The rectifiers are used to convert alternating current (AC) to direct current (DC). The advantages of increased number of phases of the AC supply are given as follows −
A smoother waveform is obtained on the DC side as the number of phases of the AC supply is increased.
The objectionable harmonics in the AC being reduced with the increase in the number of phases.
With the increase in the number of phases, the efficiency of the converter unit (rectifier) is also increased.
Therefore, because of these advantages 6-phase is preferred instead of 3-phase for rectification. In order to obtain three to six phase supply, the six-phase star connection is used.
The circuit arrangement of the six-phase star connection is shown in the figure.
Here, the secondary windings have centre taps which are joined together to form the neutral on the 6-phase side. In the case of six-phase star connection, three 1-phase transformer or one 3- phase transformer may be used for the 3-to-6 phase transformation. The primary windings are connected in delta, but these may also be connected in star.
The six-phase load terminals 1, 2, 3, 4, 5, 6 are connected to the six secondary winding terminals r1, b2, y1, r2, b1, y2 of the transformer respectively.
The phasor diagrams of the primary and secondary windings are shown in the figure below.
The connection arrangement of the diametrical connection is shown in the figure. This connection is also called as diametral connection.
In case of diametrical connection, no centre tappings are required i.e. no neutral terminal is taken out. A true 6-phase supply is only obtained using the diametrical connection, when the six secondary terminals are connected to the terminals of the suitable 6-phase load.
Here, the transformer terminals r1, r2, y1, y2, b1, b2 being connected to the load terminals 1, 4, 3, 6, 5, 2 respectively. Since the neutral terminal is not present, hence, the diametrical connection cannot be used for the rectifier circuits. The phasor diagram of the diametrical connection is shown in the figure below.