Switch, series or parallel ?

Discussion in 'General Electronics Chat' started by Man_in_UK, Oct 24, 2012.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I sometimes have to replace contactors in a control box that powers single phase AC motors. These control box's come from the USA and can be configured for 100 or 240. Long ago these contactors were open frame Allen & Bradley with two N/O contacts to run the motor and two N/C for the brake.They always came wired with both normally open contacts in series.
    I was told then to wire them up in parallel to split the current over both sets of contacts.

    I still to this day convert them to parallel and have always wondered why they came in wired in series.

    Why would they do this?
    Any ideas?
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    are you talking aux contact, overload contacts, or line contacts. a schematic would be handy.
     
  3. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    There really is not much of a schematic to show you. The contactor is just two normally open switches that operate together. I have drawn a diagram but two switches in series is not very exciting.

    The drawing shows how it is wired from new.

    [​IMG]
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    240VAC in North America is 240VAC center-tapped. The center-tap is neutral.
    Doesn't the product come wired for 240VAC in North America with a contact in series with each 120VAC? Then when it is turned off, nothing is "live".
     
  5. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    Not in this case.
    This part of the circuit is not changed when it gets converted for Europe. Both switches are always wired in series with either 110 or 240 being switched to the motor.
    The motor does obviously get its main windings setup for 240.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    It is silly to have two relay contacts in series.
    Oh, maybe if they are garbage contacts that weld closed then the other contact can still open.
     
  7. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    That is what we thought about it.
    If I don't get a reply from here with anything positive to say about such switch configuration, I might contact the people who make the equipment ......... UNLESS.

    Unless they wanted the contacts to have a limited life so they can supply spares!
     
  8. crutschow

    Expert

    Mar 14, 2008
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    I can think of two possible reasons to connect two contacts in series.

    One is that is would reduce the arc time across each contact since the arc would be shared between two contacts, increasing the contact life.

    Two is for reliability. If one contact would weld together, the other would still work.
     
  9. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    Good point. Question: if they were wired in parallel, would they still share the arc time in the same way?


    This has got to be the best answer so far.
    Or I should say ..... the only plus point anyone has come up with yet.

    EDIT - Sorry. Audioguru did bring this up earlier - he wins the lollypop!
     
    Last edited: Oct 25, 2012
  10. Audioguru

    New Member

    Dec 20, 2007
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    My young dog's name is Loli (Lolita or Lolipop)!
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Probably not. Since one contact would likely open slightly before the other, the arc would only occur on the one that opens last.
     
  12. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    Then would that not translate perfectly over to series by stating that ... it only occurs on the one that opens first?
     
  13. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Is it a simple "and" gate. Do two particular events have to happen before both contactors close and it does whatever it does?
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    Last edited: Oct 25, 2012
  15. crutschow

    Expert

    Mar 14, 2008
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    No. In series the first one to open starts to arc but when the second opens it is also carrying the current so it will likely arc also.

    In parallel the first one to open will not arc since the second is still carrying the current. And the arc voltage when the second opens is not enough to ignite an arc at the first that is already open.
     
  16. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Safety relays use contacts in series, here's the datasheet for one I installed this morning. Eaton esr5.
    Note: these contacts are in series inside the relay, you cannot change the configuration to parallel.
     
    Last edited: Oct 26, 2012
  17. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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  18. Matter45

    New Member

    Dec 13, 2012
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    can I add to this thread by asking a question:

    SCENARIO: you want to turn something on/off that would barly switch more then 1A @ 200v. However, it will never switch on/off at currents up to 1500A.

    In other words, the current going through the relay can go up to 1500A but will only switch on/off when it is less then 1A @ 200vdc.

    Can I simply use a lower rated relay switch (cheaper one) or;
    Can I parrallel cheap lower amp rating relays (say 4 x 400a) or;
    Do I need to use one big fat 1500A rating relay?

    Im not interested in high power switching, just high power carry. the switch will never close/open during loads.

    Can anyone suggest on a cheap solution? maybe its cheaper making a fat mechanical contact?

    so if you needed to open/close a circuit that could carry 1500A, what would you use?
     
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