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The Islamia University of Bahawalpur
University College of Engineering & Technology
EEN-324
Power Electronics
Three-Phase Controlled
Rectifiers
Chp#8
1
Engr. Affifa Adeeb
INTRODUCTION
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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.
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FULL-WAVE (THREEPULSE) CONTROLLED
RECTIFIERS
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With a Resistive Load
Firing angle is measured from the intercestion of
corresponding phase voltages and not the zero
crossings of the voltage waves.
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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.
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Firing angle
α = 0º
Firing angle
α = 15º
(α < 30º)
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Firing angle 30º<=α <=150°
(α= 60º)
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CONTINOUS & DICSCONTINOUS CONDUCTION
THREE-PHASE CONTROLLED RECTIFIER
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IN
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.
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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)
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With Inductive Load (without freewheeling diode)
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With Inductive Load and freewheeling diode
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Control Charcteristics of Three-phase Half-wave
controlled rectifier
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FULL-WAVE CONTROLLED
BRIDGE RECTIFIER
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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.
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Firing Sequence for six-pulse controlled bridge rectifier
(Example 8.7)
Single-Pulse Firing Scheme
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Double-Pulse Firing Scheme
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