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If a three-phase 220V AC @ 50Hz(three lines having 127V RMS voltage if measured to neutral each) mains supply is rectified with a standard 6-diode rectifier looks like this: 3-Phase Rectifier

I believe if 25 amps is drawn from output of rectifiers with a rectified voltage of roughly 310V DC, then it can deliver about 7.7kW of power.

I know in single-phase full bridge rectification the following formula is used to calculate voltage ripple: Vripple = Iload / (2 * f * C)

QUESTIONS:

Now the challenge I'm facing is how can I reduce the ripple to less than 1%, so I could deliver a steady DC power?

Am I right to assume in three-phase full bridge rectification the following formula can be used? Vripple = Iload / (6 * f * C)

Can I get away with a bunch of capacitors or is it better practice to use either a buck, boost, or buck boost converter?

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  • \$\begingroup\$ "how can I reduce the ripple to less than 1%, so I could deliver a steady DC power?" - why do you need it? \$\endgroup\$ – Bruce Abbott Nov 30 '16 at 1:19
  • \$\begingroup\$ Because that is required by the manufacturer of an electronic equipment. \$\endgroup\$ – Mark Wright Nov 30 '16 at 19:29
  • \$\begingroup\$ ...and the 'electronic equipment' is? \$\endgroup\$ – Bruce Abbott Nov 30 '16 at 22:21
  • \$\begingroup\$ a battery charger \$\endgroup\$ – Mark Wright Dec 2 '16 at 19:32
  • \$\begingroup\$ Must be a very poor design if it can't handle any more than 1% ripple. What about mains voltage variations (may be +-6% or more)? \$\endgroup\$ – Bruce Abbott Dec 2 '16 at 23:45
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Capacitors are the simple way to do it, but you can decrease the amount of capacitance needed by putting a DC link choke in the circuit after the rectifier. A choke would not do it all, you still need caps, but you can make a choke cheaper than you can buy caps.

With the caps however, you must be careful about inrush current, so generally that is dealt with by using a "pre-charge" circuit of either a current limiting resistor that is shorted with a relay / contactor in line with the DC circuit ahead of the caps, or using an NTC thermistor (up to a certain power rating).

The multi-phase transformer method requires more diode bridges and is generally used to reduce the harmonics created on the line side, not the DC bus ripple.

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    \$\begingroup\$ But the multi-phase transformer method is able to reduce the DC bus ripple too. \$\endgroup\$ – Uwe Nov 30 '16 at 11:50
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You may use special transformers for a twelve-pulse-bridge, see here https://en.wikipedia.org/wiki/Rectifier#Three-phase_rectifiers. Your circuit is a six-pulse-bridge, but even 18 and 24-pulse-bridges are possible. The additional phases are generated by using two sets of secondary windings, one in star (wye) connection and one in delta connection or by adding the voltages of additional secondary windings of the transformer. You don't need capacitors to smooth the DC voltage, but you need the special three phase transformer and additonal recitifiers. The twelve-pulse-bridge is often used for high-voltage, direct current (HVDC) electric power transmission systems, see here https://en.wikipedia.org/wiki/HVDC_converter

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Perhaps consider a multi-phase transformer (e.g. 3 to 9), this will decrease ripple significantly with low-losses.

This Paper claims 0.8% ripple using multiphase transformer

You can augment the transformers with high-power capacitors.

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    \$\begingroup\$ This needs a bit more detail included to be a good answer. \$\endgroup\$ – brhans Nov 30 '16 at 1:35

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