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Paralleling Three Full Wave Rectifier Bridges

Discussion in 'Electronic Design' started by Anand P. Paralkar, Dec 27, 2013.

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  1. mike

    mike Guest

    Thanks, that helps a lot.
     

  2. Thank you everyone for your replies. My replies to some of the posts:

    1. I did check each one of the bridges and the capacitors for faulty
    devices, incorrect polarity, short circuits and open. Didn't find
    anything there. Also, the three bridges are identical.

    2. I would doubt that inrush current is an issue here. Each time that I
    ran this experiment, I was careful enough to increase the variac output
    voltage very slowly. (Ofcourse, connecting an uncharged capacitor bank
    to full output voltage of the variac would surely trip the circuit
    breakers on the utility mains line.)

    3. Not that it matters, but we use a 230V/50Hz mains supply with the
    installation capable of handling 15A of current. (The variac fuse would
    blow somewhere around 10V AC output with no load connected to the output.)

    4. I am not sure, but I get a feeling that some of us reading this post
    have "registered" this as "bridge blowing-up". No, it is the variac
    fuse that blows-up. Infact, there was no heating on any of the bridge
    (or capacitors for that matter).

    I didn't know about the balancing resistor scheme. As already
    indicated, it may not be the solution for this problem, but, thanks for
    the idea, simple way of controlling current hogging.

    I guess, monitoring the variac current with a clamp meter (or may be
    even an oscilloscope may be a good idea).

    Thanks once again to everyone.

    Regards,
    Anand
     
  3. Guest

    Variacs are usually auto transformers and auto transformers work best
    when the output is +/-30 % from the input (70 - 130 %) voltage. Trying
    to get out only 5 % of the input voltage may cause some problems to
    the autotransformer. If there are some advanced protection mechanism
    on the variac, this might be triggered at such low setting, even if it
    might OK around the input voltage.

    A no load full wave rectified 230 Vac will generate about 325 Vdc and
    your capacitor bank will be charged to that potential at the top of
    each half cycle. As a back of the envelope calculation, assuming the
    voltage is allowed to drop to 300 Vdc during 8 ms, until the diodes
    starts to conduct again during the next half cycle, with 6.8 mF the
    load can consume more than 20 Adc or 6.2 kWavg, quite lot for a single
    phase feed.

    If only possible, I would use a three phase 230/400 V feed and with
    standard 6 pulse rectifier, 480 Vdc(avg) will be generated with 4.2 %
    rms ripple _without_ any filtering capacitors. Reducing the ripple,
    quite small capacitors would be sufficient. If the 480 Vdc is slightly
    too much/too little, a small autotransformer will handle that change.
    Get three 12 V car headlights and connect each in series from a
    rectifier to the variac output and increase the output voltage slowly
    and monitor the illumination levels of each lamp. The illumination
    level should be quite similar and they definitively will even out the
    current in different bridges.
     
  4. Use an RMS-reading meter to see what the fuse is "measuring".


    Best regards,
    Spehro Pefhany
     
  5. Guest

    To understand why auto transformers are attractive with small up/down
    voltage conversion (say +/- 10 % to +/- 30 %) , one should remember
    that in an ordinary transformer, all the power is transferred through
    the iron core, while in an auto transformer, only the power related to
    the voltage _difference_ goes through the iron core, in this case only
    10 % to 30 %.

    You can think about the auto transformer as an ordinary transformer
    with the primary connected to the input voltage and secondary with 10
    to 30 % of the input voltage. connect the secondary in series with the
    input voltage and you get 10-30 % boost (in phase) or 10-30 % drop
    (connect in antiphase). Only a small amount of the power goes through
    the iron core.

    Trying to run an auto transformer (variac) at only 5 % of input,
    practically all power flows through the iron core of the transformer,
    might cause saturation and similar problems.
     
  6. Guest

    To the OP
    I think you mentioned that the application is to charge a battery.
    Is this true?

    Also, the others are trying to tell you that the reason the variac is blowing the fuse may be that one of the diodes is now defective.
    Just because the fuse is in the variac and the diodes don't get hot does not the diodes are good.

    Do you know how to check the diodes with an Ohm meter?

    Happy new year
    Mark
     
  7. Fred Abse

    Fred Abse Guest

    <LTspice listing snipped.>

    For reality's sake, you need some series resistance in the source.

    Why hide V1 parameters?
     
  8. Fred Abse

    Fred Abse Guest

    You still haven't told us what the variac fuse is rated at.
     
  9. Fred Abse

    Fred Abse Guest

    Huh?
    In *any* transformer, all the primary current goes through the primary,
    and all the secondary current through the secondary. The secondary amp
    turns oppose the primary amp turns, resulting (in a perfect transformer
    having no leakage inductance) in zero net amp turns hence zero flux.
    Connecting windings in series makes no difference.
    Where does the rest go?
    You cannot saturate a transformer core with secondary amp turns. Even a
    shorted secondary won't do it.
    Saturation occurs where the magnetizing inductance (inductance of primary
    with open secondary) allows enough current to flow to saturate the core.
    That's why big current transformers, such as use in distribution equipment
    buzz like hell if the secondary is O/C. That's a good danger warning.

    COTS transformers are generally designed to run as close to saturation as
    possible, at rated voltage and frequency, to economize on iron.
     
  10. Guest

    True for ordinary transformers with separate primary and secondary
    windings.
    It goes through the galvanic connection.
    Depending on the variac setting, you can run hundreds of kW at 1:1
    setting (limited only by the contact ratings), at 90 .. 110 % we are
    talking about 10 % of the nominal power goes through the core, the
    rest goes through the galvanic connection.

    At 5 %, most goes through the core. possibly 1/20 of the nominal power
    can be transferred through the iron core.
     
  11. Fred Abse

    Fred Abse Guest

    If, by that, you mean two inductors in series, how do you square that
    with; voltage ratio=turns ratio, inductance ratio=turns ratio squared?
    That's just plain wrong.
     
  12. Guest

    OP is that same eternal-september troll sicko harassing sed with stupid ignorant posts for quite some time now. Don't waste your time on "it."
     
  13. Guest

    Assume you have some AC point to point connection to a load.

    Then connect a variac at 1:1 settings to the line. What happens ?

    The actual power is still flowing to the load.

    There might be some reactive (inductive) power flowing through the
    auto transformer.

    Changing the tap settings and more and more power will flow through
    the magnetic core.
     
  14. Guest

    On Sun, 29 Dec 2013 08:59:09 -0800 (PST),
    I would disagree with you.

    The OP is definitively not a "please do my homework for me" type.

    Discussing this topic is definitively appropriate for this newsgroup.
     
  15. Fred Abse

    Fred Abse Guest

    Actually less power, for a constant load, since the voltage reduces.

    Derive an expression for the voltages and currents in an autotransformer
    tapped at, say 30%, hence the flux. Neglect winding resistance.
     
  16. Sounds like you have it connected backwards!
    Are you first connected to the AC, then output of the
    variac to the bridges?

    Jamie
     
  17. John S

    John S Guest

    Yes. What value would you recommend for this simulation?
    As JF stated, it is easy to see the parameters by simply right-clicking
    on the source. If you want to see them on the schematic permanently,
    simply click the box in the lower left corner "Make this information
    visible on the schematic" after right-clicking the source. Easy.

    Cheers,
    John S
     
  18. John S

    John S Guest

    Fred, please do not sully this thread. At this point it doesn't matter
    what the OP had in mind. There is a dialogue going on that is
    instructional to many and pleasing to others. Please leave us to enjoy
    ourselves.

    Thanks,
    John S
     
  19. Fred Abse

    Fred Abse Guest

    I knew that already.
    Shift-alt-ctrl-H will unhide it as well.

    I was interested as to why John thought it necessary to hide the
    parameters, especially in view of the fact that supply impedance has an
    effect on peak rectifier current.
     
  20. John S

    John S Guest

    You omitted what you though might be an appropriate value for the
    simulation.

    Thanks,
    John S
     
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