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Thread: THE PHYSICS OF... POWER

  1. #11
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    Quote Originally Posted by Ingenieur View Post
    ....there are three types of "power":
    No there are not.
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Besoeker View Post
    No there are not.
    Since he put power in quotes, I think he is correct.

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    Quote Originally Posted by Besoeker View Post
    No there are not.
    Does anyone other than me find it scary that a Senior Member on this forum denies that VARs and VAs are forms of power? That was Sophomore year in school man.

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    Quote Originally Posted by adamscb View Post
    Does anyone other than me find it scary that a Senior Member on this forum denies that VARs and VAs are forms of power? That was Sophomore year in school man.
    This could be a pretty good thread, but is apparently going the path of ground up/down.

    Power is the rate of doing work, measured in kW.

    kVAR, in the instantaneous case, energy is stored losslessly in capacitive elements in the electric field and in inductive elements in the magnetic field. As indicated above kVA is the vector sum of kW and kVAR.

    In the time averaged case case, energy stored in the inductive and capacitive elements is recovered losslessly on the next part cycle, so the net sum over one complete cycle is zero net work performed, so no power. This ignores the I^2R losses in the conductors due to the higher current flow for circuits with bad power factor. In fact that loss is work performed by conversion to heat, but is again measured in kW and not kVAr.

    VARs are the storage of EM in the inductive and capacitive element but the net sum is zero in the average over one cycle. No work is performed (in the average case over time). Instantaneously, great and interesting work is done, but is recovered back into the system in the average case.

    Crossing my fingers this could be a good thread.
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    Quote Originally Posted by __dan View Post
    This could be a pretty good thread, but is apparently going the path of ground up/down.

    Power is the rate of doing work, measured in kW.

    kVAR, in the instantaneous case, energy is stored losslessly in capacitive elements in the electric field and in inductive elements in the magnetic field. As indicated above kVA is the vector sum of kW and kVAR.

    In the time averaged case case, energy stored in the inductive and capacitive elements is recovered losslessly on the next part cycle, so the net sum over one complete cycle is zero net work performed, so no power. This ignores the I^2R losses in the conductors due to the higher current flow for circuits with bad power factor. In fact that loss is work performed by conversion to heat, but is again measured in kW and not kVAr.

    VARs are the storage of EM in the inductive and capacitive element but the net sum is zero in the average over one cycle. No work is performed (in the average case over time). Instantaneously, great and interesting work is done, but is recovered back into the system in the average case.

    Crossing my fingers this could be a good thread.
    Ok, so technically because VARs perform no work they are not technically considered 'power'. But why does it matter? We all know the equations and how to use them, so why does it matter if VARs are technically speaking considered units of 'power' or not? This is a useless argument, this whole thread is just an argument about semantics.

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    Quote Originally Posted by adamscb View Post
    Ok, so technically because VARs perform no work they are not technically considered 'power'. But why does it matter? We all know the equations and how to use them, so why does it matter if VARs are technically speaking considered units of 'power' or not? This is a useless argument, this whole thread is just an argument about semantics.
    It matters to the size of the engine driving the generator and its fuel consumption.

    Another example is a UPS, typically kW rating = 80% of the kVA rating. A UPS rated 100 kVA is only rated for 80 kW. It is an 80 kW unit at 100% loading but has an allowance for bad power factor (the DC bus would be rated 80 kW but the output section could deliver that into some bad PF). It is not rated for 100 kW, that is a different machine, and would be overloaded at that point.
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    Quote Originally Posted by __dan View Post
    This could be a pretty good thread, but is apparently going the path of ground up/down.

    Power is the rate of doing work, measured in kW.

    kVAR, in the instantaneous case, energy is stored losslessly in capacitive elements in the electric field and in inductive elements in the magnetic field. As indicated above kVA is the vector sum of kW and kVAR.

    In the time averaged case case, energy stored in the inductive and capacitive elements is recovered losslessly on the next part cycle, so the net sum over one complete cycle is zero net work performed, so no power. This ignores the I^2R losses in the conductors due to the higher current flow for circuits with bad power factor. In fact that loss is work performed by conversion to heat, but is again measured in kW and not kVAr.

    VARs are the storage of EM in the inductive and capacitive element but the net sum is zero in the average over one cycle. No work is performed (in the average case over time). Instantaneously, great and interesting work is done, but is recovered back into the system in the average case.

    Crossing my fingers this could be a good thread.

    A quibble or two and some additional comments.

    The energy transferred in the form of "reactive power" is positive over one quarter cycle. Which one depends on whether the dominant reactance is capacitive or inductive. The following quarter cycle is then negative, so the energy flow balances to zero over each half cycle.
    You can also have a resonant parallel or series circuit in which the energy moves back and forth between the capacitor and the inductor with zero losses in the limiting case of ideal components and lossless wiring.

    The discussion of power based on energy flow that nets out to zero gets a little weird and contributes to the opinion that what is being described by kVAR is not really power in the classic sense. The biggest problem is that kVAR assigns a "power" value for repetitive cycles to an energy flow that nets to zero over each half (and therefore each full) cycle.

    The fact that treating kVAR as a power component with the property that when you add it vectorially at right angles to the resistive power lets you calculate the RMS amperes actually measured over a full cycle makes it a useful parameter. In my opinion a useful fiction, just as centrifugal force makes the math easier for some people to understand and work with than calculating the required centripetal acceleration.

  8. #18
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    Quote Originally Posted by __dan View Post
    It matters to the size of the engine driving the generator and its fuel consumption.

    Another example is a UPS, typically kW rating = 80% of the kVA rating. A UPS rated 100 kVA is only rated for 80 kW. It is an 80 kW unit at 100% loading but has an allowance for bad power factor (the DC bus would be rated 80 kW but the output section could deliver that into some bad PF). It is not rated for 100 kW, that is a different machine, and would be overloaded at that point.
    You don't have to take me back to school. Almost everyone on this forum (I hope) knows the difference between kW and kVA; that's not what this argument is about. This argument is about if you can truly call VARS 'power' because they don't do real work. VAR's are often called 'reactive' or 'imaginary' power, but people are arguing if you can call it power at all.

    The difference between kW and kVA is important, as you noted in your post, and I completely agree. But the (stupid) argument going on here is purely about semantics.

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    Quote Originally Posted by adamscb View Post
    You don't have to take me back to school. Almost everyone on this forum (I hope) knows the difference between kW and kVA; that's not what this argument is about. This argument is about if you can truly call VARS 'power' because they don't do real work. VAR's are often called 'reactive' or 'imaginary' power, but people are arguing if you can call it power at all.

    The difference between kW and kVA is important, as you noted in your post, and I completely agree. But the (stupid) argument going on here is purely about semantics.
    Sorry about the tone, it was not intended nor did I see it. It is interesting to me, the physical effect of what is happening.

    As a test question if I asked for an explanation of the difference between kVA and kW, the ratio cos( phi ) is not what interests me. I would be blown away by someone who could give a description of the underlying physical effect, an understanding of the physical reality on which the math may then be applied.

    As you can see, if you have an "Emerson 1000" UPS, it says in 4" tall letters it's rated 1000 kVA continuous. Very few will look at the fine print on the nameplate or the specs and see it clearly says 1000 kVA, 800 kW. From that pool a cut is made of those who know the truth, exactly mathematically, how to calculate (or read) the rated load carrying capacity. It is a 1000 model for competitive marketing purposes.
    Lasciate ogne speranza, voi ch'intrate

  10. #20
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    Quote Originally Posted by adamscb View Post
    Does anyone other than me find it scary that a Senior Member on this forum denies that VARs and VAs are forms of power? That was Sophomore year in school man.
    VArs give you a measure for the reactive component hence the "r". VA gives you current rating so you can size current carrying capacity of components. Neither is a measure of power. I'm sorry if you find that scary.
    Si hoc legere scis nimium eruditionis habes.

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