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big iron toasted?

Discussion in 'Electronic Repair' started by Walter Harley, Sep 24, 2003.

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  1. I need some real-world perspective, please.

    There's a big power transformer in a power amp I'm working on. The amp is
    supposed to produce 1000W RMS (though we all know how amp specs are...).
    The fuse at the input is 10A for 120V mains, and the label says 1100VA. So
    the transformer is somewhere around 1kVA.

    Now, when I hook this transformer up to 120V, with no load connected, it
    hums loudly and it draws 1.1A from the line.

    Interestingly, the current (remember, no load connected) is only about 0.5A
    at 105V, and there is almost no hum. The hum and the current go up very
    steeply as I increase the voltage to 120V, using a variac.

    The secondary voltages measure what I'd expect: 138VAC rms across the outer
    taps, 31VAC rms across the inner taps. DC resistance, subtracting the
    resistance of shorted probes, is 0R37 across the outer taps, 0R58 across the
    inner taps, and 0R38 across the primaries (in 120V mode, which has the two
    primary windings in parallel).

    Does this seem right? Has this transformer gotten toasted somehow, or is
    this normal? I've never seen that much no-load current draw or hum, but I
    don't usually work with amps this powerful, either.

    Thanks!
     
  2. It sounds like it's saturating near full line voltage. Are you running
    a 60 Hz on 50 Hz by chance?

    However, may be normal if the transformer was designed to always be operated
    with a load. If the amp is Class A or AB, there would always be a guaranteed
    load on the transformer.

    Microwave oven transformers are also like this.

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  3. Guest



    It could be that you have a few shorted/poorly insulated turns in one
    of the secondary windings, I would recommend taking it to a
    transformer manufacturer and having it insulation & flash tested to be
    certain, or at the very least perform an insulation test yourself with
    a megger, both between windings and to the core. The initial current
    drawn by these types of transformer is usually fairly high, as is
    generated mechanical noise so I wouldn't worry too much about that,
    however, the rise in core temperature should not become appreciably
    high when it is off-load. If this is happening I would look for a
    replacement. It has generally been my experience that the last
    component to fail in high power amps would be the transformer, but as
    with all things, that would very much depend on what happened to the
    amp to begin with. For instance, if the amp was given 230v
    accidentally, whilst being set for 120v that may cook it, as could a
    shorted bridge rectifier or dead short smoothing block etc.


    regards
    Alastair
     
  4. Thanks, Sam. No, it's definitely on 60Hz.

    Class AB, so there would always be at least a bit of load. But it also does
    this when the load is connected - that is, when I hook it up to the rest of
    the supply circuitry and hook that up to the rest of the amp. I'm not sure
    whether it quiets down when it's actually pushing a load.

    Back-story: The amp is a Carvin FET1000. This is a class-AB MOSFET stereo
    amp, with each channel consisting of a paralleled trio of MOSFETs. (The
    MOSFETs are paralleled without the benefit of current-sharing resistors,
    which is part of what caused the failure that landed it on my workbench.
    But that's another story.) It's biased at 120mA bias current on each side,
    plus a bit extra for the control circuitry, so when idle I'd expect it to be
    drawing about 0.3A of current plus whatever gets wasted in heating up the
    transformer.

    Sounds like your underlying thought is that it's nothing to worry about?
     
  5. Thanks. It did indeed see a dead short for a little while; that,
    delightfully, is the failure mode of this particular amp design when the
    output transistors blow.

    If it's dissipating 130W (=1.1A * 120V) with no load, I would think it's got
    to get pretty hot; but I haven't left it hooked up for more than 20 seconds
    with full voltage, so I'm not sure.

    But, I'm confused as to how the tests you suggested would find a shorted
    turn? That does seem like exactly the thing I want to test for.

    -walter
     
  6. Hal Murray

    Hal Murray Guest

    If it's dissipating 130W (=1.1A * 120V) with no load, I would think it's got
    What's the power factor? My guess is that it's far from 1.

    If there is a shorted turn, it will smoke pretty quickly. (That's
    the same as shorting the output of a 1 turn secondary.)


    I'd probably put a dummy load on it for a while and see how hot
    it gets. Light bulbs might be a convenient technology.

    Does it smell like it was smoked?
     
  7. Good point. I don't know what the phase relationship between current and
    voltage is, nor do I know what the waveforms look like. The meter I've been
    measuring current with is not a true-RMS meter (though I do have one). That
    would certainly explain the steep rise in (apparent) current as I get close
    to 120V.

    It smells and looks okay. But it's a bit hard to tell; other parts of the
    amp were smoked, plus the amp has been used to do PA in clubs, so there are
    other scents interfering. There's definitely nothing obvious.

    Putting it on a dummy load for a while is a good idea. Couple of 100W light
    bulbs in parallel, maybe.
     
  8. I don't know about this particular transformer but I think that you are
    wasting your time?
    It is normal for a transformer to draw current even though there isn't any
    load on the secondary side. It is just a big coil on the primary side. Take
    a step up/down transformer or even an isolation transformer , they draw
    quite a bit of current even without a load, it is just a big coil.

    Look for your problem elsewhere.
     
  9. Not necessarily. I'd expect them not to skimp on the transformer. Unlike
    a microwave oven where cost is everything, it wouldn't be likely they'd
    let the transformer go into saturation just to save a couple bucks on
    copper.

    However, if the transformer doesn't get hot with the amp idling, then
    it's probably OK. If it cooks just sitting there with no audio input,
    then it could be as someone else suggested - an insulation breakdown
    or intermittent shorted winding or a few. The buzz isn't the issue,
    it's the behavior of current with respect to input voltage.

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  10. This depends on the transformer design. A "big coil" as you say is an
    inductor. If the inductance is high, the current will be low. Various
    losses and parasitics can contribute to primary current.

    In the case of this transformer, I believe the behavior is non-linear which
    suggests either core saturation or some sort of breakdown at higher input
    voltage.
    As has been suggested, run it no load or with a dummy load for awhile
    to see if it gets hot. If it doesn't get more than somewhat warm,
    it's probably fine.

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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  11. I'll fire it up with a light load for a while tonight at full voltage, and
    see if it cooks.
     
  12. I didn't have time to rig up a dummy load, so I just ran it for 30 minutes
    or so with no load. It hums and vibrates, but it doesn't get hot at all.
    Evidently the power factor is very low.

    So, I'm going to presume that it's just saturating at the very peaks, and
    not worry about it. Probably under load it'll quiet down.

    Thanks, everyone, for your advice!
     
  13. Sounds fine. Still may need to tighten it up or add glue or something
    to quiet it down.

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  14. default

    default Guest

    The current rising sharply does suggest saturation. Is there a tap on
    the xformer for say 105 volts? that it may be connected to? That
    would explain it neatly.

    One amp does seem like a lot but may not be - it is an inductor under
    no load and part of that current is reactive and out of phase with the
    voltage so doesn't use power . . . does it get hot just being plugged
    in for several hours?

    I've a couple of 1kw Crown amps, they eat about 3 amps at 120 at idle.
    They operate class AB and are class A up to about 100 watts.
     
  15. Asimov

    Asimov Guest

    "Sam Goldwasser" bravely wrote to "All" (24 Sep 03 17:39:10)
    --- on the heady topic of "Re: big iron toasted?"

    SG> From: Sam Goldwasser <>
    SG> This depends on the transformer design. A "big coil" as you say is an
    SG> inductor. If the inductance is high, the current will be low.
    SG> Various losses and parasitics can contribute to primary current.

    SG> In the case of this transformer, I believe the behavior is non-linear
    SG> which suggests either core saturation or some sort of breakdown at
    SG> higher input voltage.
    SG> As has been suggested, run it no load or with a dummy load for awhile
    SG> to see if it gets hot. If it doesn't get more than somewhat warm,
    SG> it's probably fine.

    A big transformer can draw a lot of magnetizing current. Anyways, why
    doesn't anyone suggest he do a "ringing test" to determine if there are
    shorted turns? An insulation leakage test may be useful too.

    Asimov
    ******

    .... I know flyback, ultor, and 47 other dangerous words.
     
  16. The power dissipation is probably nowhere near that. The 1.1 A is
    probably all reactive current, with nearly 90 degrees phase shift with
    respect to voltage. The only power dissipation is the reactive current
    multiplied by the resistance of the primary winding, which is probably
    only a few ohms.

    Dave
     
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