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new small relay breaks 300A at 400V

Discussion in 'Electronic Design' started by Winfield Hill, Jan 27, 2006.

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  1. John Larkin

    John Larkin Guest

  2. 400V *DC* Yes, that's quite impressive, even though its operation life
    (in the 100's of operations only) is rather limited.

    Note the strange operating current specification-- 40-45W for 100msec
    then it drops down to 4W (for the high current version only). I wonder
    what's *really* in there- not just a coil, I'll wager.


    Best regards,
    Spehro Pefhany
     
  3. John Larkin

    John Larkin Guest

    Probably one of those boosted solenoid things, with a power coil and a
    holding coil and a limit switch, like the overdrive solenoid on an
    Austin-Healey.

    John
     
  4. Jim Thompson

    Jim Thompson Guest

    An arc-sweeping "magnet" is mentioned. Maybe it's an electromagnet?

    ...Jim Thompson
     
  5. Carl Smith

    Carl Smith Guest

    I wonder what its life would be if it was seldom switched under
    load.

    You might ask, what good is a 300A relay if you never switch a
    load with it...

    In a previous life I designed motor control electronics at a
    forklift company. The forklift had a 600 amp main power
    contactor that switched power to the motor controllers. But
    under normal operation the main power contactor was never opened
    or closed with the motors powered. In the software, the motor
    controllers were all shut off first, then the contactor was
    opened. So the contactor only opened in an error situation
    where the software determined there was something wrong, such as
    a failed motor controller.
     
  6. It could even be an electronic switch of some kind-- they say it
    doesn't need any external snubbing (snubbering?) so maybe they had to
    add something internally.

    They also warn against using a simple diode on the lower-current
    version, for the usual reasons.


    Best regards,
    Spehro Pefhany
     
  7. John Larkin

    John Larkin Guest

    Arc quenching happens in the turnoff direction, so that can't be it.

    John
     
  8. John Larkin

    John Larkin Guest


    I did something like that. It's a power amp for MRI imaging. There's a
    BIG 3-phase transformer with multiple secondary windings and
    rectifiers to give, basicly, floating bipolar power supplies of 100,
    50, and 25 volts. A bunch of smallish relays switch power supply
    sections in and out, so we wind up with a dual 3-bit DAC, 25 volts per
    LSB, 100 amps max. The relays are selected to set the power amp
    voltage rails, depending on the expected load. To keep the relays from
    exploding, the firmware makes sure they're switched at zero current.

    There is an aux supply to precharge the filter caps so the relays
    don't see the surge. It's the world's dumbest switcher: transformer,
    rectifier, mosfet, and a big power resistor, with the uP turning the
    fet on/off as needed.

    This all works great, and when, rarely, something goes wrong, we get
    smoke and stuff.

    John
     
  9. Carl Smith wrote...
    Resulting in... what exactly, an explosion?
     
  10. FChoquette

    FChoquette Guest

  11. It's to run a compressor that blows the spark/arc out.

    Graham Holloway
     
  12. John Larkin

    John Larkin Guest

    Aren't there actually explosive things to do that? And I think I've
    seen tanks of freon or SF6 or something that are used to blow arcs
    out.

    John
     
  13. Carl Smith

    Carl Smith Guest


    I should have said "So the contactor only opened under load in
    an error situation"

    The result was just a bit of a spark. The contactor could
    switch the full load of the motor controllers, it just makes a
    spark and causes wear on the contactor tips. Older electric
    forklifts, such as those that use the GE EV1 controller, switch
    the contactors under load in normal operation. But it is a
    regular maintenance item on these forklifts to replace contactor
    tips.
     
  14. Phil Hobbs

    Phil Hobbs Guest

    Nah, in the forward direction it's rated >10**3 cycles, so it's a mere
    30 cents per cycle.

    Contact erosion in DC relays is asymmetrical.

    Cheers,

    Phil Hobbs
     
  15. Compressed air most likely, if you are thinking about utility type
    transmission breakers. The SF6 (and other exotic gasses) are sealed
    systems, so while there might be some arc-blowing going on within the
    switch, replenishing the gas isn't a big problem requiring large
    tankage.
     
  16. From the '70s until he sold it a few years ago, my dad ran a business
    rebuilding those compressors (among other electric utility related stuff).
    They're pretty nifty devices - designed to run in very remote locations
    under severe environmental conditions, so they do things like periodically
    change their own oil (every 'X' hours of operation). The air is compressed
    to around 2000psi, if I remember, and of course has to be very dry.

    But I think that SF6 is becoming more common even for the run-of-the-mill
    utility breakers, except in locations too remote for routine restocking. A
    couple of tanks of SF6 is cheaper than trying to keep a fancy compressor
    running.
     
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