Three-phase diode rectifier convert AC balanced signal into stable DC signal where power output is higher than 15kW. Three-phase diode rectifiers can be two types – star rectifier and bridge rectifier.

Star diode rectifier

The three-phase star rectifier is depicted on the figure below.

What is three-phase diode rectifier

What is three-phase diode rectifier

What is three-phase diode rectifier

This rectifier works by the principle, that the phase with the biggest voltage through it , comparing to two other phases, conducts. And conduction angle is 2π3.

Here Vdc=32ππ65π6Vmaxsinθdθ=3π32Vmax=0.827Vmax, and VL=32ππ65π6(Vmaxsinθ)2dθ=32π(π3+34)Vmax=0.84Vmax,    IS=VmR12π(π3+34)=0.485VmR.

The most important characteristics of three-phase star rectifier, listed in the datasheets, are:

  • Peak repetitive reverse voltage VRRM;
  • output ripple frequency fr;
  • Diode average current IF(AV);
  • Rectification ratio;
  • RMS input voltage per transformer leg VS;
  • Transformer rating primary VA;
  • Transformer rating secondary AV;
  • Peak repetitive forward current IFRM;
  • Ripple factor;
  • Diode RMS current IF(RMS);
  • Form factor;
  • Form factor of diode current IF(RMS)IF(AV).

In some conditions 3-phase diode rectifier can experience core saturation. In order to prevent this effect, star diode rectifier schematic can be modified, adding the secondary winding. This connection is also called inter-star. Every secondary winding has phase displacement π3.

Bridge diode rectifier

The three phase bridge rectifier is presented below. They are used in the applications demanding the highest transformer utilisation factor of three-phase system.

3 phase bridge rectifier

The average values for voltages and currents are:

VDC=3ππ32π33Vmaxsinθdθ=33πVmax=1.654Vmax. RMS value of output voltage VL=9ππ32π3(Vmaxsinθ)2dθ=Vmax32+934π=1.65Vmax.

RMS current through each transformer winding IS=Im2π(π6+34)=0.78Imax. Thr RMS current value through every diode is ID=1π(π6+34)Imax=0.5ImaxImax=1.73VmaxR.

For instance, Digi-Key Electronics offers great variety of diode rectifiers.

[1] 3 phase diode rectifiers, “Power Electronics Handbook”, p.154,  Muhammad H. Rashid.