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Transformers

Discussion in 'Electrical Engineering' started by Rusty, Sep 16, 2004.

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  1. Rusty

    Rusty Guest

    Forgive my ignorance. Is a DC transformer different than AC transformer?
     
  2. You need AC for a passive transformer to work (actually you need a magnetic
    field that changes over time). DC voltage cannot be transformed by a
    passive transformer. You have to chop it up, transform the chopped up
    waveform, and then smooth it out again. In other words it takes an active
    circuit.

    So are they different? Yes in that a DC transformer doesn't really exist.

    Charles Perry P.E.
     
  3. Rusty

    Rusty Guest

    In that case, do they make bridge rectifiers for 2000+ volts AC?
     
  4. I have seen adjustable speed drives at 4000 volts so I would assume they do.

    Charles Perry P.E.
     
  5. Stephen B.

    Stephen B. Guest

  6. John G

    John G Guest

    Please give us a few simple clues about what you really want then maybe
    we could properly answer your question.
     
  7. Rusty

    Rusty Guest

    I have a transformer from a microwave oven and I want the output DC
     
  8. Guest

    What are you going to do with the DC - aside from possibly
    killing yourself? My fear is that you may not recognize the
    dangers or know how to work with high voltage safely.
     
  9. bushbadee

    bushbadee Guest

    Sorry Chuck you are wrong on that one.
    DC transformers do exist and they are used in most satelites.
    They use two cores and require an energizing source and are used to measure
    DC currents.
    They are some times refered to as Mag Amps.
    You will find them explained in Rotors.

    I also invented a dc transformer which is extremely accurate some years ago.
    I can acturally measure with an accuracy of about 1 part per million or even
    more.
    The higher the current, the more accureate it is.
    The offset is fixed, no matter what the current is.
    I delivered a paper on it at the IEEE but it was a bit advanced and I do not
    think any one even understood what was going on.
    It uses a single core.
    Basically the DC drives a very narrow loop material way into saturation.
    I mean like a thousand times the width of the hysterieses loop.
    Now a voltage is applied to the search coil so as to bring the core out of
    saturation.
    When the core comes out of saturation and the core travels back up the loop
    there is a voltage pulse out of the sense loop. This pulse then is a
    measure of how deeply into saturation the core was and a measurement of the
    current flowing.
    Because the saturated state of the core is so large compared to the loop
    width, the measurement can be very accurate.
    I actually build and tested the device.

    Advantage is just one core and extreme accuracy.
     
  10. Miles

    Miles Guest

    Yes - I've seen ones up to around 500kV (they call them HVDC links, but they
    are probably a little outside your price range... :)
     
  11. Not really. I said there were no passive transformers. Magamps are active.

    Also, he is trying to transform DC for power use, not measurement.

    Charles Perry P.E.
     
  12. Rusty

    Rusty Guest

    I'm gpoing to use it to charge capacitor banks
     
  13. Rusty

    Rusty Guest

    I'm going to use it to charge capacitor banks
     
  14. Dave Holford

    Dave Holford Guest


    Sounds like a potentially lethal device.

    Dave
     
  15. bushbadee

    bushbadee Guest

    I misunderstood.
    I thought he wanted to measure the current.
    In any case, these magamps are called DC transformers even though they are
    just for measurements.
     
  16. bushbadee

    bushbadee Guest

    I'm going to use it to charge capacitor banks

    That is even more dangerous.
    Do you know any thing about HV.
    Do you know how far a spark can jump.
    Do you understand the effect of a sharp point on the high voltage end.
    In any case, work with one hand in your pocket if you will not be disswaded.

    If you have to ask questions about HV DC that means you do not have enough
    knowledge to work with it.
    Nothing substitutes for a couple of years of apprenticship where you learn
    the things not in the books.
     
  17. bushbadee

    bushbadee Guest

    I have a 5000 volt meter.
    It is a direct reading meter and is not an electronic one.
    It reads 5000 volts directly and does not require any special probes.
    The interesting thing about it is that if the conditions are right the
    needle sticks to the glass.

    I used it when I was designing 28 to 5000 volt isolated, regulated, power
    supplies that could fit into a 20 cigarette package.
     
  18. daestrom

    daestrom Guest

    We also used this type of feedback. It is just a rate-sensitive feedback to
    the regulator. The output is zero in the steady-state, but when the field
    current is changing (as it would when a large load is applied to the
    generator), the signal from this transformer would be non-zero and
    proportional to the rate of field current change. Signal was used to dampen
    oscillations in the system caused by a somewhat high-gain regulator and a
    long time-constant in the field current (due to field winding's inductance).

    We also used another form of 'DC' transformer that was really a form of
    mag-amp. An SSR (self-saturating reactor) would be placed in the half-wave
    rectified supply to the generator field. The control winding of the SSR
    would carry a DC signal from the regulator. After just a couple of cycles
    with no control-winding current, the rectified DC of the power circuit would
    saturate the reactor (i.e. self-saturating) and the reactor would present
    low impedance to field current. As generator voltage rose, the DC current
    in the control-winding would increase (fed from regulator). DC
    control-winding current would 'de-saturate' the reactor so that the reactor
    was a high-impedance on the subsequent rising half-cycle. At some point
    during cycle, the half-wave current would again saturate the reactor and the
    impedance would suddenly drop again. End result is a 'chopped' half-sine
    wave current to the field, much like you see today with phase-controlled SCR
    bridges. The exact point of 'turn-on' was a function of the DC control
    current.

    'Bushbadee' could probably give a much better explanation of mag-amps, but
    they were small, controlled a lot of power, handled extremes in temperature
    with little trouble and pre-dated SCR's and power-electronic devices.

    daestrom
     
  19. bushbadee

    bushbadee Guest

    No your description is pretty good.
    I put two saturable reactors in the control circuit of the Vela satelite.
    They worked at about 7Kz.
    They had the input voltage on them so that if the input voltage rose, they
    saturated sooner and turned off the power transistors sooner.
    Fantastic regulation with just 6 db gain in the loop.
    That allowed a 12 db per octave roll off and the circuit could regulate way
    out in frequency.
    Even with 12 db per octave roll off we had a fixed 6 degree margin which was
    set by the turns ratio so the cirucit was unconditionally stable.
     
  20. daestrom

    daestrom Guest

    A large change in the load on the generator output (DC or AC) will change
    the voltage output of the generator. Hence the need for a voltage regulator
    in the first place. But to get the desireable regulation, the regulator
    unit must have a pretty high gain (field current must more than double when
    the generator goes from no-load to full-load).

    Such high gain comes at a price. A high gain coupled with a long
    time-constant on the field winding of the generator (DC current through a
    massively inductive load) can result in sustained oscillations. Of course
    there are several ways to counteract such oscillations. One might be to
    sense the rate of change of the generator voltage and use that as a second
    input to the regulator, giving you PD (proportional and derivative) control.
    Another method (the one chosen in this case) is to sense the regulator
    output and its rate of change. Use the regulator's output rate of change as
    a form of feedback.

    So, one could sense the regulator's output any number of ways. A simple,
    rugged method chosen was to use the dI/dt of the field current. But the
    current in the power stage of the output needs to be electrically isolated
    from the regulator. A 'DC transformer' is used since it's secondary voltage
    is proportional to the dI/dt of the 'primary' (the generator field current).
    It also provides the electrical isolation all in one package.
    Any number of control mechanisms *could* have been used, just have to meet
    the requirements. The power needed to supply the field ranges from 30-40
    amps at no-load to ~100 amps at full load. The entire unit must be
    self-contained and capable of starting up with no external electric source.
    It must be operable in some extreme temperature ranges (~20F to ~150F) and
    must withstand the severe vibration of the diesel engine and, believe it or
    not, being depth-charged (severe shocks of several g's).

    Nowadays, it could be done with solid state devices, but in the '60's and
    '70's, mag-amps and SSR's were the way to go. Also used in motor-generator
    sets used for 'vital busses'.

    daestrom
     
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