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Isolation transformer draws excessive current under no load condition

Discussion in 'Electronic Repair' started by JW, Feb 10, 2012.

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  1. JW

    JW Guest

    Hi all,

    Is there any reason that a 120VAC to 120VAC isolation transformer would
    draw 2.54A on its primary when there is no load present on it's secondary?

    Part reference:


    I finally got around to wiring this transformer, and I noticed something
    that doesn't seem quite right to me. I have it wired for 120VAC (H1
    connected to H3 and H2 connected to H4) and it seems to be working as I
    get about 120VAC on the secondary, ( wired X1 to X3 and X2 to X4) but with
    no load on the secondary, the transformer is drawing 2.54 Amps.

    Looking for a sanity check I guess. I'm beginning to think the thing may
    be defective... Email to the Temco has produced no response as of yet.
  2. N_Cook

    N_Cook Guest

    If there are shorted turns then there would be a difference in DC ohms of
    each coil, assuming they should be the same for 120/120
  3. mike

    mike Guest

    It's not unusual for a transformer to draw some amps when unloaded.
    That's what inductors do. The current should be out of phase with the
    What you care about is POWER. Measure the RMS power consumed by the
    transformer. That's what counts.

    If it's actually dissipating 300W, it will get very warm.
  4. 2.5 amps seems like an awful lot of current, in-phase or not.

    It sounds as if the unit is "defective" in some undefined. Or it might be
    that the transformer's inductance is too low.
  5. mike

    mike Guest

    Sure you do. It's on the outside of your house and goes
    round and round...They send you a bill every month.
    My base load is 200W, so an additional 300W would be
    easy to see.
    The newer ones simulate round and round
    with an lcd display and have a flashing IR led that can
    be used to measure actual power quite accurately.
    If you have an old Palm III vintage PDA, I can send you
    a program that lets you point the IR window at your power
    meter and graph consumption...but a stopwatch counting
    the display works as well.

    It's best to turn off everything you can to reduce the base
    load and improve the precision of your measurement with and
    without the transformer connected.

    If you wanna spend a few bucks, the P3 Kill A Watt meters
    are very handy in this application.

    As a go/no-go test, you can put an incandescent light bulb
    in series with the transformer and see how bright it gets.

    But at 300W, you shouldn't have any trouble sensing transformer
    temperature rise with your hand.
    Wrap the transformer in insulation
    and plot the temperature vs time...weigh the transformer to guess
    at heat capacity and calculate the power from that.

    So, several ways to "do that".
  6. mike

    mike Guest

    First guess, I'd have to agree. My Old Stancor 250W transformer
    draws 9W 18VA .15A RMS unloaded.
    One more thing to try.
    Make sure you're not paralleling a primary and secondary winding.

    Hook up one winding to the AC. Measure all 4 voltages very accurately.
    How you connect the windings can make a difference. Small changes
    in coupling due to the arrangement of the windings can produce
    small differences in the output. When you start paralleling them,
    you can get one winding fighting another. If that's the case,
    energizing only one of the 4 windings with the others disconnected should
    make the problem go away.
    It's not impossible that a manufacturing defect screwed up the turns
    count on one of the windings.

    In a transformer this big, if you have a shorted turn, you should be
    able to smell the result.

    Depending on how close the transformer is to the box, the metal
    box represents a shorted turn. All the flux is SUPPOSED to be inside
    the core....supposed to be. I don't expect taking it out of the box
    would make any significant difference, but I'd try it before scrapping
    the device.

    But the real diagnostic is to measure the real power consumed.
    You can chase your tail trying to solve a problem that doesn't exist.
    Try the series light bulb.
  7. mike

    mike Guest

    Federal Pacific SE120N1F FT2036 - Isolation Transformer 120 x 240
    Primary 120/240V Secondary - 1 kVA 60Hz

    2.5 A suggests
  8. David Lesher

    David Lesher Guest

    Buy a Kill-o-watt; a useful tool to have around.
  9. Ian Jackson

    Ian Jackson Guest

    In message
    The symptoms described sound very similar to using a 60Hz transformer on
    a 50Hz supply (as might happen if you are using American equipment in
    Europe), and the transformer hasn't got enough iron in it - so it's
    saturating. However, that's not what you are doing.

    If saturation IS the problem, you can usually confirm it (under no-load
    conditions) by winding the supply voltage up on a variac, and measuring
    the current the transformer draws. It will rise suddenly when the core
    starts to saturate. Although the problem is much more likely to be
    shorted turns, a quick test for saturation might be interesting.
  10. mike

    mike Guest

    Not sure how repeating myself helps, but here goes.
    I'd plug it into a Kill A Watt meter. That's how I got the other
    results I mentioned in this thread.

    I also disclosed several other techniques.

    If you would read the parts you snipped, I also gave you several other
    ways to estimate the power consumption of a transformer.

    If you don't have a readable utility meter or a stopwatch or a
    light bulb or a voltmeter or a scale or a thermometer or a hand, you
    should look elsewhere for advice. I'm all out.

    I think I figured out how to do it with a used tea bag, the chime
    from Big Ben and a plate of fish and chips.
    Nope, you're still gonna need the hand.
  11. Phil Allison

    Phil Allison Guest

    "Robert Macy"

    If it bothers you, you can add a high quality AC cap in parallel
    around 55 uF. That should 'resonate' out the reactive current assuming
    120Vac, 60Hz yields around 127 mH.

    ** You are a total ignoramus about transformers.


    Magnetising current is not inductive - it is highly non linear with peaks
    at each zero crossing.

    ..... Phil
  12. Phil Allison

    Phil Allison Guest


    ** Is that 2.54 amps RMS or not ?

    In any case, is it getting rather hot and making a humming noise ?

    If not, then all is OK.

    The magnetising current will drop significantly when a full load is


    Adding capacitance in parallel has no effect on the tranny whatsoever -
    and it will make the PF worse.

    ..... Phil
  13. Phil Allison

    Phil Allison Guest

    "Robert Macy"
    Phil makes a good point about the terms being thrown around here.

    ** The one doing the chucking about is YOU - pal.

    Magnetizing current is NOT core inductance current.

    ** But includes it.

    Back to the core' inductance...The core's inductance is usually 5 to
    10 times the impedance of full load.

    ** Wrong - it is way more than that.

    The NON LINEAR magnetising current increases with applied voltage and rise
    sharply as the max rating is approached.

    Now, the core winding's inductive reactance .....

    ** Is so high the resulting current flow barely matters.

    At 60Hz that core's inductance will be about 382 mH.

    ** It is far more likely to be many Henries.

    To get rid of such inductive reactive currents, which cause a
    'lagging' power factor, it is possible to add a parallel capacitor
    essentially in resonance taking it to near zero, thus 'correcting' the
    power factor.

    ** FFS - give up on this crap.

    You cannot correct the PF of an unloaded AC supply tranny with a parallel
    cap !!!

    It is of note that the current is ALWAYS there, even at full load, in
    parallel with your load. If the transformer is made properly, the
    waveform will be fairly linear, if the core is starting to saturate,
    the current at the peaks will increase due to that saturation

    ** In reality, the ONLY significant current flow occurs around each
    voltage zero crossing.


    Toroidal AC supply transformers are different.

    Inductance is way high and I mag is almost non existent up to the rated
    input voltage at the rated frequency.

    ..... Phil
  14. mike

    mike Guest

    I'm intrigued. How do you use a Passport to measure
    transformer losses? If you don't have a hand, you can't
    even pick it up.
  15. Franc Zabkar

    Franc Zabkar Guest

  16. JW

    JW Guest

    Yes. I've measured it with the same result using both a Watts Up meter and
    a Fluke 87.
    Making a humming noise, yes. It's not getting hot, the external
    temperature rise of the case is only about 10c.
    Nail. Head.

    My test setup and how this all started:

    120V in is being fed to a variac, then to the isolation transformer, then
    the load. On the secondary of the transformer I added a toggle switch that
    would connect the secondary in series or parallel depending whether I
    wanted 120V or 240V out. When I first wired up everything as I stated in
    my OP, being on the cautious side I placed a 1.5A fuse into the variac so
    if I mis-wired something there'd be no damage. Well, on first power up I
    slowly raised the voltage on the primary and at a little over 110V the
    fuse blew.

    The actual wattage when the primary is at 120V is only 42W as measured by
    the Watt meter. Here's why - the power factor is really crappy at just
    about .15. This would explain everything. I wonder if this is typical of a
    1KVA transformer? I never thought to check the wattage being drawn...
    Noted, and thanks.

    Also, thanks to everyone else for their input - some very good thoughts.
    It would appear that there's no problem after-all.
  17. JW

    JW Guest

    It's potted inside the box, so that's not possible. In any case it would
    seem that there's no problem (see my other post), thanks.
  18. Out of curiosity, I took out my Tenma 72-545 1.5A isolation transformer.

    On my Kill A Watt, it draws 60 mA unloaded, 3W / 7VA, with a power factor of

    I assume the current without a load varies with the primary inductance, not
    the rated capacity. So the 2.45A drawn by the OP's transformer seems to be
    unnaturally high.
  19. Phil Allison

    Phil Allison Guest

    "Robert Macy"

    ** Is there any way to shut fools like you up ??

    Adding caps will indeed have NO effect on the tranny, but WILL improve
    the PF as seen by your AC mains,

    ** Absolute BULLSHIT !!!!!!!!!

    which means the amount of power you
    drop in your wiring [and pay for] will be less.

    ** Absolute IDIOCY !!!!!!!

    Adding caps is a STANDARD way to adjust power factor to 1.

    ** But never used with an off load transformer - because that idea is 100%

    YOU are an obsessed IDIOT with a one track mind.

    Piss off.

    .... Phil
  20. Phil Allison

    Phil Allison Guest

    "Phil Allison"
    ** Good.

    ** Sounds about normal for a large e-core tranny

    ** That is pretty high actually - a typical 1KVA tranny has about 6 to 8 %
    power loss at full load.

    ** Not everything.

    You have to know that the current waveform is distorted - THIS fact is
    causing the poor PF when unloaded.

    .... Phil
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