Working of Three Phase Uncontrolled Full Wave Rectifier

• The connection diagram for three phase full wave uncontrolled rectifier using Delta – star transformer is shown in the figure A. 
• There are two diodes used for each phase and load is connected between common anode terminal and common cathode terminal of diodes. 
• There are two three phase half wave rectifiers which are made by diode D1, D2 and D3 and diodes D4, D5 and D6 respectively.

Working

• The three phase supply voltage waveform is shown in the figure B.
• The diode can conduct when phase voltage ( either positive or negative ) is maximum out of three phases.

Point a 

• The voltage of phase R becomes zero. 
• The voltage of phase B becomes nearly maximum positive whereas the voltage of phase Y becomes negative therefore diode D3 from phase B positive half leg and diode D5 from phase Y negative half leg conducts. 
• The current flows from phase B – diode D3 – load resistance R_L – diode D5 – phase Y.

Point b

• The voltage of the phase R increases towards positive maximum whereas the voltage of phase B decreasing towards positive to negative. 
• The voltage of the phase Y already negative as earlier. Therefore the diode D1 from phase R leg conducts whereas the diode D5 continues its conduction. 
• Therefore the current flows from phase R – diode D1 – load resistance R_L – diode D5 – phase Y.

Point c

• The voltage of the phase R becomes positive maximum. 
• The voltage of the phase B decreases towards negative maximum whereas the voltage of phase Y increases from negative to positive.
• Therefore the diode D1 from phase R positive leg continues its conduction whereas the diode D6 from phase B negative leg conducts.
• The load current flows from phase R – diode D1 – load resistance R_L – diode D6 – phase B.

          

• This will result in one diode from positive group D1, D2 or D3 and one diode from negative group D4, D5, or D6 conducts at a same time e.g. D3 and D5, D5 and D1, D1 and D6, D6 and D2, D2 and D4. 
• Each diode conducts for 120^o. It should be noted that two diodes of a same phase never conduct otherwise the supply should be short circuited.

THREE PHASE FULL WAVE  
UNCONTROLLED RECTIFIER.PDF

THREE PHASE FULL WAVE 
UNCONTROLLED RECTIFIER 
WAVEFORMS.PDF

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