3 Phase 4 wire

Discussion in 'General Electronics Chat' started by cryosar, Aug 17, 2009.

  1. cryosar

    Thread Starter Active Member

    Apr 2, 2009
    40
    0
    Hi all,
    Am developing a 10W SMPS. my input voltage is 415V, having 3 phase 4 wire. Can anyone help me how to connect the input bridge using diode for 3 phase 4 wire and i wanted to know its operation also. Thanks in advance
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Four wire would (most likely) be Three phase + Neutral or (possibly) just Three phase plus Earth.

    You may find you actually have both Neutral and Earth, as often Earth is assumed to be present and not counted. (But it is still a separate core / pin in a plug-in cable so some people may count it..)

    A three phase bridge rec uses the same principle as a sigle phase one:
    Each AC input has a diode to DC out + and a diode to DC out -.

    Where you have four diodes for single phase with two AC terminals, you have six diodes for the three phase AC input.

    You MUST use properly rated, high-voltage ceramic fuses before the bridge rectifier. A fault could otherwise start a fire or worse.

    Once rectified, you are dealing with around 600 - 700V DC.
    Both the 415V and the DC side are EXTREMELY DANGEROUS - Touch anything and you may DIE...

    For a device that only needs 10W, you should consider using other methods for prototyping such as a current limited and isolated DC bench supply rather than the rectifier.

    Still very dangerous but not as bad as an unlimited mains supply (you could still be dead but at least not smoking and crozzled).
     
  3. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    You mentioned that the power supply would provide 10 W. How
    many volts would be the output of this supply?

    Before rectifying, run the electricity through a transformer. This will
    isolate it from the mains and, since you only need 10W of power,
    using a small transformer will limit it to safer levels.

    Since you need relatively little power, it might suffice to use
    only one out of the three phases. The resulting DC wouldn't
    be as smooth, but that may or may not be an issue for what
    you are doing.
     
  4. cryosar

    Thread Starter Active Member

    Apr 2, 2009
    40
    0
    I have attached the bridge circuit. I can understand the operation of 3 phase 3 wire (RYB) but with RYBN bridge am not able to understand. Can anyone explain me?
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Either you ignore the neutral and the DC Out is +/- 300V or so to Neutral (& Ground), or you only use three diodes, one from each phase to +DC with Neutral as the 0V.

    Having said that, capacitors with a voltage rating to handle rectified 415V are rare.

    It is common to use two caps in series to get a suitable voltage capability. Normally you use bleed resistors across each to equalise the voltage, but if you have Neutral available this becomes the center point of the capacitors with a full bridge rec.

    Think of it as two, three-diode half bridges, one supplying positive and one negative.
    (You should still use bleed resistors to safely discharge the caps.)
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Without an isolation and step down transformer, this project is just a fatal accident waiting to happen. If you only need 10 watts, use one phase to feed a wall transformer, although the line may make finding a suitable one interesting.

    Line voltages over 240 are very hard to handle safely.
     
  7. cryosar

    Thread Starter Active Member

    Apr 2, 2009
    40
    0
    Isolation of around 2 KV will be provided.Continuous will be 4W & momentary power will be 10W. once am through with my circuit i ll post it.
     
    Last edited: Aug 18, 2009
  8. cryosar

    Thread Starter Active Member

    Apr 2, 2009
    40
    0
    can anybody tell me what is the formula to calculate DC after full wave bridge rectification of three phase supply?
     
  9. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    If you use the 'two series caps with midpoint to neutral' setup, it is simply twice the single phase rectified value.

    If you don't use neutral, I think it will be slightly less but it's too early in the morning for me to get my head round it, all I can think of is coffee...
    Calculation time..

    The peak on the caps should be (the voltage you are rectifying) x 1.414 if I remember correctly, so:
    With 2 caps & neutral, that would be 2 x 230 x 1.414 = 650.44V
    Without neutral 415 x 1.414 = 586.81V

    (Assuming 230V per phase, 415V 3 phase)
    That is not a definitive answer but I believe it's a good approximation.

    Remember industrial supplies are often very unstable voltage-wise due to varing loads. At one industrial site I go to, the single phase '240V' supply to the offices varies from 230 to 262V. That's simply one phase from the incoming three phase.
    The DC will vary in proportion to the actual AC supply.

    Idea: Unless a key part of your project is the high voltage stuff, for a very low power like 10W you could use a converntional transformer / rectifer / smoothing to an appropriate low voltage, then something like a National Semiconducter 'Simple Switcher' circuit to do the stabilisation.

    That's likely far simpler and cheaper to build, and definitely far safer.
     
Loading...