Three-phase commutation with six-diode bridge
In practical power electronic converter circuits, the commutation follows the same basic sequence outlined above. Figure 6.8 shows a typical six-pulse rectifier bridge circuit to convert three-phase AC currents IA, IB, and IC, to a DC current ID.
This type of circuit is relatively simple to analyze because only two of the six diodes conduct current at any one time. The idealized commutation circuit can easily be identified. In this example, the commutation is assumed to be taking place from diode D1 to D3 in the positive group, while D2 conducts in the negative group.
In power electronic bridge circuits, it is conventional to number the diodes D1 to D6 in the sequence in which they are turned ON and OFF. When VA is the highest voltage and VC the lowest, D1 and D2 are conducting.
Similar to the idealized circuit in Figure 6.8, when VB rises to exceed VA, D3 turns on and commutation transfers the current from diode D1 to D3. As before, the commutation time is dependent on the circuit inductance (L) and the commutation voltage (VB – VA). As can be seen from the six-pulse diode rectifier bridge example in Figure 6.4, the commutation is usually initiated by external changes.
In this case, the three-phase supply line voltages control the commutation. In other applications, the commutation can also be initiated or controlled by other factors, depending on the type of converter and the application. Therefore, converters are often classified in accordance with the source of the external changes that initiate commutation. In the above example, the converter is said to be line-commutated because the source of the commutation voltage is on the mains supply line. A converter is said to be selfcommutated if the source of the commutation voltage comes from within the converter itself. Gate-commutated converters are typical examples of this.