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current leading or lagging the voltage

Discussion in 'Mathematics and Physics' started by PG1995, Jan 16, 2012.

  1. PG1995

    PG1995 Active Member

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    Thank you, Ratch, MrAl.

    I'm studying phasors which deals with the angle at t=0; i.e. if v(t)=cos(wt+20), then phasor is V=1<20. So, I think I need to need to know how to determine which wave is leading or lagging at zero crossing. Please now a look on the attachment and help me with it. Thank you.
     
    Last edited: Jan 18, 2012
  2. Ratchit

    Ratchit Well-Known Member

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    PG,

    As the diagram shows, phasors are assumed by convention to be rotating in a CCW direction. Phasors are drawn relative to each other, usually at the 0° line so they can be easily measured. Therefore, if a particular voltage or current phasor is more CCW than another phasor, that phasor is designated to be "leading". And remember, voltages and currents stay within their 180° limit. Its direction of rotation determines whether it is lagging or leading. It cannot be more concise than that.

    Ratch
     
  3. PG1995

    PG1995 Active Member

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    Thank you, Ratch.

    Okay. One last question. If the phasor V is at 30 degree angle and phasor I is 200 degree from the +ve x-axis. Then, which one is leading? Notice that 200-30 is less than 180 degree, i.e. 170 deg.

    Now if the phasor V is at 20 deg and I is at 330 deg, which one leading?

    I'm so sorry for taking this long to understand this. I hope you don't mind. Thanks.

    Best wishes
    PG
     
  4. dave

    Dave New Member

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  5. Ratchit

    Ratchit Well-Known Member

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    PG,

    Back up the V phasor to the 0° line and notice the I phasor is at the 170° line. Since the I phasor reaches 170° before the V phasor does, the I phasor is leading. You will never find that situation in a series or parallel circuit because the V and I are never more that 90° from each other.


    Remember the phase difference is never more than 90° difference between V & I, or 180° between like voltages or like currents in a series or parallel circuit. So I is at -30° and V is at 20°. Therefore, since V has reached 20° before I does, V is the leading phasor in this case.

    Ratch
     
  6. PG1995

    PG1995 Active Member

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    Thank you very much.

    But what if I back up the V phasor to the 0 deg line and then I is at 310 deg. Now the I phasor has reached 310 deg mark before the V phasor, therefore, isn't I the leading one? Please let me know and I have almost understood it.

    Best wishes
    PG
     
  7. Ratchit

    Ratchit Well-Known Member

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    PG,

    No, the I phasor is at -50°. We keep the angles at less than 90° for I and V. You cannot build a circuit that exceeds that limit, remember?

    Ratch
     

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