Phase Angle between voltage and current

Discussion in 'Homework Help' started by rvb53, Jun 30, 2008.

  1. rvb53

    rvb53 Thread Starter Member

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    hi everyone,
    I need to know that by calculating phase between volatge and current.....

    what information does the phase angle can give.........?

    like voltage or current lagging or leading, inductance....etc? but I need to know more....?:confused:
  2. silvrstring

    silvrstring Active Member

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    rvb53,

    When your circuit's current phase angle is leading, your circuit is capacitive. If your current phase angle is lagging, the circuit is inductive.

    What this means is if you were to draw the Thevenin equivalent of the circuit, you would have an inductor OR a capacitor--not both.

    If it is application you are looking for, this will reveal its importance when you want to adjust a circuit to be more efficient concerning power and current. Often, an example circuit will point out the waste of power that can happen when a circuit is too inductive--VAR(inductive). You will use a capacitor to balance the reactive power your circuit is using, and try to bring reactive power as close to zero as you reasonably can, so that only real power is used--minimizing power loss/waste.

    You might want to read more about power factor if you are interested in applications.

    Hope this helps.
  3. Sparky

    Sparky Well-Known Member

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  4. CDRIVE

    CDRIVE Senior Member

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    Actually, that statement is a bit misleading. Since a purely reactive circuit consumes no power at all. Phrased more precisely, phase angle is brought as close to zero as possible to insure that all available power will be delivered to the load.
  5. rvb53

    rvb53 Thread Starter Member

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    hi everyone,
    thanx for ur suggestions...
    I need more information that phase angle infer to ,other than above all...
  6. CDRIVE

    CDRIVE Senior Member

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    Do you have a good grasp of Ohms Law? If you do we can take it from there. By the way, don't feel bad about not fully grasping this subject. I've been a professional in this industry since 1966 and I remember vividly when phase angle was first introduced to me and how confused I was!;)
  7. theamber

    theamber Active Member

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    Current is the only quantity here and voltage is just a potencial diference. Look in the time axis.
    Although both the current and the voltage oscillate sinusoidally in an AC circuit they will not necessarily rise and fall simultaneously with each other in each circuit element or the circuit as a whole. The current and voltage will oscillate with the same frequency but they will (in general) be out of phase with each other. The exception being when the circuit is in resonace (phase angle of the circuit is 0) or if there is only resistors in the circuit.
    In a pure inductor, the current lags the voltage by 90 degrees so if you look at the two wave forms (voltage and current) you will see the voltage at maximum at the same time the current is zero. As you introduce resistance into the circuit, the current lags by less.
    In a capacitive circuit the current is said to lead the voltage although the reality is slightly different because a current happening before a voltage is applied is crazy. The truth is a negative current lagging by 90 degrees but the current leading idea works because the waves continue repeating.
    Both reactances offer opositions to changes in voltage or current that is why always one lags the other.
    It is useful to think of the phase angle as the angle by which the peak voltage leads or lags the current. If the phase angle is positive the voltage leads the current (inductive) by offering oposition to current changes. The oposite happens with a pure capacitive circuit. In a pure inductive or capacitive circuit the power dissipative is 0 but that never happens in the real world with all the variants.

    I hope you get it. At least this is the way I understand it.
    Last edited: Jul 1, 2008
  8. rvb53

    rvb53 Thread Starter Member

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    hi everyone,
    thanx for ur information and suggestions....
  9. arrie

    arrie New Member

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    This subject can be really confusing. What you need to keep in mind, when talking about current leading or lagging and phase angles, you are working with alternating current. The magnitudes of voltage and current varies continually, and this variation takes the form of a sine wave.
    As soon as you hear sine wave you hear trigonometry, good luck. The different components have different effects on the phase between voltage and current, because of their characteristics, some has to charge up, other produce opposite potential, etc. except the resistor, it basically chucks your current in the form of heat.
    If you think of a sine wave, it varies, say it starts at 0, increases to a max, then decreases to a zero and through into a negative value and finally back to zero.
    So somewhere you will have no current at all, elsewhere it will be at maximum, and elsewhere a negative current, meaning the polarity has reversed, this can be e.g. because the inductor is now driving your circuit, etc. There are many other scenarios depending on what the circuit looks like, and at what point in the circuit you are looking into it.
    Go find some good books: Hughes electrical and electronic technology, Delmars textbook of electricity, there is thousands.

    good luck
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