Graetz bridge problem

Discussion in 'Homework Help' started by kamel2913, Dec 28, 2015.

  1. kamel2913

    Thread Starter New Member

    Mar 6, 2015
    15
    0
    Hello,

    can anyone tell me where am I wrong please:
    we have:

    [​IMG]

    the firing angle of Thyristors=120°, E=100v, V(t)=220sin(angle), f=50hz, , R=10ohm, dissipation angle = 90°


    I want to know Uc(angle), for me I have found:


    120°+a -->180°-a° Uc=V(t)
    180°-a°-->270°-a° Uc=0
    270°-a-->300°+a Uc=E
    300°+a-->360°-a Uc = -V(t)
    360°-a-->450°-a Uc=0
    450°-a-->480°+a Uc=E

    wtih a = sin^-1(E/Vm)
     
    Last edited: Dec 28, 2015
  2. sailorjoe

    Member

    Jun 4, 2013
    361
    63
    It's hard to read, but I believe Uc is the voltage across the E, L, and R series components.
    You need to realize that the Uc angle is modified by the inductor and resistor in the circuit, so those variables should be in your final equation.
    I don't follow your math, I'm afraid. Your "-->" operator is unclear as to what it means, but more importantly, you haven't started with a fully formed equation based on the circuit.
    Also, it helps to think about exactly what is happening. A sine wave of 220vrms comes in, but doesn't get switched through the thyristers until it at 120 deg. The signal through the bridge (thyristers) is significantly clipped. So coming out at Uc you have these little 60 degree pulses, two per input cycle. Each cycle starts with an inrush of current that the inductor will resist, so the reactance of the inductor is important. Also, the battery has an effect on the incoming pulses, by clipping off their tails.
     
  3. kamel2913

    Thread Starter New Member

    Mar 6, 2015
    15
    0
    Hello thank you for your reply,
    yes Uc is the voltage across the E,L, and R.
    and --> it's mean from-to (exmpl: 120°-->180°, from 120° to 180°)
    so that will be:

    from120°+a to 180°-a° Uc=V(t) (Th1 and Th4 are on)
    from180°-a° to 270°-a° Uc=0 (Dr is on)
    from270°-a to 300°+a Uc=E (all Ths and Dr are off)
    from300°+a to 360°-a Uc = -V(t) (Th2 and Th3 are on)
    from360°-a to 450°-a Uc=0 (Dr is on)
    from450°-a to 480°+a Uc=E (All Ths and Dr are off)

    wtih a = sin^-1(E/Vm)


    I do have problem with Single Phase Half/full Controlled Rectifiers with R,L,E series components, I will be very thankful if you can explain me or give a web site that explain when the Battery will effect on the angles and when not, for exemple if we remove the Dr, the battery will have no effect on the angles, I don't know why.
     
  4. sailorjoe

    Member

    Jun 4, 2013
    361
    63
    Ah, yes, thank you for your explanation of -->, I understand now your intentions.
    I recommend you start with some graph paper and draw the incoming sine wave. The draw the output of the thyristors, taking into account the fact that the battery keeps one or more of them reversed biased at some point in the cycle.
    Can you do that?
     
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