1. SINGLE PHASE HALF CONTROLLED BRIDGE RECTIFIER WITH RESISTIVE LOAD

2. For symmetrical operation:-
CIRCUIT DETAIL:-
The fig. shows circuit diagram of single phase half controlled bridge rectifier with resistive load. In this circuit, a single gate pulse can be used for triggering either of SCR. The SCR which is in forward blocking state is only triggered at instant of firing.

3. SYMMETRICAL CONFIGURATION:

4. CASE 1:
During positive half cycle L is positive with respect to N .Then SCR T1 is in forward blocking state and diode D2 is ON. When gate pulse is applied at SCR T1 then current flow through path L-T1-RL-D2-N.
CASE 2:
During negative half cycle, N is positive with respect to L. Then SCR T2 is in forward blocking state and diode D1 is conducting. When gate pulse is applied to SCR T2 then current starts to flow through path N-T2-RL-D1-L.

5. For Asymmetrical Operation:-
CIRCUIT DETAIL:
The figure shows circuit diagram of asymmetrical configuration in which the SCRs T1 & T2 are connected in same arm of bridge where as diodes D1 & D2 are in series and connected in other arm of bridge .

6. ASYMMETRICAL CONFIGURATION:

7. CASE 1:
During positive half cycle, L is positive with respect to N. The SCR T1 is in forward blocking state and D2 is forward biased .When gate pulse apply at α angle at gate of T1 ,the current flows through path L-T1-RL-D2-N.
CASE 2:
During negative half cycle, L is negative with respect to N. The SCR T2 is forward blocking state and D1 is forward biased .When gate pulse apply to SCR T2 then current flow through path N-D1-RL-T2-L.

8. WAVEFORMS:- Ɯt 3Π 2Π Π Ɯt α α Ɯt

9. 3 phase half controlled bridge converter (basic construction)
The 3 phase half controlled bridge converter is as shown in the fig.(1)in this there are all 6 devices,3 SCR’s and the remaining 3 diodes.
The load is connected between the common cathode point of the thyristors and the common anode of the diodes.
The bridge circuit in fig. shows that the neutral point N of the input 3 phase ac supply is not connected anywhere in the ckt.
The half wave converter can be redrawn as shown in fig(2). The SCR’s S1,S2 & S3 form one half wave control rectifier and the diodes D1,D2 &D3 form another rectifier ckt.
In this way half wave converter is equivalent to addition of two half wave ckts.
The load is fed via the upper 3 phase half wave rectifier & the return path for the current is provided via the lower half wave rectifier to one of the 3 supply lines.
No neutral is required.
At a time two devices conduct simultaneously one from each half wave rectifier. One SCR & one diode will conduct simultaneously.
The load vtg therefore will be a line vtg. & not the phase vtg. Which line vtg. Appears across the load is decided by which pair of devices conduct at the given instant of time.

10. 3 Phase Semiconverter

11. Equivalent ckt. For Conduction mode.

12. 3 phase half converter with purely resistive load.
With a purely resistive load (R) the load current will be in phase with the load vtg.
The instantaneous load current will be equal to the instantaneous load vtg divided by the resistance R.
Naturally the shape of the load current waveform will be same as that of the load vtg. Waveform .
Depending on the value of firing angle “α”,this ckt can operate in two different modes of operation.

13. Important points:
6 PULSE OUTPUT: the load vtg waveform contains 6 pulses per cycle of the input vtg. As shown in fig(4). The ripple frequency of the output vtg waveform is therefore 6 times the supply frequency i.e,300Hz.
CONDUCTION ANGLE: each device SCR or diode conduct for 2π⁄3 radians or 120⁰.
NO FREEWHEELING: as load is resistive, it cannot store energy.therfore freewheeling action doesn’t take place in any of two modes of operation with resistive load. ::

14. Operation with purely resistive load
The voltage & current waveforms through purely resistive load are as shown in fig.
The load voltage waveform is same as that with the load inductive load. The load current however will not be ripple free.
Instead the instantaneous load current is equal to the instantaneous load voltage divided by the load resistance R.

15. Therefore the shape of the load current waveform is same as that of the load voltage and it is in phase with the load voltage.
The sequence in which the diodes are turned on & off also remains same as that with the inductive load. The outgoing diodes are turned off due to line communication.
Each diode conducts for a duration of 120°or 2 π/3 radians and the current through each diode during its conduction period is equal to the instantaneous load current during that period.

16. 3 phase full controlled bridge rectifier with purely resistive load:

17. RESULT:
the three phase bridge rectifier & fullwave circuit have the advantages of higher average output voltage ,lower rippel factor, higher rippel frequency, better rectification efficiency & better transformer utilization factor.
the condution angel for diode is 120° or 2𝜋 3 radians for half wave & bridge but 60° for full wave circuit.
The peak inverse voltage also remains same i.e 𝑣 𝑚(𝑙𝑖𝑛𝑒) or 3 𝑣 𝑚(𝑝ℎ) or √6 𝑣 𝑠(𝑟𝑚𝑠) for all the three configuration.