Circuit Operation

The operation of the semi-controlled rectifier circuit displayed in Fig. 1 is now explained. The waveform of the bridge output voltage for a firing angle of 30o is shown in Fig. 2. .It can be seen that the trace of the positive rail of output voltage is that of a controlled rectifier, since only the top-half of the bridge is controlled. The trace of the bottom rail is that of an uncontrolled bridge rectifier, since the bottom half contains only diodes.

Fig. 2

Let the 3-phase supply be defined as shown in equation(1).

Given that the firing angle is 30^o, SCR S₁ is triggered when wt = 60^o . The conduction range of the SCRs in the top half can be is now expressed in equation (2).

As long as the firing angle a remains less than 60^o, the expression for output voltage over one output cycle can be expressed as follows.

Substituting for v_R, v_Y, and v_B from equation (1), we get that

When the firing angle a is higher than 60^o, the expression for output voltage over one output cycle can be expressed as follows, if conduction through the load is continuous.

Substituting for v_R, and v_B from equation (1), we get that

Depending on the firing angle, the bridge output repeats itself every 120^o. The applet shown below plots the output voltage, given the firing angle. It is assumed that conduction through the load impedance is continuous.

If the firing angle a is less than 60^o, when SCR S₁ is triggered, SCR S₁ and diode D₆ conduct during the period (a + 30^o) £ wt < 90^o. Figure 3 shows the path of conduction that would exist during this period. The pairs that conduct vary depending on the firing angle. Table 1 show the pairs that would conduct when the firing angle a is less than 60^o, whereas Table 2 shows the pairs that would conduct when the firing angle a is greater than 60^o.