Polyphase AC circuits question.

Discussion in 'Homework Help' started by manaxmizery, Apr 16, 2013.

  1. manaxmizery

    Thread Starter New Member

    Nov 7, 2012
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    0
    3. A star-connected resistive load is connected to a three-phase supply of 400V, determine the line current and the resistance per phase if it consumes 9 kW. The same resistive load is now arranged in delta connection and connected to the same three-phase supply of 400V, determine the phase current flowing in the resistor and also the line current
    (Ans: Star IL = 13.04A, Rper phase = 17.64Ω, Δ IPhase = 22.67A, IL = 39.27A)


    I'm stuck at doing this question since there is no power factor given.Please take a look at it and I really appreciate if you guys take the time to solve this question.And please do,explain in simple terms since I'm still a beginner in the Electrical/Electronics course.Thanks in advance.


    Regards,
    Steve
     
  2. User_axpro

    New Member

    Apr 15, 2013
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    0
    Well for a start the PF is kind of given in the question, in a purely resistive load (balanced star) we know PF = cosΦ = 1
    and we know:
    VL = 400v
    P consumed = 9kw
    ( Apparent power = TruePower as CosΦ = 1)

    So: P = √3*VL*IL*cosΦ
    IL = P/(√3*VL*cosΦ)
    IL = 9000/(√3*400*1)
    IL = 12.99A

    I think from here you know where to go ?
     
    Last edited: Apr 16, 2013
  3. manaxmizery

    Thread Starter New Member

    Nov 7, 2012
    7
    0
    I considered power factor to be 1 but I lacked the explanation in order to arrive to this conclusion.So,if they stated in the question that it's a purely resistive load,do we always assume the pf to be 1?
     
  4. User_axpro

    New Member

    Apr 15, 2013
    8
    0
    Yes, In AC P = V*I (as ignoring Cosθ) is only true for resistive loads so PF must be equal to one.
    Now you have the resistance per phase just redraw in Delta configuration and work from there, quite simple, for backround reading to reinforce you knoweldge i refer you to AllAboutCircuits Text book:
    http://www.allaboutcircuits.com/vol_2/chpt_10/5.html
     
  5. manaxmizery

    Thread Starter New Member

    Nov 7, 2012
    7
    0
    Oh,I see.I'll check that textbook out and thank you so much for helping me out with this question.Cheers mate ! :)
     
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