I have three identical sinusoidal voltages, R, S and T, whose phases are spaced by 120o difference. I want to obtain three different DC (unfiltered) voltages from these tree AC signals for some reason. What is the proper way of doing this?

I considered using one of the two rectifier formations below, but I'm not sure if they are correct or not.

Circuit #1:
enter image description here

Circuit #2: enter image description here

  • 1
    \$\begingroup\$ Won't this imbalance the line if you're drawing different amounts of power from each phase? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 12 '13 at 7:26
  • \$\begingroup\$ @IgnacioVazquez-Abrams: Minor phase imbalance is OK and expected in a commercial or residential context. \$\endgroup\$ – Li-aung Yip Nov 12 '13 at 12:34
  • \$\begingroup\$ The three-phase voltage is supplied from a separate personal three-phase generator, so the imbalance will remain as a personal problem. This will not be used in public power networks. \$\endgroup\$ – hkBattousai Nov 12 '13 at 13:08
  • \$\begingroup\$ First - How much current?, Second You do know that Neutral is connected to GND/earth at the utility supply? I don't recormend either of the circuits. Isolate your various DC from earth. Your earth leakage is going to be big and would possibly trip the earth leackage detectors. Wait! is GND the Protective earth or just an internal reference ...? \$\endgroup\$ – Spoon Nov 12 '13 at 13:24
  • \$\begingroup\$ @Spoon OK, I will explain the details. The power source is a wind generator. The circuit charges some batteries, in other words, it drains very high power from the generator. The problem is, it doesn't rotate at low wind speeds because of high current drain. To solve this problem, I decided to turn off DC2 and DC3 until the current drained from DC1 is above some limit. This requires a different rectifier model as I explained in my question. Can you please share your opinion on this? \$\endgroup\$ – hkBattousai Nov 12 '13 at 13:42

Your underlying premise is flawed, so none of your circuits are a good idea.

The simplest solution is to rectify all phases together to make a single DC supply. This mean one pair of diodes for each power feed line. It is unclear whether that will be 3 or 4 pair because it's not clear exactly what "neutral" is in your 3 phase system.

This resulting DC is going to be the input to a switching power supply that eventually produces current to charge the batteries. There is no reason to play games drawing power from only one phase at low torque. In fact that's a bad idea since it will increase the torque ripple. Instead, just modulate how much power you take from the raw DC supply. You can have the switching power supply stop drawing power altogether below some frequency, or otherwise draw power as a function of frequency. Of course you have to limit this to the maximum the battery can take too.

For a better quality but more complicated and costly scheme, you rectify each phase separately so that you can regulate the power factor each phase sees. However, you still try to draw about the same power from each phase. Separating the phases is only to allow for good PFC control per phase, not to allow for a different load from each phase. All the PFC front end supplies will dump power onto the same DC output, with a buck converter from there making the desired battery current. That buck converter takes into acount the frequency (rotation speed) as before. The loading the generator will see will be a function of how much power this buck converter is trying to draw from the DC voltage produced by the three PFC front ends.

The advantage of the PFC scheme is that, if balanced properly, will put a constant torque load on the generator. With the simple scheme, the generator will see some torque ripple at 6x the power frequency.


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