RECTIFIERS

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

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

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

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

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

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

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

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

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

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

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

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