# Three-phase rectifier circuits

The 3-phase supply sources are used usually when the load currents are high. In such situations the multi-phase rectifying systems are applied. They can be divided into two basic types:

• unidirectional rectifying systems
• bidirectional (bridge) rectifying systems

In the first case the rectifying system, supplied from m-phase transformer, contains one diode in each phase, so the number of diodes is (m). All the diodes are directed consistently and their cathodes are shorted and connected with receiver’s terminal. Each phase conducts for 1/m of period, when the voltage value is higher than voltages of the remaining phases. As a result of that the current flows continuously in the load and it has a pulsating character.
In multi-phase unidirectional systems the voltage constant component on the load can be determined according to the following formula:

where:

• UOS – value of voltage constant component
• Uf – effective value of phase voltage (secondary winding)
• Ufm = √2 Uf – maximal value of phase voltage
• m – number of phases  (m≥0)

The constant component increases with the increase of phase number. The dependence between UOS/Ufm relation and the number of phases in unidirectional rectifying system is presented in the table below:

 m 1 2 3 4 6 12 ∞ Uos/Ufm 0,32 0,64 0,81 0,90 0,96 0,99 1

The maximal and effective value of the phase current are as follows:

,

Power efficiency:

Pulsation coefficient is calculated in accordance with the following equation:

Several diagrams of unidirectional rectifiers with various output parameters. When selecting an appropriate system attention should be paid both to above-mentioned output parameters and economical reasons (cost of rectifier and supply transformer) and then the most optimal solution should be selected.

Fig. 1 Diagram of unidirectional, 3-pulse rectifier.

Fig. 2 Diagram of unidirectional, 6-pulse rectifier.

Fig. 3 Diagram of unidirectional, 6-pulse rectifier consisting of two 3-pulse rectifiers connected in parallel, through a compensating choke.

In unidirectional systems the currents in transformer secondary windings, connected to a converter, flow only for a part of the period. Therefore the typical power of transformer exceed considerably the power value of the direct current circuit. In addition to that an unfavourable phenomenon of core magnetization, decreasing the efficiency of converter and the entire system, occurs in unidirectional systems. This phenomenon is caused by the constant component of secondary windings current. The 6-pulse system with a compensating choke is the most favourable in respect of transformer utilisation.
In case of the second type of multi-phase rectifying systems, i.e. in bidirectional (bridge) system each phase is connected with two diodes: one belonging to the first group (D1, D2 and D3) and one belonging to the second group (D4, D5 and D6). The current of load connected between linked anodes and cathodes of both diode groups, flows always through two diodes and two phases of transformer secondary winding. The voltage run on the load results from momentary values of phase-to- phase voltage. The current of each phase consists of two pulses with the duration 1/m of the period. The direction of their flow is opposite, so the phase current does not contain the constant component and does not cause the core magnetization. It allows to obtain better utilisation of transformer and bigger watt-hour efficiency  ηp of rectifier. Mostly used bidirectional rectifying systems are presented below:

Fig. 4 Diagram of bidirectional 6-pulse rectifier in bridge system.

Fig. 5 Diagram of 12-pulse rectifier, consisting of two bridge 6-pulse rectifiers connected in series.