion from 3 phase to six phase can be obtained by having two similar secondary windings for each of the primaries of the three phase transformer. This is showing in the Fig. 1.

The three phase supply is given to primaries of the three transformers and six phase output can be obtained from the six secondaries as shown. There are many ways of connecting these secondaries. Some of them are i) double delta ii) double star iii) dimetrical. The dimetrical connection is generally used in practice.

The double delta connection is shown in the Fig.2.

Fig. 1

Fig. 2

As seen from the Fig.2 this arrangement of secondaries connections are taken from points 1, 3, 5 and in the second set, the connection are taken from points 2, 4 and 6.

The double star connection for obtaining 6 phase supply is shown in the Fig.3.

Fig. 3

Six equal voltages with a phase displacement of is obtained which is indicated in the phasor diagram shown in the Fig. 4. The primary windings may be either connected in delta or in star. If the primaries are connected in star then care must be taken that the neutral point of the star connection must be connected to the neutral of the generator to avoid the difficulties due to floating neutral and triple frequency currents.

Fig. 4

In the case of six phase rectifier, the star point or the neutral point formed by the coils serves as the neutral point of the d.c. supply from the rectifier.

The most commonly used connection is the dimetrical connection for three phase to six phase transformers. It is shown in the Fig. 5. On each single phase transformer there is only one winding on the secondary side. The two secondary leads are connected to diametrically opposite points on the armature windings of the synchronous converter. The middle points of the secondary windings may be connected together to form a neutral for the three wire circuit. The high voltage or primary windings are either connected in star or delta. But delta connection is generally preferred because of the triple frequency harmonics of voltages which are introduced in star-star connection.