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THE PHYSICS OF... POWER

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    #16
    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.
    Lasciate ogne speranza, voi ch'intrate

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      #17
      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.

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        #18
        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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          #19
          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

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            #20
            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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              #21
              Originally posted by Besoeker View Post
              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.
              No energy is dissipated in the 100% reactive components. Thus the only power dissipated is in the resistive part of the load. A resonant tank circuit (i.e. a pure capacitor and inductor in parallel) simply bounces back and forth the same energy packet stored as an electrical field in the capacitor with the inductive energy stored magnetically in the inductor. At resonance since Xc = Xl this energy packet has a periodic time of 1/2*Pi*sqrt(LC).

              I could give you a fluid analogy but I do not want to be beaten up and demonized once again as I was a couple of months ago when I did so. I upset those who do not need to really understand the physics and have resorted to not going further that simply learning the math model by heart. Nikola Tesla truly despised those types of 'con by rote' characters.

              Comment


                #22
                Originally posted by dionysius View Post
                No energy is dissipated in the 100% reactive components. Thus the only power dissipated is in the resistive part of the load. A resonant tank circuit (i.e. a pure capacitor and inductor in parallel) simply bounces back and forth the same energy packet stored as an electrical field in the capacitor with the inductive energy stored magnetically in the inductor. At resonance since Xc = Xl this energy packet has a periodic time of 1/2*Pi*sqrt(LC).

                I could give you a fluid analogy but I do not want to be beaten up and demonized once again as I was a couple of months ago when I did so. I upset those who do not need to really understand the physics and have resorted to not going further that simply learning the math model by heart. Nikola Tesla truly despised those types of 'con by rote' characters.
                no one (except you) is 'upset'
                no need for insults or backbiting just because your feelings were (apparently still are) hurt
                we understand the physics, you appear not to, hopefully you learned a little
                the hydraulic analogy for an AC ckt is not useful, hence not done, it makes it more complex and difficult to understand

                Tesla developed some of the math models, as you call them, most would call them the math representation of the physical system


                reactive power IS dissipated AND absorbed by reactive loads
                it performs no net work though
                Last edited by Ingenieur; 08-29-16, 09:52 AM.

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                  #23
                  Originally posted by Ingenieur View Post
                  reactive power IS dissipated AND absorbed by reactive loads
                  it performs no net work though

                  As I understand it, reactive power is the time average rate at which energy is stored in reactive components such as capacitors and inductors. And real power is the time average rate at which energy is delivered from the source to the load.

                  Apparent power is simply what you get, when you do a raw multiplication of the voltage and amperes, neglecting the fact that they may be out of synchronization. And primarily has the application of transmission component sizing, because it helps you keep track of the volts you have to insulate and the current you have to account for to prevent overheating.

                  Comment


                    #24
                    Originally posted by Carultch View Post
                    As I understand it, reactive power is the time average rate at which energy is stored in reactive components such as capacitors and inductors. And real power is the time average rate at which energy is delivered from the source to the load.

                    Apparent power is simply what you get, when you do a raw multiplication of the voltage and amperes, neglecting the fact that they may be out of synchronization. And primarily has the application of transmission component sizing, because it helps you keep track of the volts you have to insulate and the current you have to account for to prevent overheating.
                    the way I visualize it
                    active AC power: the electrons (or holes) oscillate at 60 Hz and migrate so they have a net displacement, hence work is performed
                    reactive power: the electrons (or holes) oscillate at 60 Hz but do not migrate so they have no net displacement, hence no work is performed

                    this is a pretty good description

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                      #25
                      Originally posted by Ingenieur View Post
                      the way I visualize it
                      active AC power: the electrons (or holes) oscillate at 60 Hz and migrate so they have a net displacement, hence work is performed
                      reactive power: the electrons (or holes) oscillate at 60 Hz but do not migrate so they have no net displacement, hence no work is performed

                      this is a pretty good description
                      Quite to the contrary, if they migrate to have a net displacement you have a DC component to the current. Nothing at all to do with work related versus reactive current.
                      I also have a problem with the statement in the PDF that reactive power is the maximum value of the rate of energy transfer into/out of a reactive component. It is actually a measure of the rate at which energy is moved whose value is the time average of the absolute value of the amount of energy stored. (Multiply the 1/4 cycle integral of current x voltage by four and divide by the period of the AC).
                      Last edited by GoldDigger; 08-29-16, 12:09 PM.

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                        #26
                        Originally posted by Ingenieur View Post
                        reactive power IS dissipated AND absorbed by reactive loads
                        It is not dissipated.
                        Si hoc legere scis nimium eruditionis habes.

                        Comment


                          #27
                          Originally posted by GoldDigger View Post
                          Quite to the contrary, if they migrate to have a net displacement you have a DC component to the current.
                          Quite right. With AC they oscillate back and forth - a very tiny distance.
                          Si hoc legere scis nimium eruditionis habes.

                          Comment


                            #28
                            Originally posted by Besoeker View Post
                            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.
                            When I try to explain how low pf makes equipment sizes bigger with no increase in ability to do work I show them the beer mug analogy. pf is the ratio of foam to beer. Mug sized to handle both but a pf closer to unity maximizes your drinkable beer for the same mug size.
                            Attached Files

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                              #29
                              Originally posted by jtinge View Post
                              When I try to explain how low pf makes equipment sizes bigger with no increase in ability to do work I show them the beer mug analogy. pf is the ratio of foam to beer. Mug sized to handle both but a pf closer to unity maximizes your drinkable beer for the same mug size.
                              A delicious analogy!

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                                #30
                                Originally posted by GoldDigger View Post
                                Quite to the contrary, if they migrate to have a net displacement you have a DC component to the current. Nothing at all to do with work related versus reactive current.

                                I also have a problem with the statement in the PDF that reactive power is the maximum value of the rate of energy transfer into/out of a reactive component. It is actually a measure of the rate at which energy is moved whose value is the time average of the absolute value of the amount of energy stored. (Multiply the 1/4 cycle integral of current x voltage by four and divide by the period of the AC).
                                wrong
                                the migration can be oscillatory and out of phase with the reactive component, with a displacement greater in the direction of power/current flow


                                wrong, their definition is correct, yours' is wrong
                                take it up with the authors, lol, I'm sure they care


                                http://www.failureelectrical.com/resume.html
                                Ph.D., Electrical Engineering, Massachusetts Institute of Technology, 1971
                                M.S., Electrical Engineering, Massachusetts Institute of Technology, 1968
                                B.S., Electrical Engineering, University of Massachusetts, 1966

                                lol

                                post yours

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