1. RECTIFIERS

2. RECTIFYING CIRCUITS
A rectifier circuit is one which links an a.c. supply to a d.c. load, that is, it converts an alternating voltage supply to a direct voltage.
The rectifier broadly classified
According to AC supply
Single phase
Three phase
According to process of rectification
Half-wave(rectification on one half of input voltage cycle)
Full-wave(bridge)(rectification on +ve and –ve half of input voltage cycle)
According to control characteristics
Uncontrolled(only diodes)
Fully controlled (only thyristors)
Half-controlled (diodes and thyristors)
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

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

4. SINGLE-PHASE HALF-WAVE RECTIFIER
Uncontrolled single-phase half-wave (Diode)
a) R load
During the positive half cycle of input voltage, the diode is forward biased , it therefore conducts from Ѳ =0 to Ѳ =π
Load Voltage vL = vs
Load current iL = vs /R
During the negative half cycle of input voltage, the diode is reverse biased , it is therefore turned off and goes to blocking state.
Load Voltage vL =0
Load current iL = 0
Peak Inverse Voltage(PIV)= V max
Figure 1:Single phase half wave circuit
Mean value of half wave sine wave with R load
(1)
Figure 2:Waveforms for pure resistive load
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

5. Uncontrolled single-phase half-wave (Diode) b) R L load
Almost all d.c. loads contain some inductance; the waveforms shown in Fig. 4 are for an inductive load having the equivalent circuit shown in Fig. 3.
The current iL continues to flow even after the source voltage vs has become negative, this is because of the presence of inductance L in the load circuit.
The instantaneous load voltage
(2)
enables the waveshape of the load current iL
Figure 3: Load Equivalent Circuit
Figure 4 :Waveforms for RL load
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

6. b) R L load (cont..d)
The diode will conduct upto Ѳ =Ф until the current though the load falls to zero, this is due to the nature of the RL load
When iL = 0 at Ѳ =Ф
Load Voltage vL =0
Supply voltage vs =0
Vmean = Vmax 2π (1-cos Ф) (3)
Mean value of half wave sine wave with R L load
Figure 5 :Waveforms for RL load
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

7. FREE-WHEELING/ COMMUTATING DIODE
Many circuits, particularly those which are half or uncontrolled, include a diode across the load as shown in Fig. 6.
This diode is described as a free-wheeling / commutating diode, as its function is to commutate or transfer load current away from the rectifier whenever the load voltage goes into a reverse state.
The commutating diode serves one or both of two functions; one is to prevent reversal of load voltage and the other to transfer the load current away from the main rectifier, thereby allowing all of its thyristors /diode to regain their blocking state.
Figure 6: Block diagram of rectifier with commutation/ free-wheeling diode
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

8. ANALYSIS OF RECTIFIER DC output voltage for half wave
Ripple Factor(RF) : it is the lack of smoothness of the waveform.
For half wave rectifier ripple factor =1.2
Peak Inverse voltage: It is the maximum instantaneous voltage which occurs across the rectifying diode in the reverse direction.
For a half wave rectifier PIV = Vm
Rectifier Efficiency ( Rectification ratio): It is defined as the ratio of dc load power to the ac power input
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

9. Controlled single-phase half-wave (Thyristor)
Figure 7: Single-phase Half-wave Controlled Circuit
The thyristor will only conduct when its voltage vT is Positive and it has received a gate firing pulse ig.
The thyristor conducts at an angle α(FIRING ANGLE) ,where the pulse ig is applied to gate of thyristor.
Without commutation diode the output waveform is similar to the figure 4(diode rectifier with RL Load).
The presence of a commutating diode which prevents the load voltage reversing results in the waveforms shown figure 8.
Mean value of output voltage
Figure 8: waveforms for Small firing day angle and continuous current
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

10. BI PHASE HALF WAVE RECTIFIER
Figure9 : Bi-phase Half-wave circuit
Mean value of output voltage
Figure10 : Bi-phase Half-wave connection waveforms
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

11. SINGLE PHASE FULL WAVE BRIDGE RECTIFIER
Uncontrolled Rectifier(Diodes)
Figure11 Uncontrolled Rectifier circuit
Figure12 Uncontrolled Rectifier waveforms
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

12. Fully Controlled bridge Rectifier(Thyristor )
Figure13 Controlled Rectifier circuit
Mean value of output voltage
Figure14 Controlled Rectifier waveform
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition

13. Half-controlled bridge Rectifier(Thyristors and diodes)
Figure15 Half-controlled Rectifier circuit
Mean value of output voltage
Figure16 Half-controlled Rectifier waveform
Reference: 'Power Electronics', Cyril C Lander, McGraw-Hill Book Company, 3rd Rev Edition