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What is reactive power?

  1. Nov 22, 2014 #1
    Complex power has two components the "active" and the "reactive". I am comfused. What is the physical interpretation of the complex component?

    Also what is the reactance?
  2. jcsd
  3. Nov 22, 2014 #2
    Reactive component of complex power represents power (energy) which oscillates between source and load to-and-fro. This component is not dissipated like active power.
  4. Nov 22, 2014 #3
    Can you please elaborate, what does oscillating power mean?
  5. Nov 22, 2014 #4
    Power = energy/time. For reactive power this means in first semicycle of sine oscillation the energy is delivered from a source to a load, and in next semicycle returned back from load to the source.
  6. Nov 22, 2014 #5


    Staff: Mentor

    This used to be a running joke in college. The electrical engineers would say to the mechanical engineers, "I'll explain reactive (imaginary) power if you will explain entropy."

    There is a very simple way to visualize it. Think of sinusoidal voltage V and current I. Power is V*I.

    If V and I are in phase, then V and I are positive for the first half of a cycle, and both are negstive for the last half of a cycle. V*I is positive for both halves, so power is positive over the whole cycle.

    If V and I are out of phase by 90 degrees, then V*I is positive for half the cycle and negative for the other half. Add it up over a whole cycle and you get zero. Thst is reactive power. There is a whole lot of stuff going on, but added up,over whole cycles, the net power is zero.

    Choose out of phase angles other than 0 or 90 and you can get different combinations of real and reactive power?

    It might help if you draw pictures of V, I, and V*I.
  7. Nov 23, 2014 #6


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    So called reactive power is due to the energy that's stored in the reactive components. Personally, I don't like the term as I think it is used to describe something 'the opposite' to the power that is dissipated. It isn't really Power at all. All you need is to follow the Maths and avoid the name.
  8. Nov 23, 2014 #7


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  9. Nov 23, 2014 #8
  10. Nov 23, 2014 #9
    I had the same question regarding reactive power as the OP.

    This is what I have understood so far, so please correct me if I am wrong or confirm (and also answer the remaining questions) what I have posted below is all correct:

    There are three types of power in an AC current:
    Apparent Power
    Active Power
    Reactive Power

    Apparent Power is the sum of Active Power and Reactive Power. What units is Apparent Power measured in?
    Active Power is measured in Watts (Joules/sec). Is this is the power that gets used up by the destination load like a lamp in an AC current?
    Reactive Power is measured in VARS (What are the units for this?). Is this the power that goes to the destination load and then returns back to source load because of the Alternating Current?

    Also what about the power that is lost by the resistors, or other heat losses, that are in between the AC circuit of a source and destination load? What is that power called?
  11. Nov 24, 2014 #10


    Staff: Mentor

    Apparent power is measured in Volt Amperes.

    Losses are simply called "losses". Not much mystery there. There can be real and reactive parts of losses.
  12. Nov 26, 2014 #11
    But aren't heat losses theoretically 'Active/True Power' because they are getting used up in some sense, by the environment? The environment acting like a load such as a lamp. Because my understanding of reactive power is that it keeps bouncing back and forth between source and load and never gets used up. Anything that is used up is Active Power, whether it is given of as heat in the environment or used up by the load. Now granted anything that get lost into the environment would have to be from the reactive power part. So the reactive power transforms in true power, when considering heat losses into the environment. Would that be the correct understanding?

    Or is there a different way of using terminology for heat losses into the environment?
  13. Nov 26, 2014 #12


    Staff: Mentor

    See my post #5 in this thread about the power reversing direction for half of each cycle, I think that's what you mean by bouncing back and forth.

    But that's not the whole story, a load such as a motor must be supplied with some reactive power as well as real power. It "consumes" both kinds. Reactive power is also consumed by the inductive impedance of the power lines causing the voltage to drop at the far end of the line. In some cases we must provide capacitors near the ends of those lines to replace those lost VARs and keep the voltage up.

    Industrial customers are charged penalties by utilities if their loads draw too much plus or minus reactive power. That is called low power factor. Regarding your question though, those loads "consume" reactive power.

    Devices called synchronous condensers are like generators that make zero real power but they make a controllable amount of plus or minus VARs. They are used to help control voltage. We also use shunt reactors (inductance between line and ground) or shunt capacitors to add plus or minus VARs to help control voltage.

    So, the Primary purpose of the grid is to supply real lower, but reactive power is a necessary secondary need. Management of reactive power is the primary means of controlling voltage magnitude on the grid.
  14. Nov 27, 2014 #13
    I think I am confusing reactive power with the fact that in an AC current, electrons are flowing back and forth. So I am thinking with the electrons, the reactive power is also going back and forth with the electrons and never gets used up. Only the Real Power gets used up. Maybe this thinking is incorrect?

    Also can their be a AC current that has no reactive power?
  15. Nov 27, 2014 #14


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    Only real power is used up by the load as 'work' but the generator must also supply the reactive power component of complex power needed for non-completely resistive loads like a motor that cause phase shifts in the fields to operate. That extra reactive component still circulates on the lines and contributes to resistive losses in the lines above those of a completely resistive load.

    A resistive load like a electrical space heater core would have almost no reactive power.
    Last edited: Nov 27, 2014
  16. Nov 27, 2014 #15


    Staff: Mentor

    With AC power, the electrons go back and forth with both real and reactive power.

    They key thing you are missing is the phase angle between voltage and current; that is what determines the real and reactive components of power. It sounds like you are trying to visualize current only. That won't work.

    Here is a YouTube video, that might help.
  17. Nov 27, 2014 #16
    I think this is what was confusing me. I thought the the real power always got used up by the load, such as a lamp where it given off as heat and light, but what came back to the source was the reactive power. So this must be incorrect interpretation? Both the real and reactive power go back and forth in AC current, but then what part of the power gets used up by something like a lamp?

    Also can there be an AC current without any reactive power?

    Last, the phase graphs of voltage and current that is being shown in the videos, at what two points exactly is the voltage being measured at? And also the current?
  18. Nov 29, 2014 #17
    If you think of a incandescent light bulb then it is purely resistive without any reactive component, as the filament is nothing more than just a wire with different chemical properties than the typical ones and with a higher resistance so current heats it up until it glows aka light.

    Basically it works like this , when voltage and currrent climb and decrease together the moment when the voltage is at its peak the current is the strongest as it should be because it has the volts behind it to push, in reactive power the current reaches its maximum after voltage has already reached its maximum and is declining so the current is unable to do usable work.
    Kinda like towing a car using a elastic string. This is how I understand it.
  19. Nov 29, 2014 #18
    So if we have an AC circuit with a resistor only, such as a bulb as you said, in that circuit there will be no reactive power?

    Also I keep hearing about reactive power that causes heat losses? Doesn't active power also cause any heat loss?
    And what about a DC current, wouldn't that also have heat losses?
    Or all these heat losses just due to the resistance of the wire, where we can treat the wire itself as a resistive load consuming/giving off energy?

    Wouldn't that technically mean that the reactive power converts into active power, when it is lost as heat?
  20. Nov 29, 2014 #19


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    Yes, the losses are real but it's not real power delivered to the load in any case. It's in the form of reactive power circuit losses above those that would have been if the load was completely resistive.
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