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.

Full-Wave (Three-Pulse) Controlled Rectifiers

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

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.

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

Firing angle 30º<=α <=150° (α= 60º)

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)

With Inductive Load (without freewheeling diode)

With Inductive Load and freewheeling diode

Control Charcteristics of Three-phase Half-wave controlled rectifier

Full-Wave Controlled Bridge Rectifier

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.

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

Double-Pulse Firing Scheme