Introduction of three phase full wave uncontrolled 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.

Three phase full wave rectifier circuit diagram

Working of three phase full wave uncontrolled rectifier

The three phase supply voltage waveforms are 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.

Waveform of three phase full wave uncontrolled rectifier

The waveform of three phase supply, diode conduction and voltage is given here. Here, 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.