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Homework Help: Three Phase system question?

  1. Jan 9, 2013 #1
    Hi There

    I have attempted the following question can anyone confirm what i have done and help with the rest of the question. Thanks


    Derive the numerical relationship between the line and phase currents for a balanced three-phase delta-connected load.

    Three coils are connected in delta to a three-phase, three-wire, 400V, 50Hz supply and take a line current of 5A 0.8 power factor lagging. Calculate the resistance and inductance of the coils.

    If the coils are star-connected to the same supply, calculate the line current and the total power.

    Calculate the line currents if one coil becomes open-circuited when the coils are connected in star

    This is what i have;

    EL = 400V
    IL = 5 A
    P.F cos0 = 0.8 lagging

    Iph = IL/√3 = 5/√3 = 2.887 A

    Impedance of each phase, Zph = Eph/Iph = 400/2.887 = 138.6 Ohms

    Resistance of each phase, Rph = Zph cos0 = 138.6 x 0.8 = 110.8 Ohms

    Reactance of each phase, Xph = Zph cos0 = 138.6 √1 - (o.8)^2 = 83.2 Ohms

    Therefore inductance = Xph/2πf = .265 H

    Coils connected in star;

    Eph = EL/√3 = 231

    Iph = Eph/Zph = 231/138.6 = 1.667 A ( Iph = IL )

    Using P = √3 x EL x IL cos o.8
    P = √3 x 400 x 1.667 x 0.8
    P = 923.9 W

    one coil becomes open-circuited; ?

  2. jcsd
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