zero crossing switching in three phase systems

Discussion in 'General Electronics Chat' started by m121212, Sep 13, 2015.

  1. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    In a single phase AC system, it would make sense to disconnect the load at a zero current crossing (and to activate it at a zero voltage crossing).

    What about in a three phase system? If one phase is at zero, the other two are not. Is one phase just selected to spare the switching element for that phase?)

    Are all phases switched simultaneously?

    If zero crossing switching is indeed useful for three phase fixed frequency systems, is it also useful for three phase variable frequency systems?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    See page 8
    https://www.fairchildsemi.com/application-notes/AN/AN-3004.pdf
     
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  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    Please note that zero crossing circuits only trigger the triac at zero volts. The triac itself turns off when the current drops below a threshold.
     
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  4. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Thanks for the comments.

    I guess if it's a heater or a motor, it doesn't matter if the phases are disconnected at different times.

    In figure 16 on that app note, why do the switches go on when the voltage is non-zero?
     
  5. Johann

    Senior Member

    Nov 27, 2006
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    Well, the circuit is under power (functional), therefore whenever any phase is not zero, its appropriate switch will turn on as the triac will be triggered after the zero transition. Remember, the 3 phases follow up at 120 degree intervals in time.
     
  6. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    I thought the whole point was to trigger such that the triac gates on _exactly_ at the zero transition.
     
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