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using step-down transformer as step-up

Discussion in 'Electronic Design' started by [email protected], Feb 11, 2009.

  1. Guest

    At first blush, it seems like you should be able to use a power
    transformer as either step-down or step-up, simply by feeding the
    input into either the primary or secondary. However, is this really
    the case? For example, consider one of those pole transformers that
    normally step down, say, 12kV to 220V. If the secondary is not
    connected to anything, I would expect the primary current to be quite
    small. However, if you were to feed 220V into the secondary with the
    primary open, wouldn't quite a bit of current be drawn by the 220V
    secondary? (I've never tried this, I'm running on intuition here, so
    I would welcome any insight.)

    A similar situation: one of those Weller soldering guns. It has a
    transformer with many turns on the 120V primary, and a ONE-turn heavy
    copper secondary going to the tip. Surely if you reverse that and
    feed in 1V (or whatever the secondary voltage is) it will be almost a
    direct short at 60Hz, even if the primary is unloaded?

    Is my intuition right, or am I totally off base here?

    If I am right, what is it that makes the transformer asymmetrical?
    That is, different when used as step-up or step-down? How would a
    step-up transformer that is meant to convert 220V to 12kV differ in
    construction from the step-down pole transformer, and why?

    Robert
     
  2. Phil Allison

    Phil Allison Guest

    ** It is - with some provisions.

    ** Enough current would have to flow in the secondary to magnetise the iron
    core.

    That level would be 12,000/220 times the normal level.


    ** Basically - you are wrong.


    ** Basically, they are not.

    Problems can arise with small AC supply transformers that have high
    magnetising current levels and poor regulation factors.

    These will still work OK in reverse, but at a somewhat lower voltages than
    when used the normal way.



    ...... Phil
     
  3. In <>,
    My wild guess is feed "normal secondary voltage" into what is normally
    the secondary, with what is normally the primary being open, and the
    voltage at what is normally the primary will be low-90's % of "nominal",
    when the transformer is a bigger one of several to 10's of KVA. My
    experience is that ones of a KVA or two achieve upper 80's % in that area.
    I consider this a fairly inefficient transformer. I do not have one
    handy to try. Though my guess is that running one of those in reverse
    needs several amps to maybe around 10 amps to supply "magnetizing current"
    from the low voltage side at "normal secondary voltage" (in addition to
    soldering tip load current), to achieve output voltage from the line cord
    maybe around 75-80 volts or so output voltage from the line cord. (Though
    I would not be surprised with the line cord putting out to open circuit
    or a voltmeter anywhere from 60 to 105 volts).
    Loss, which is probably at least somewhat evenly split into the two
    directions.
    Due to losses, I expect 12KV-to-220V (more like 240V) to have turns
    ratio a little less than 50 (12,000/240), probably more like 48 maybe 47.

    Step-up from 240V to 12 KV means probably turns ratio of at least 53
    even in 1-KVA capacity ballpark.

    - Don Klipstein ()
     
  4. Guest

    For example, consider one of those pole transformers that
    Interesting....so does that mean that a transformer designed to step
    220V up to 12kV would have more turns on the 220V side than a similar
    step-down transformer, so that the current drawn by the 220V winding
    in the absence of a load is smaller? Robert
     
  5. Phil Allison

    Phil Allison Guest


    Interesting....so does that mean that a transformer designed to step
    220V up to 12kV would have more turns on the 220V side than a similar
    step-down transformer, so that the current drawn by the 220V winding
    in the absence of a load is smaller?


    ** Your whole notion is false.

    Transformers are essentially symmetrical devices.




    ....... Phil
     
  6. Guest

    No. It means that the wire used on the 220V side would be thicker (to
    carry the magnetising current without getting too hot) in a
    transformer designed to be used to step up a 220V source to provide a
    12kV output, as compared with a transformer designed to step down a
    12kV source to provide a 220V output.

    The turns raito for either transformer may well be slightly higher
    than the nominal 54.5 to compensate for the voltage lost driving the
    magnetising current through the winding resistance, but this ratio
    should be the same for transformers driving the same output load in
    either direction. In practice, it isn't always possible to wind a high
    voltage winding with the optimal wire thicknesss - thin wire breaks
    easily.
     
  7. Guest

    ** Enough current would have to flow in the secondary to magnetise the
    You ARE right. Transformers ARE essentially symmetrical. I think I'm
    starting to see that was troubling me had nothing to do with symmetry
    at all, but rather with the no-load current drawn by the winding used
    as the primary.

    If I plug the 220V winding of that 25kVA step-down transformer into my
    220V wall socket, it'll probably blow the fuse even if nothing is
    connected to the other side. But that's not asymmetry. It just means
    that the transformer wasn't meant to be used like that. If it was, it
    would have more turns on the 220V side (and a correspondingly larger
    number of turns on the 12kV side to maintain the ratio) so that the
    now greater inductance would draw less current while magnetizing the
    core.

    So it's not the transformer itself that is asymmetrical in behaviour,
    but rather the requirements placed on the windings, depending on
    whether you use them as primary or secondary. A primary winding
    shouldn't draw much current when unloaded, and for a secondary, fewer
    turns is probably an advantage since it gives lower impedance so the
    voltage won't drop as much under load.

    Does that make sense, or am I still wrong? Robert
     
  8. Guest

    ** Enough current would have to flow in the secondary to magnetise the
    You ARE right. Transformers ARE essentially symmetrical. I think I'm
    starting to see that was troubling me had nothing to do with symmetry
    at all, but rather with the no-load current drawn by the winding used
    as the primary.

    If I plug the 220V winding of that 25kVA step-down transformer into my
    220V wall socket, it'll probably blow the fuse even if nothing is
    connected to the other side. But that's not asymmetry. It just means
    that the transformer wasn't meant to be used like that. If it was, it
    would have more turns on the 220V side (and a correspondingly larger
    number of turns on the 12kV side to maintain the ratio) so that the
    now greater inductance would draw less current while magnetizing the
    core.

    So it's not the transformer itself that is asymmetrical in behaviour,
    but rather the requirements placed on the windings, depending on
    whether you use them as primary or secondary. A primary winding
    shouldn't draw much current when unloaded, and for a secondary, fewer
    turns is probably an advantage since it gives lower impedance so the
    voltage won't drop as much under load.

    Does that make sense, or am I still wrong? Robert
     
  9. Phil Allison

    Phil Allison Guest

    You ARE right. Transformers ARE essentially symmetrical. I think I'm
    starting to see that was troubling me had nothing to do with symmetry
    at all, but rather with the no-load current drawn by the winding used
    as the primary.

    If I plug the 220V winding of that 25kVA step-down transformer into my
    220V wall socket, it'll probably blow the fuse even if nothing is
    connected to the other side. But that's not asymmetry. It just means
    that the transformer wasn't meant to be used like that. If it was, it
    would have more turns on the 220V side (and a correspondingly larger
    number of turns on the 12kV side to maintain the ratio) so that the
    now greater inductance would draw less current while magnetizing the
    core.

    So it's not the transformer itself that is asymmetrical in behaviour,
    but rather the requirements placed on the windings, depending on
    whether you use them as primary or secondary. A primary winding
    shouldn't draw much current when unloaded, and for a secondary, fewer
    turns is probably an advantage since it gives lower impedance so the
    voltage won't drop as much under load.

    Does that make sense, or am I still wrong?


    ** You are still wrong.

    When I say transformers are essentially symmetrical - I mean they CAN be
    used in both the normal and reverse directions.

    Why not simply post a REAL question - ie one where YOU are not posing mad
    theories and making up false examples.

    Eh ????


    ...... Phil
     
  10. Guest

    Why not simply post a REAL question  - ie one where YOU are not posing mad
    I'm sorry. I didn't mean to upset anyone. This is a real question to
    me since I need a high-voltage step-up transformer for an RF amplifier
    and was planning to use a power distribution transformer backwards,
    but it's expensive so I wanted to make sure it will do what I want
    before I buy one. Again, sorry I'm being a pain. I won't ask any
    more dumb questions. Robert
     
  11. Phil Allison

    Phil Allison Guest

    <
    I'm sorry. I didn't mean to upset anyone.


    ** Not upset, but you were going no-where with your absurd Q.


    This is a real question to
    me since I need a high-voltage step-up transformer for an RF amplifier
    and was planning to use a power distribution transformer backwards,
    but it's expensive so I wanted to make sure it will do what I want
    before I buy one. Again, sorry I'm being a pain. I won't ask any
    more dumb questions.


    ** If you had just posted that simple Q right off - with a link to the
    tranny you are looking at - you would have very likely had your answer
    long ago.





    ...... Phil
     
  12. Guest

    Trying again with a "simple" question :)
    Here's a link to pictures and some info about a transformer:
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=400028867104
    (You have to scroll down past some irrelevant stuff. Then there are
    pictures and a close-up of the nameplate.)
    It's a 25kVA 4160/7200/12470V to 240V 60Hz step-down transformer.

    What I'm wondering is, if I use this as a step-up transformer by
    applying 240V to what is normally the secondary, what would be the no-
    load current drawn by the 240V winding?

    I suspect there isn't enough information here to determine that, but
    perhaps someone who is familiar with transformers of this type would
    have some idea?

    Robert
     
  13. Rich Grise

    Rich Grise Guest

    On Wed, 11 Feb 2009 18:24:46 -0800, renenkel wrote:
    That's been done before with "pole-pigs"; typically hams using 3300/240
    trannies (local dist. lines were often 3300V back in those days). They
    had no problem with excessive primary current - the magnetizing current
    in the core is sufficient either way - consider a neon sign transformer.

    Have Fun!
    Rich
     
  14. Phil Allison

    Phil Allison Guest


    ** Such large transformers have low * percentage * losses and the iron and
    copper loss ( at full load) are about the same. The 240 volt winding is
    rated a tad over 100 amps - so I reckon I mag might be 5 to 10 amps or so
    on the 240 volt side.

    But you will never be able to switch it on without using a big variac or a
    well designed "sort start "circuit and you will never get more out than you
    put in.

    Clearly, you have some totally hair-brained, LETHALLY DANGEROUS and ILLEGAL
    scheme in mind.


    Kindly GO STRAIGHT TO HELL.

    Bloody lunatic.



    ...... Phil
     
  15. Guest

    I've built a few low power RF amplifiers in the past. I "rolled my
    own" impedance matching transformers for coupling between stages. I
    don't think that transformers designed for 50- 60 Hz are going to wrok
    well at RF frequencies. What frequency and power levels are you
    working at?

    George Herold
     
  16. Baron

    Baron Guest

    If he is talking about a PSU for a big (1KW) SSB linear amp I've used
    microwave oven transformers to generate 3KV @ 1 amp. Its a low duty
    cycle application. The transformers are free from the local dumpster !
     
  17. whit3rd

    whit3rd Guest

    Yes, but only ALMOST a direct short; the secondary winds around
    a magnetic core, and has, thus, some series inductance.
    If one loads the primary, the effective impedance on the
    secondary will go down. If you short the primary, it
    acts like most of the series inductance vanishes, though
    the primary has significant internal resistance which will
    prevent the short circuit from being completely effective.
     
  18. Guest

    I've built a few low power RF amplifiers in the past.  I "rolled my
    Actually, when speaking of running a step-down transformer backwards I
    was thinking of the power supply transformer. I'm thinking of making
    a high-powered induction heater with something like 100A at 240V input
    to the transformer, and a 20kW water-cooled power triode as an
    oscillator. Maybe my saying "RF" was a bit of an exaggeration; I was
    thinking of something under 100kHz.

    As for the "Darwin awards" (thanks, John :), I guess I must have been
    trying ever since I stuck my fingers into an energized light bulb
    socket at the tender age of 6, but somehow I'm still around :)

    As for Phil's lovely admonition to "go straight to hell", I hardly
    think this is "illegal", or even "lethally dangerous" if handled
    correctly. Units with far more power than I am proposing are used
    routinely commercially. For example, here's a 150kW unit:
    http://cgi.ebay.com/Inductoheat-150...06518207QQcmdZViewItemQQptZLH_DefaultDomain_0
    I'm sure even that is small potatoes in some applications.

    Also lurking in the back of my mind for some future day is a low-
    frequency radio transmitter for, say, 8kHz. I believe frequencies
    that low are unregulated, so there's no legal limit to the amount of
    power one could use. (Please no flame wars if I'm wrong...just a
    polite correction would be fine! :) The only problem is antenna
    efficiency, but that's probably a topic for another thread....

    Robert
     
  19. Guest

    If he is talking about a PSU for a big (1KW) SSB linear amp I've used
    Thanks for the tip. I'm looking for something more powerful, but this
    is a great idea for other projects. I see microwaves put outside on
    the curb all the time, and never thought of cannibalizing them for
    their transformers! I suppose the magnetron might be useful to some
    folks as well, but perhaps it would be burnt out in a discarded
    microwave. (Although, you'd be surprised how much perfectly
    functioning stuff gets thrown out by consumers and industry alike. I
    remember when I was a grad student, the amount of good stuff I
    scavenged from the university's garbage bin was astounding! And later
    when I worked at a big company too. Until they got wise and locked
    the garbage. To make sure the perfectly good equipment actually DID
    get turned into landfill or whatever. Sad, sad, sad....) Robert
     
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