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I don't like this transformer!

Discussion in 'Electronic Design' started by Chris Carlen, Nov 15, 2003.

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  1. Chris Carlen

    Chris Carlen Guest


    About my 500VA Hammond 115VAC 50/60Hz line isolation transformer again.

    Today I left it powered for a few hours, running only a lamp with a 20W
    compact fluorescent lamp, then unplugged the lamp and let it sit
    unloaded for another hour or so.

    This thing has become too hot to touch!

    This is not right. I realize that there are reasons why core losses can
    be larger at no load than with rated load, but this is just too hot. I
    have never seen a transformer get more than mildly warm when idling for
    indefinite time periods.

    BTW, the line voltage here in Livermore, CA is about 122-123V most of
    the time. Remarkably, it was exactly the same in Bethlehem Pennsylvania
    where I used to live.

    Now I am powering the isolation transformer from 115V from my Variac, to
    see how much cooler it might run at its rated voltage. But I would
    expect any respectable transformer to run only mildly warm at up to
    about 130V, which is the upper end of what is usually considered the
    line voltage range over which equipment should function (I usually
    consider it to be 105-130V).

    What would this thing do at 50Hz? Hard to say. The core loss might be
    less, so it would be cooler. This most certainly isn't I2R loss, as the
    current over the 0.4ohm winding R is about 0.5A, which is an
    undetectable 100mW.

    I think I might try another brand of isolation transformer. Allied
    (where I bought this thing) has a lovely dual primary and dual secondary
    500VA tranny, that can be wired to do 1:1, 2:1, or 1:2. But its open
    frame, and I really didn't want to spend time making a box for it. They
    also have a Stancor 500VA with cord and covers. I have had good
    experience with Stancor, so maybe I'll buy that and return this one. I
    have two 200VA Stancor trannys in my home made dual power supply, and
    they only get barely warm when idling.

    I'll post an update when I see how the Hammond unit is at 115V.

    Good day!
  2. I read in that Chris Carlen <[email protected]_FIELD.ea
    No it wouldn't. The induction is 20% higher at 50 Hz for the same input
    voltage and that more than compensates for the lower frequency.
  3. Marc H.Popek

    Marc H.Popek Guest


    Sounds like you continue to have fun and challenging work within the labs!


  4. I've got a monster 3kVA isolation transformer that does the same thing.
    It's never shown any other sign of trouble under any sort of load, but it
    always terrified me with how hot it got. Took a while to heat (it weighs, I
    dunno, maybe 75lbs) but after it was on for a while I'd be worried about it
    eventually carbonizing the wood platform I had it mounted on. I haven't
    used it for a while.
  5. John Fields

    John Fields Guest

  6. Jeff

    Jeff Guest

    I was looking at some large (45 -150 kVA) 3 phase transformers - they
    operate the same way unless you get the newer energy effiicient ones. The
    insulation is rated to at least 175 deg C, often more.
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