Three-phase diode rectifier

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.

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 $\frac{2\pi }{3}$.

Here $V_{dc}=\frac{3}{2\mathrm{\pi }}{\int }_{\frac{\mathrm{\pi }}{6}}^{\frac{5\pi }{6}}{V}_{max}\sin \theta d\theta =\frac{3}{\pi }\frac{\sqrt{3}}{2}{V}_{max}=0.827V_{max}$, and $V_L=\sqrt{\frac{3}{2\mathrm{\pi }}{\int }_{\frac{\mathrm{\pi }}{6}}^{\frac{5\mathrm{\pi }}{6}}{\left(V_{max}\sin \theta \right)}^{2}d\theta }=\sqrt{\frac{3}{2\mathrm{\pi }}(\frac{\mathrm{\pi }}{3}+\frac{\sqrt{3}}{4})}{V}_{max}=0.84V_{max}, I_S=\frac{V_m}{R}\sqrt{\frac{1}{2\mathrm{\pi }}(\frac{\mathrm{\pi }}{3}+\frac{\sqrt{3}}{4})}=0.485\frac{V_m}{R}$.

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

• Peak repetitive reverse voltage $V_{RRM}$;
• output ripple frequency $f_r$;
• Diode average current $I_{F\left(AV\right)}$;
• Rectification ratio;
• RMS input voltage per transformer leg $V_S$;
• Transformer rating primary VA;
• Transformer rating secondary AV;
• Peak repetitive forward current $I_{FRM}$;
• Ripple factor;
• Diode RMS current $I_{F\left(RMS\right)}$;
• Form factor;
• Form factor of diode current $\frac{I_{F\left(RMS\right)}}{I_{F\left(AV\right)}}$.

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 $\frac{\pi }{3}$.

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.

The average values for voltages and currents are:

$V_{DC}=\frac{3}{\mathrm{\pi }}{\int }_{\frac{\mathrm{\pi }}{3}}^{\frac{2\mathrm{\pi }}{3}}\sqrt{3}{V}_{max}\sin \theta d\theta =\frac{3\sqrt{3}}{\mathrm{\pi }}{V}_{max}=1.654V_{max}$. RMS value of output voltage $V_L=\sqrt{\frac{9}{\mathrm{\pi }}{\int }_{\frac{\mathrm{\pi }}{3}}^{\frac{2\mathrm{\pi }}{3}}{\left(V_{max}\sin \theta \right)}^{2}d\theta }=V_{max}\sqrt{\frac{3}{2}+\frac{9\sqrt{3}}{4\mathrm{\pi }}}=1.65V_{max}$.

RMS current through each transformer winding $I_S=I_m\sqrt{\frac{2}{\mathrm{\pi }}(\frac{\mathrm{\pi }}{6}+\frac{\sqrt{3}}{4})}=0.78I_{max}$. Thr RMS current value through every diode is $I_D=\sqrt{\frac{1}{\mathrm{\pi }}(\frac{\mathrm{\pi }}{6}+\frac{\sqrt{3}}{4})}{I}_{max}=0.5I_{max}$. $I_{max}=\frac{1.73V_{max}}{R}$.

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

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