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3 phase rectifier

Discussion in 'Electronic Design' started by EnigmaPaul, Dec 12, 2008.

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  1. EnigmaPaul

    EnigmaPaul Guest


    If I need a solid 350VDC output from a 3-phase generator, can someone
    point me to the equations to use to figure out how much peak to peak
    voltage I must maintain on the output of the generator to achieve the
    DC voltage out using a 3-phase rectifier?

  2. MooseFET

    MooseFET Guest

    What sort of filter is after the rectifier?

    Assuming a standard 3 phase bridge:

    The bottom of the ripple is at 3/4ths the peak to peak minus the drop
    in the diodes. If you need to maintain a voltage without filtering,
    this is the point you need to worry about.

    The peaks of the ripple are at 0.866 of the peak to peak minus the
    drop of the diodes. If this is a capacitive filtered case, this is
    the peak of the ripple on the output.

    A good estimate for the capacitive filtered ripple amplitude is:

    V = I / ( 360 * C)
  3. Eeyore

    Eeyore Guest

    It won't be solid. It'll still have ripple, but 3 phase is much better in
    that respect than single phase.

  4. EnigmaPaul

    EnigmaPaul Guest

    There is nothing designed yet. What we're trying to do is to
    determine how much 3 phase AC input we'll need to be able to rectify
    down to 350VDC. We can design the rectifier and filter to be whatever
    we like and we're probably use whether the best state-of-the-art might
    be. Assuming that, what is the relationship between the best DC out
    versus AC p-p in?
  5. Rich Grise

    Rich Grise Guest

    1. Do your own math - that's what school is for.

    2. Don't top-post

    Good Luck!
  6. Eeyore

    Eeyore Guest

    Rectify DOWN ?

    To be honest it doesn't sound like you should be playing with ANY
    electricity, never mind 3 phase !

  7. Eeyore

    Eeyore Guest

    WHICH output ? Phase to phase or phase to neutral ?

  8. Jamie

    Jamie Guest

    This depends on the source at hand.
    If you have a floating Delta supply, then you are stuck with
    the phase to phase common practice which in general is a basic
    DC peaks = RMS * 1.414
    minus all the other little crap i'll explain later.

    If you decide to use a grounded (Y)/Star type source, then you could
    use a simple 3 Diode full wave with respect to ground/common.

    Since 3 phase xformers are designed with a 120 degree offset
    in mind, each leg is actually generating more than what you
    measure from leg to leg. This is needed to make up for the degree
    offset so that you can achieve required voltage leg to leg how ever,
    in systems where the (Y)or 1 leg of a delta is ground and thus using
    a common as the low side. You need to up the actual when converting
    to single phase/DC.

    in which case.
    Vrms = Vrms * (1/0.8666);
    for example.
    in systems that employ 480V 3 phase to common point.
    Vrms = (480 /2)*1.153 = (480 * 1.153) /2 = 276.72

    We'll just round that off to 277.

    systems are measured in RMS (mostly), you then calculate your Peak.

    Pk = 277*1.414 = 391.68

    diodes have a loss of ~ .7 per unit.
    so, if you use a full bridge in a floating 3 phase, you'll
    have ~ 1.5 loss which gives you a total of ~390.0

    after that is all said and done, you have some ripple
    from the caps..
    I guess if one was to rely on the standard system
    for better ripple control, one can use that extra voltage
    to be dropped via a resistor on the reservoir capacitor.

    So in the end, it all works out :)
    The dropping R must be calculated via the expected load and
    the ripple error. The R will drive the reservoir cap. Also
    keep in mind that crappy caps with high ESR can give you some
    bad ripple."
  9. Rich Grise

    Rich Grise Guest

    Hey, Guy! What's this guy paying you to do his homework for him? ;-p

  10. Rich Grise

    Rich Grise Guest

    Have Fun!
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