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Phasor Diagrams in single phase series circuits

  1. Feb 4, 2013 #1
    In single phase series circuits, should the current always be drawn along the x axis and the voltage then drawn with respect to it? E.g. Should Vr be drawn in line with I, VL leading by 90 degrees (pointing upwards) and Vc lagging by 90 (Pointing down)? The reason I ask is because ive seen some diagrams for a purely capacitive circuit where the voltage is along the x axis and the current is pointing upwards? but I always thought you drew the diagrams with the common factor on the horizontal?
     
  2. jcsd
  3. Feb 4, 2013 #2
    If the sinusoidal voltage is given to you in the form
    [tex]v(t)=V_msin({\omega}t+\phi)[/tex]
    then when you represent it on the phasor diagram, it has that angle([itex]\phi[/itex]) with respect to the positive, real axis. If all you know is the current leads/lags the voltage, then set one of the phasors as your reference, and the other phasor will either lead or lag that phasor depending on the orientation that you went with. I usually have voltage as my reference, and if the current lags (purely inductive), then the current phasor will be -90 degrees from the reference voltage phasor. If it leads in a purely capacitive network, the opposite is true.
    For example, [itex]V_{RMS}e^{j0}[/itex] and [itex]I_{RMS}e^{-j90}[/itex] is the same as [itex]V_{RMS}e^{j90}[/itex] with [itex]I_{RMS}e^{j0}[/itex] because ultimately the phasor is rotating counterclockwise about the origin, at 90 degrees with respect to each other.
     
    Last edited: Feb 4, 2013
  4. Feb 5, 2013 #3
    Thanks for the reply. Is it acceptable to use either method as I've always been taught to work from the principle that current is same everywhere in series circuits and hence should go on the positive real axis.
     
  5. Feb 5, 2013 #4

    jim hardy

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    Soon enough you'll be ambidestrous, swapping back and forth to whichever convention better suits the problem at hand.

    It's just like learning language.
     
  6. Feb 6, 2013 #5

    vk6kro

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    You would plot voltages on a screen that only showed voltages, not currents.

    In a series circuit, if there is a resistor present, the voltage across the resistor will be in phase with the current, so this will be plotted along a horizontal line and other voltages will be plotted relative to this.

    The current in all components of a series circuit is the same, but the phase of the voltage will vary depending on the component.
     
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