Power Electronics

# 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 ${V}_{dc}=\frac{3}{2\mathrm{\pi }}{\int }_{\frac{\mathrm{\pi }}{6}}^{\frac{5\pi }{6}}{V}_{max}\mathrm{sin}\theta d\theta =\frac{3}{\pi }\frac{\sqrt{3}}{2}{V}_{max}=0.827{V}_{max}$, and .

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:

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

RMS current through each transformer winding ${I}_{S}={I}_{m}\sqrt{\frac{2}{\mathrm{\pi }}\left(\frac{\mathrm{\pi }}{6}+\frac{\sqrt{3}}{4}\right)}=0.78{I}_{max}$. Thr RMS current value through every diode is ${I}_{D}=\sqrt{\frac{1}{\mathrm{\pi }}\left(\frac{\mathrm{\pi }}{6}+\frac{\sqrt{3}}{4}\right)}{I}_{max}=0.5{I}_{max}$${I}_{max}=\frac{1.73{V}_{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.