How it works
Half-wave (single SCR): Vdc = (Vm/2π)(1 + cosα); only positive half-cycle used. Single-phase full converter (4 SCRs): Vdc = (2Vm/π)cosα, range 0 to 180°. Single-phase semiconverter (2 SCRs + 2 diodes): Vdc = (Vm/π)(1 + cosα), cannot go negative. Three-phase full converter (6 SCRs): Vdc = (3√3Vm_phase/π)cosα = (3Vm_LL/π)cosα; for 415 V line, Vm_LL = 586 V, so Vdc_max = 560 V at α = 0°. Free-wheeling diode (FWD) across the load prevents Vdc going negative during current commutation with inductive loads, improves waveform, and provides a path for load current when all SCRs are off.
Key points to remember
Firing angle α is measured from the natural commutation point (NCP) — the point where the incoming phase naturally becomes more positive than the outgoing. Form factor FF = Vrms/Vdc; ripple factor RF = √(FF² − 1) — lower is better. Input displacement factor, distortion factor, and power factor all decrease as α increases. With a highly inductive load, current is nearly constant (Id) and ripple is small. Without FWD and with an inductive load, output voltage has negative excursions during commutation that reduce average output and stress the SCRs.
Exam tip
Every Anna University power electronics exam has a numerical on three-phase fully controlled rectifier — memorise Vdc = (3√3/π)Vm cosα and know how to calculate average and RMS output voltage for a given firing angle and supply voltage.