Confusing with this text... Pls help

Discussion in 'General Electronics Chat' started by powersys, Jan 10, 2006.

  1. powersys

    Thread Starter New Member

    Jan 10, 2006
    5
    0
    We know that:

    Lagging power factor means that current lags voltage, implying an inductive load.
    Leading power factor means that current leads voltage, implying a capacitive load.

    An inductive load is represented by R + jX
    An capacitive load is represented by R - jX


    The attached text was copied from a textbook. Do you think there are mistakes in the text and figures?

    For example:

    ... line-5 of paragraph P1
    For a load of capacitor VAr is produced and considered negative or lagging as in Figure 3.10.


    ... line-2 of paragraph P2
    A capacitive load on the other hand will be represented by R-jX as shown in Figure 3.12 and hence produces lagging VArs.

    Pls check if there are other mistakes.

    Please advise. I'm confused now....

    Thanks

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  2. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    Hi,

    You are correct when you state that the text is confusing. It is confusing because they are considering two conditions with respect to VArs.

    1) The capacitor is charging and the VAr is the power stored by the capacitor as an electric field (absorbed Var or leading VAr).

    2) The capacitor is discharging and the VAr is being exported by the capacitor (produced VAr or lagging VAr).

    At the instant the capacitor starts charging, the current peaks while the voltage rises from zero. Current is flowing into the capacitor. The capacitor is absorbing VAr.

    At the instant the capacitor starts discharging, the voltage is peak and the current rises from zero. Current is flowing out of the capacitor. The capacitor is producing VAr.

    So the text is actually considering four concepts
    1) leading power factor
    2) lagging power factor
    3) leading VAr
    4) lagging VAr

    It would have been simpler to understand if the author had not scrambled the two concepts:
    1) leading and lagging VAr, and
    2) leading and lagging power factor.

    Section 3.2.8 summarizes it. Don't confuse the two separate concepts.

    John
     
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