1. CLASSIFICATION OF THREE PHASE RECTIFIER
HALF WAVE
(SINGLE WAY)
UNCONTROLLED
(DIODES)
CONTROLLED
(THYRISTORS)
6 PHASE HALFWAVE ( THYRISTORS)
BRIDGE RECTIFIER
(DOUBLE WAY)
FULLY CONTROLLED
HALF CONTROLLED
(DIODES AND THYRISTORS)
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

2. THREE PHASE RECTIFIER HALF WAVE RECTIFIERS Three phase uncontrolled
rectifiers using diodes
Mean value of output voltage
Figure1Three phase Half wave uncontrolled Rectifier circuit
Assuming level d.c. load current IL, the diode currents shown in Fig. 2 are each blocks one-third of a cycle in duration. Using the r.m.s. value of the diode current for the required rating purposes for each diode
Also by taking the square root of the mean of the sum of the (current)2 over three equal intervals in the cycle, that is
Peak Reverse voltage of diode
PRV= 𝟑 𝑽 𝒎𝒂𝒙 (𝒑𝒉𝒂𝒔𝒆)= 𝑽 𝒎𝒂𝒙 (𝒍𝒊𝒏𝒆)
Figure2 Three phase Half wave uncontrolled Rectifier waveforms
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

3. Three phase Half wave Controlled rectifiers using Thyristors
Figure6 Three phase Half-controlled Rectifier circuit
Figure7 With large and small firing angle
Figure5 Three phase Half-controlled rectifier waveforms
PRV of thyristor= 𝟑 𝑽 𝒎𝒂𝒙 (𝒑𝒉𝒂𝒔𝒆)= 𝑽 𝒎𝒂𝒙 (𝒍𝒊𝒏𝒆)
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

4. THREE PHASE RECTIFIER BRIDGE RECTIFIERS
Full wave uncontrolled bridge rectifier (diodes)
Figure8 Three phase full wave uncontrolled Rectifier circuit
The three-phase bridge connection is most readily seen as a full-wave connection by reference to the circuit layout shown in Fig. 8
The load is fed via a three-phase half-wave connection, the return current path being via another half-wave connection to one of the three supply lines, no neutral being required.
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

5. Modes of operation of Three phase uncontrolled bridge rectifier
Input voltage waveform
Load output voltage waveform
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

6. Full wave uncontrolled bridge rectifier (diodes)
Figure9 Three phase full wave uncontrolled Rectifier circuit
Mean value of output voltage Vmean
RMS Current rating of diode= 𝑰 𝑳 𝟑
Secondary rms current= 𝟐 𝟑 𝑰 𝑳
PRV of Diode=
𝟑 𝑽 𝒎𝒂𝒙 (𝒑𝒉𝒂𝒔𝒆)= 𝑽 𝒎𝒂𝒙 (𝒍𝒊𝒏𝒆)
Figure 10. Three phase full wave uncontrolled Rectifier circuit
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

7. To derive the load-voltage wave­form, consider that the two diodes which are conducting are those connected to the two lines with the highest voltage between them at that instant.
This means that when va is the most positive phase diode D1 conducts, and during this period first vb is the most negative with diode D6 conducting, until vc becomes more negative when the current in diode D6 commutates to diode D2
The load voltage follows in turn six sinusoidal voltages during one cycle, these being va – vb , va – vc , vb – vc , vb - va, vc – vb , vc - vb, all having the maximum value of the line voltage, that is, 𝟑 times the phase voltage.
The diode current waveforms shown in Fig. 10 reveal that each diode conducts the full-load current for one third of a cycle, the order of commutation determining the numbering of the diodes in the circuit.
The diode voltage vD1 waveform can be determined as the difference between the phase voltage va and the voltage at the top of the load relative to the supply neutral N.
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

8. Modes of operation of Three phase controlled bridge rectifier
Input voltage waveform
Load output voltage waveform
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

9. Full wave Controlled bridge rectifier (Thyristors)
Figure11 Three phase full wave Controlled Rectifier circuit
RMS Current rating of Thyristor = 𝑰 𝑳 𝟑
Secondary rms current= 𝟐 𝟑 𝑰 𝑳
PRV of Thyristor=
𝟑 𝑽 𝒎𝒂𝒙 (𝒑𝒉𝒂𝒔𝒆)= 𝑽 𝒎𝒂𝒙 (𝒍𝒊𝒏𝒆)
The three bridge rectifier can be made into fully controlled by connecting six rectifying elements with thyristors as shown in fig 11.
The mean load is controlled by delaying the commutation of the thyristors by firing angle 𝛂.
The current waveform shapes are similar to diode FWR except they are delayed by angle 𝛂 given in figure 12.
Figure12 Three phase full wave Controlled Rectifier circuit
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

10. 3
OVERLAP
Current transfer between diodes/thyristors has been assumed to be instantaneous
However, in practice current transfer takes a finite time due to presence of supply inductance
Current commutation is delayed with incoming and outgoing devices conducting simultaneously over the ‘overlap’ period.
The figure shows that at commutation there is an angular period 𝜸 during which both the outgoing and incoming diode are conducting . This period is known as ‘overlap’.
Figure 1
Figure 2
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

11. OVERLAP AND OUTPUT VOLTAGE4
The phenomenon is explained using three phase half wave rectifier. As shown in Figure 1,2,3
During the overlap period the output voltage is the average of the incoming/outgoing phases.
OVERLAP AND OUTPUT VOLTAGE
Overlap reduces the mean output
The reduction is proportional to:-
–Supply inductance (reactance)
•more inductance longer overlap period
–Current •more current current transfer takes longer
‘Area Loss’ in output voltage waveform = L IL
Figure 3
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition