Three Phase Full Wave Controlled Rectifier
The 3-phase half wave converter combines three single phase half wave controlled rectifiers in one single circuit feeding a common load. The thyristor S1 in series with one of the supply phase windings ‘a-n’ acts as one half wave controlled rectifier. The second thyristor S2 in series with the supply phase winding ‘b-n’ acts as the second half wave controlled rectifier. The third thyristor S3 in series with the supply phase winding acts as the third half wave controlled rectifier. Figure bellow shows three phase fully controlled rectifier.
• When thyristor S2 is triggered at ωt=(5π/6α), S1 becomes reverse biased and turns-off. The load current flows through the thyristor and through the supply phase winding ‘b-n’. When S2 conducts the phase voltage vbnappears across the load until the thyristor S3 is triggered.
• The 3-phase input supply is applied through the star connected supply transformer as shown in the figure. The common neutral point of the supply is connected to one end of the load while the other end of the load connected to the common cathode point.
• When the thyristor S1 is triggered at ωt=(π/6 + α)=(30° + α) , the phase voltage Van appears across the load when S1 conducts. The load current flows through the supply phase winding ‘a-n’ and through thyristor S1 as long as S1 conducts.
• When the thyristor S3 is triggered at ωt=(3π/2 + α)=(270°+α) , S2 is reversed biased and hence S2 turns-off. The phase voltage Van appears across the load when S3 conducts.
• When S1 is triggered again at the beginning of the next input cycle the thyristor S3 turns off as it is reverse biased naturally as soon as S1 is triggered. The figure shows the 3-phase input supply voltages, the output voltage which appears across the load, and the load current assuming a constant and ripple free load current for a highly inductive load and the current through the thyristor T1.
• For a purely resistive load where the load inductance 'L = 0' and the trigger angle α >(π/6), the load current appears as discontinuous load current and each thyristor is naturally commutated when the polarity of the corresponding phase supply voltage reverses. The frequency of output ripple frequency for a 3-phase half wave converter is fs, where fs is the input supply frequency.The 3-phase half wave converter is not normally used in practical converter systems because of the disadvantage that the supply current waveforms contain dc components.