TABLE OF CONTENTS Three-phase controlled rectifiers have a wide range of applications, from small rectifiers to large high voltage direct current (HVDC) transmission systems. They are used for electrochemical processes, many kinds of motor drives, traction equipment, controlled power supplies and many other applications. From the point of view of the commutation process, they can be classified into two important categories: line-commutated controlled rectifiers (thyristor rectifiers) and force-commutated pulse width modulated (PWM) rectifiers.
Figure 12.1 shows the three-phase half-wave rectifier topology. To control the load voltage, the half-wave rectifier uses three common-cathode thyristor arrangement. In this figure, the power supply and the transformer are assumed ideal. The thyristor will conduct ( ON state), when the anode-to-cathode voltage v AK is positive and a firing current pulse i G is applied to the gate terminal. Delaying the firing pulse by an angle ? controls the load voltage. As shown in Fig. 12.2, the firing angle ? is measured from the crossing point between the phase supply voltages. At that point, the anode-to-cathode thyristor voltage v AK begins to be positive. Figure 12.3 shows that the possible range for gating delay is between ? = 0 and ? = 180 , but because of commutation problems in actual situations, the maximum firing angle is limited to around 160 . As...
