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Full wave controlled rectifier RL Load 90° Firing angle

  1. Jan 12, 2015 #1
    For a full wave thyristor controlled rectifier, if the firing occurs at 90°, the average output voltage is zero. Therefore the average output current will be zero also I assume? If considering idealised current waveforms, should all the currents also be zero (thryristors etc.) be zero or should they have some amplitude because surely if they were zero, the thyristors wouldn't latch?
     
  2. jcsd
  3. Jan 12, 2015 #2

    jim hardy

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    Ahh

    Single phase ?
    with resistive load it's straightforward, instantaneous current is E/R and average of a symmetric waveform will be zero even though there is current actually flowing.

    with inductive load it's more interesting. Slow your thinking down to a frame by frame movie.
    We'll start at positive peak when first thyristor fires:

    When thyristor fires, current begins to increase per di/dt = E/L, so a current is established during first quarter cycle.
    When voltage sinewave crosses zero , current does not cease as with resistive load.
    That's because you have established a current through an inductor.
    Inductance now pushes current by Lenz's law in same direction, through the same thyristor and back into source. The thyristor remains forward biased due to the inductor's counter EMF..
    Current decreases by same phenomenon, di/dt = E/L.

    If current reaches zero(or more precisely thyristor's holding current) the thyristor can commutate.

    At negative peak when next thyristor fires, current will commence(or continue) to move negative per di/dt=E/L.
    And so on.

    That's only a qualitative answer, i know,
    The point of confusion is usually with the inductance returning power to source during part of the cycle, when current appears to flow backward through the thyristor.
    But think about it with no thyristors at all, just an inductor and an AC source: - isn't that why the inductor draws no power?

    Surely you're working with a textbook that derives the equations? I'd be hard pressed to find mine.

    hope this helps

    old jim
     
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