2. Introduction
Diodes in three-phase rectifier circuits of chp 7 are replaced by THYRISTORS
Circuits in chp 8 are fully controllable
average output voltage can be varied by controlling the triggering input to the SCR gates in suitable manner.
The three gate pulses are displayed by 120° relative to each other, giving same delay angle to each SCR.
Each SCR conducts for 120° after triggering and remain off for 240º.

3. Full-Wave (Three-Pulse) Controlled Rectifiers

4. With a Resistive Load
Firing angle is measured from the intercestion of corresponding phase voltages and not the zero crossings of the voltage waves.

5. When α is varied, the segments of phase voltage that fabricate the load voltage change, providing a control over the average value.
Negative exursions of load voltage are possible, depending upon the nature of load.

6. Firing angle α = 0º
Firing angle α = 15º
(α < 30º)

7. Firing angle 30º<=α <=150°
(α= 60º)

8. Continous & Dicscontinous Conduction in Three-Phase Controlled Rectifier
For resistive load
0°<=α<=30º, output voltage is continous.
30°<=α<=120º, output voltage is discontinous and has some intervals in which output voltage is zero
α >150°, output is zero.
For Inductive load
There is no discontinous conduction mode for three-phase controlled rectifier if L>>R.
But if L approx R or firing angle is very large, discontinuities can be seen in output as output voltage can become zero in certain intervals (those intervals in which inductor has quickly desipated its energy and firing angle hasn’t reached)

9. With Inductive Load (without freewheeling diode)

10. With Inductive Load and freewheeling diode

11. Control Charcteristics of Three-phase Half-wave controlled rectifier

12. Full-Wave Controlled Bridge Rectifier

13. Circuit Diagram
Two SCRs, one from positive group and one from negative group, must be triggered instantaneously.
SCRs 1&4 are for phase A, SCRs 3&6 for phase B, SCRs 5&2 for phase C.

14. Firing Sequence for six-pulse controlled bridge rectifier
(Example 8.7)
Single-Pulse Firing Scheme

15. Double-Pulse Firing Scheme