Reactive Power -Q-

Discussion in 'Homework Help' started by mo2015mo, Jan 24, 2014.

  1. mo2015mo

    Thread Starter Member

    May 9, 2013
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    Hi guys,,
    I wonder :eek: why we transmitted reactive power if the load just consumed the real power??
    Can anyone explain that to me clearly??
     
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  2. crutschow

    Expert

    Mar 14, 2008
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    The reactive power is not intentional. It's a result of the various capacitances and inductances in the circuit. In the case of a transmitting antenna it can be a result of impedance mismatch between the RF amplifier, drive cable, and antenna impedance. If they are all matched then there is no reactive power.
     
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  3. mo2015mo

    Thread Starter Member

    May 9, 2013
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    Okay,, thanks thanks my friend for new information :)
     
  4. mo2015mo

    Thread Starter Member

    May 9, 2013
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    What the advantages of "Q"?? Are there benefits of Q?
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    As crutschow said, reactive power is unintentional. If we could figure out ways to do what we want without producing it, we would. But we usually can't, so instead we find ways to minimize it's detrimental impact. Often that involves generating opposing Q to cancel out the unavoidable Q, so you could say that this is a benefit of Q.
     
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  6. mo2015mo

    Thread Starter Member

    May 9, 2013
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    okay my friend,, I have understood what are you talking
    BUT ,, i mean that
    lets assume that ... there are system as attached photo
    A generator has S1 = -1000+j268
    A Motor has S2 = -1000-j268
    and a transmission line(T.L.) between them has impedance is Z= 0+j5 Ω and the current through T.L. is 10.35 ampere
    thus P = -1000 -(-1000) =0 W
    Q = 268 -(-268) = 536 Var = I^2 * X

    By power value ,P, and current direction through (in or out) machine , I can conclude the type of machine (Generator or Motor)
    By Reactive power ,Q, value What can i conclude by this value ?? and what means this value Q= -268 or 268 or 536 Var ??
     
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  7. WBahn

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    Mar 31, 2012
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    The sign of the reactive component tells you whether you are looking at net inductance or net capacitance.
     
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  8. mo2015mo

    Thread Starter Member

    May 9, 2013
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    okay,, thats mean Q for net inductive(absorbs) and -Q for net capacitive(supplied)
    so i can said motor and T.L. is net inductive , generator is capacitive or What??
    How generator is net capacitive ??
    please explain more my friend with examples for that :) :) :)
     
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    Last edited: Jan 27, 2014
  9. WBahn

    Moderator

    Mar 31, 2012
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    The source is generally considered ideal (though possibly with a source impedance associated with it to account for the major non-idealities) and so is neither inductive or capacitive. The phase angle between voltage and current in the source is dictated by the load. It's the load that is either inductive or capacitive.

    If you assign the current so that it is going into the positive terminal of the device, then you are treating the device as a load and positive reactive power means the load is net inductive.
     
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  10. mo2015mo

    Thread Starter Member

    May 9, 2013
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    okay ,, i.e. for previous attached photo the machine on the right side of sys is generator and -Q means capacitive load (leading P.F.).
    and the machine on the left side is motor and +Q means inductive load(lagging P.F.) .
    thanks very much my friend ,, I have 3 Q's as written below ...

    -T.L. is inductive ??(and always all the time)

    -If we need to correct P.F. ... firstly where we are used it?? In motor , by adding capacitive load to load??

    -In industry,if we're correcting P.F. ==> reducing our electricity cost bill because reduces T.L. losses ?
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    We don't "transmit" reactive power, it is a result of the load's characteristics. In fact, the power companies put giant capacitors on the pole to correct the power factor to offset the effect of the inductance in all of the appliances being fed power.
     
  12. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Your example is somewhat confusing. if S1 & S2 both have a negative real power component then the related machines (generator & motor) must both be sending real power into the network. The motor would have to be in regenerating mode. I'm not sure where you get the zero net power result from.

    I take your point that if a motor is taking real power from the network then clearly (excluding motor losses) the mechanical load driven by the motor demands that equivalent power to accomplish a particular task - such as pumping water. This power must come from one or more generators attached somewhere to the AC network.

    The attendant Q values must simply reflect the fact that no physical motor has unity power factor and if it is operating at a certain output power then some of the current supplied to the motor must be flowing in equivalent reactive elements within the motor - such as (but not limited to) the reactive power required for energizing the motor's magnetic circuit.

    Power factor correction is not limited to the placement of capacitors near the loads connection to a system. A consumer may opt to install PFC capacitors at their point of common coupling to reduce their reactive power draw from the power utility. Power utilities may charge consumers for reactive power draw.

    Large scale power system operators may themselves take steps to regulate or control reactive power distribution within an AC network. See this link .... http://en.wikipedia.org/wiki/Synchronous_condenser
     
  13. mo2015mo

    Thread Starter Member

    May 9, 2013
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    Really thanks for you
    bountyhunter t_n_k WBahn

    my teacher answered me and said "we really needed to reactive power Q to establish the magnetic filed flux between the primary and the secondary coils "
     
  14. WBahn

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    Mar 31, 2012
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    While I would say that this is not a complete answer, it may well have been quite reasonable within the context of which it was being given.
     
  15. mo2015mo

    Thread Starter Member

    May 9, 2013
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  16. WBahn

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