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Homework Help: Calculating Voltage Source in AC Circuit

  1. Apr 24, 2015 #1

    I'm currently working on a problem involving the calculation of an unknown voltage source of a circuit containing a resistor, inductor and load in series.

    Unfortunately, the problem sheet does not give an answer at the end, so I am unsure as to whether my approach is correct -- hence this post. I suppose I'm really looking for guidance on how to tackle this kind of problem.

    1. The problem statement, all variables and given/known data

    Calculate the source voltage and draw the voltage and current phasor diagram for the single phase circuit.

    Screenshot (202).png

    2. Relevant equations

    [tex]Real Power, P = \widetilde{|V|}\widetilde{|I|}cosφ[/tex]
    [tex]Complex Power, S = \widetilde{V}\widetilde{I}^*[/tex]
    [tex]Apparent Power, |S| = \widetilde{|V|}\widetilde{|I|}[/tex]
    [tex]Power Factor, cosφ = \left(\frac{P}{|S|}\right)[/tex]

    3. The attempt at a solution

    I was undertaking the following approach:

    From the load rating and power factor given, the apparent power and phase angle can be deduced; thus the power can be calculated from the power factor relationship.

    The current may then be determined from the real power equation.

    Now, voltage across each component can be calculated and the phasors summed to determine the source voltage.

    Any input would be greatly appreciated. Thanks.
    Last edited: Apr 24, 2015
  2. jcsd
  3. Apr 24, 2015 #2


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    Gold Member

    For a series circuit like this, all components have the same current. Start by drawing a current phasor as the reference for everything else. The current is found from 10MVA / 10kV = 1 kA. Then draw the voltage across each component, in-phase, leading or lagging as appropriate. The phasor sum of all the voltages will be the supply voltage.
  4. Apr 24, 2015 #3

    Thank you for your reply; it helped confirm my understand of the problem solution.
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