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inrush surge

Discussion in 'Electronic Repair' started by William Sommerwerck, Nov 21, 2013.

  1. Back around 1980 I had to repair a ReVox deck. It kept blowing its fuse --
    even though the fuse was the correct value and speed.

    I watched the fuse as I turned on the unit. So much current was drawn that the
    fuse heated up and bowed, then came back to normal as it cooled. After a few
    cycles, the fuse wire grew sufficiently fatigued that it broke.

    I don't remember how I "solved" this. I might have gone to a slo-blo, or
    increased the value slightly. Anyhow, it didn't come back.

    Thoughts? (Including "You're an idiot!" Well, there were no stories about the
    owner's house burning down.)


    "We already know the answers -- we just haven't asked the right questions."
    -- Edwin Land
     
  2. John-Del

    John-Del Guest

    It's unlikely on a ReVox deck, but some more advanced equipment I've workedon over the years has had multiple relays or SCRs in a timed power up sequence. One RPTV I worked on years ago would occasionally blow fuses on start (but never when running), and it turned out to be a stuck relay (prevented one SMPS from being power up until the first one did).
     
  3. N_Cook

    N_Cook Guest

    Add an anti-inrush NTC thermistor made for the purpose, assuming no
    actual fault situation .
     
  4. "N_Cook" wrote in message
    That, of course, was the issue. There wasn't anything obviously wrong with the
    unit.
     
  5. Phil Allison

    Phil Allison Guest

    "William Sommerwerck"
    ** That is just what a FAST fuse does when subjected to current surges.
    ** Fuses in the AC supply always need to be "slo blo" or "T" types.

    There are darn few exceptions to this rule.

    BTW

    Fuses with a flat metal strip inside are inherently slow and made of tin
    not copper too.


    ..... Phil
     
  6. **Premium fuses from known suppliers usually open at the specified
    current and within specified times. Cheap, Chinese fuses are often
    unknown quantities. Often-times a package of fuses will be all
    incorrect. They don't care. They don't put their brand name on the fuse.
     
  7. Guest

    The version of this I've seen is on the DC input (274 V nominal, 100 A)
    to a big variable-frequency drive. There are two relays; the "little"
    SPST one that operates first has about a 10 ohm ceramic resistor in
    series with the contacts. The "big" one that operates second has no
    resistor, and is DPST, for both sides of the DC bus. You can hear the
    "tick, tick" of both relays operating in a second or less when you power
    on the machine, and the "tick" of the big one releasing when you power
    it off.

    There is also a 125 A fuse on the DC input. It is rated at 100 sec
    max at 250 A (2x rated), 15 sec max at 375 A (3x), and 1 sec max at
    625 A (5x).

    Matt Roberds
     

  8. Could it be the fuse was rated for 240 volt operation (at 1/2 the value) and
    being used on 120 volts?

    I often see this - 240 volt operation ha a fuse 1/2 the value of that for
    120 volt operation.


    Mark Z.
     
  9. Phil Allison

    Phil Allison Guest

    "Mark Zacharias"

    ** I see the reverse sometimes, mainly with 1960s Fender guitar amps.

    The AC tranny has been replaced long ago to suit use in Australia - but the
    fuse holder labelling is still for the USA.


    .... Phil
     
  10. Guest

    The more I think about it (and look at the wiring diagram), the more I
    think the "big" relay is really two SPST relays operated at the same
    time.
    In this case, the operator position is not convenient to where the
    relays need to live, so the relays get switched by 12 VDC control
    signals. There are some other safety functions between the operator and
    the relays as well; the computer can decide that bad things are
    happening and drop out the control signals to the relays.
    I think this system is set up to fail safe if one set of contacts gets
    welded. The normal startup sequence is something like

    1. Close main relay in - line.
    VFD should not yet have DC.
    2. Close pre-charge relay in + line.
    VFD has current-limited DC. Input caps charge.
    3. Close main relay in + line.
    VFD has current-unlimited DC.
    4. Open pre-charge relay in + line.

    and the normal shutdown sequence is something like

    1. Open main relay in - line.
    The input caps should start discharging, and the voltage at the VFD
    should start dropping.
    2. Open main relay in + line.

    If the pre-charge relay contacts or main relay + line contacts are
    welded, then the VFD will see DC as soon as it does startup step 1. It
    can then open the main relay - line and be safe.

    If the main relay - line contacts are welded, it probably can't detect
    it at startup, but it can detect it at shutdown when the voltage fails
    to drop off; it can open the main relay + line and be safe.
    Roughly 34.5 kW at nominal voltage. The motor that this thing drives
    is rated at 33 kW, or just a touch under 25 hp.

    Physically, the fuse is only about 1.5" (38 mm) long and maybe 0.75"
    (19 mm) diameter, with big lugs sticking out of the ends to bolt it
    down. In use, it lives behind a polypropylene cover that you have to
    remove an interlock and a screw to get at.
    Well, technically, all fuses just "melt". :)

    Matt Roberds
     
  11. Guest

    Part of the reason the system I am describing has more than one set of
    contacts is that switching the "line" power is also its job; it's not
    like your power supply where the user is expected to provide the line
    power switching.
    The VFD, here, has the ability (and authority) to command the motor to
    zero speed, so it can arrange for there to be no load before it starts
    trying to open the relays. Both + and - lines float with respect to
    "ground", so it doesn't really matter which one it opens.
    Good point. There is a current transformer on the "line" side of the
    DC input, before any relay contacts. I don't know if it uses that in
    its startup/shutdown strategy, but there isn't any reason why it
    couldn't.
    I can guarantee that you've seen one of these, just not at this level of
    detail. They sold a lot of them in sunny California.
    Dunno. It's got a label or wrapper around the whole thing, and I've
    never peeled it back. I've never blown one, and I've never unbolted one
    from its holder to shake it. It has a 2,000 A "breaking capacity", if
    that implies anything about its construction.
    Charles Darwin never met *you*, apparently. :D
    Somehow, that's not usually specified for screwdrivers, unless they
    are deliberately sold with a very high resistance.

    Matt Roberds
     
  12. all the lambda stuff I've come across is bizarre in design and
    construction.

    I don't recall the model, but one was just a plain switching supply of a
    few hundred watts with a gigantic wirewound power resistor in it. Nobody
    else does that, nor do most places change their name every 5 weeks either-
    lambda, nemic, veeco, tdk, wtf?
     
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