Talk:AC power

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Contents 1 Real, reactive and apparent power 2 Notation 3 Math markup 4 Proposed mergers 5 Watts, volt-amps, and power meters 6 Reactive Power

[edit] Real, reactive and apparent power

Real power, reactive power and apparent power should be combined into one article. This is a mess - repetitive and I'm not sure the three articles are even consistent with each other. --Wtshymanski 04:30, 3 Apr 2005 (UTC)

agreed, assuming appropriate redirects. fyi, you seem to imply that the apparent power is the product of the Vrms and Irms only for a sine wave. This is not true. The rms calculation is not waveform dependent, although the phase separation calculation is based on a sine wave. I also find your use of the term "linear" in this context to be somewhat quirky. Specifically, the juxtaposition with containing only... as you might recall, resistance, capacitance, and inductance is a basis set for a waveguide or conductor with linear response. Moreover, it is the equivalent transfer function that is linear, not the network itself. I admit this is a slight nitpicking of usage but linear is a dangerously overloaded term. (unsigned comment by Pearlg)

we probablly wan't to bring in complex power as well especially since apparent power is just the magnitude of complex power. Plugwash 00:39, 22 Apr 2005 (UTC)

ok well i've done it what do you think of the result?

I am very happy with all you've done with this article so far. I have one objection to "Apparent power is handy for rough sizing of generators or wiring, especially when the power factor is close to 1." As it happens apparent power is very important when the power factor is far from 1. Indeed, when the power factor is close to 1, an engineer need only consider the real power for sizing, but as the power factor declines, these calculations must be made on the basis of the apparent power. The turbines suppling energy to a generator operating at 100% efficiency MUST be conveying energy at a power equal to the real power + losses due to the reactive power. From the stand-point of a person or turbine turning the generator the apparent power affects how much power must be generated to deliver the real power. --Pearlg 03:10, 24 Jun 2005 (UTC), --Pearlg 23:07, 24 Jun 2005 (UTC) (note: this comment was edited by its authour AFTER it was replied to)

sorry but that doesn't make sense. reactive power is just a quirk of the way electrical cuircuits work it doesn't represent ANY net transmission of power. Indeed in a balaced 3-phase system it doesn't even represent any instantanious trasfer of power. so if the power flowing into the generator really is equal to the apparent power then the generator would make insane amounts of heat with low power factor loads. do you have any sources for the claim you just made?! Plugwash 03:40, 24 Jun 2005 (UTC)

No, it is more than a quirk of the way electrical circuits work. Though *ideally* it does not represent any net transmission of power, in practice it does. Unfortunately, if you look at the magnitude of Z of a material through which the power is conveyed or used, you will find that there is resistance (ac) at the oscillation frequency. Though experiment: take a ring of copper. Induce a current in it. The current wave will die out overtime. Repeat with a superconductor: you can store energy for a very long period of time. So it goes with reactive power. Pushing the reactive power back and forth along the conductor induces losses. Thus, as the power factor degrades, you must size up all your components... the amounts of your active materials and your dielectrics on the basis of S. The joule loss of an m-phase balanced system is given by dP = m*r*(S/V)^2 where V is the line-neutral voltage, S is the apparent power, and r is the magnitude of Z.

"Every elementary nonactive component of S has two basic attributes:

• It causes power loss in the system
• It is the amplitude of an oscillation of power which does not contribute to a continuous unidirectional transfer of energy" (Emanuel, A., "On the Definition of Power Factor and Apparent Power in Unbalanced Polyphase Circuits with Sinusoidal Voltage and Currents" IEEE Transactions on Power Delivery, Vol 8., No. 3, July 1993

"The hottest spot temperature in transforms, alternators and cables is a function of [the Apparent Power]" (ibid)

So, the load seen by the generator depends on not only the energy transferred but on the energy losses from both the energy transfer and the oscillation. --Pearlg 23:07, 24 Jun 2005 (UTC)

---

The concepts of reactive and apparent power may be extended to multi-phase systems by summing up the apparent powers of the individual phases. It should however be noted that in an unbalanced multi-phase system the overall apparent power may be higher than where P and Q are the total real and reactive powers. -- I'd like a cite for this.

Apparent power is used to describe the power load as seen by a generating source. It is the vector sum of the real power, which represents energy transferred from the source to the load, and reactive power, which represents energy that circulates between the source and the inductive and capacitive energy storage elements of the load. It is typically of most interest in power transmission and distribution. | S | ² = P² + Q²

move later to better spot complex power is a complex quantity which captures information about both the magnitude and type of power consumed by a load. It can be defined as the product of the phasor representation of rms voltage and the complex conjugate of the phasor representation of current. Real power is the real part of complex power, reactive power is the imaginary part of complex power and Apparent power is the magnitude of complex power. S = P + jQ

[edit] Notation

"(In this section overline will be used to indicate phasor or complex quantities and letters with no annotation will be considered the magnitude of those quantities.)"

Is this a standard notation? - Omegatron 23:16, August 14, 2005 (UTC)

well its what my university lecturers use i dunno if its a standard beyond that though. Plugwash 23:19, 14 August 2005 (UTC)

It wasn't standard when I was studying or practising electrical engineering (although that may just indicate how old I am!). A one-side-head horizontal arrow over a symbol was sometimes used to indicate a vector quantity, but most texts used boldface symbols for that. I always used a simple overline in the statistics sense - a mean or average (rms of course). Magnitude was denoted with matrix notation e.g. |z|. Plain symbols were used only for DC. Of course we used e for electromotive force and i for current, which drove the mathematicians nuts because they had totally different meanings for them. To top it off, because i was already in use, we used j for what mathematicians call i. I'm certain we can never find a notation that satisfies everyone, so we have to clearly specify the one that we use, then keep it consistent across related articles. Boldface and || have the advantage that they can be expressed directly within WP - no CSS or PNG required. JohnSankey 17:54, 5 October 2005 (UTC)

That's standard enough. As long as we clarify in an article whether:

• V = phasor, |V| = magnitude ([1] [2] [3])
• V = magnitude, = phasor

Actually, it seems the only uses of overline are WP mirrors... :-\ Including complex number, which uses overline to mean complex conjugate. - Omegatron 01:01, August 15, 2005 (UTC)

I suspect that the use of overline for phasors is borrowed from some kind of vector notation (after all you could consider a phasor to be a two dimensional vector rather than a complex number its just easier to do the required calculations with it treated as a complex number). As for overline not being used much on the web i suspect that is a case of simply noone knowing that its availible (its only availible through CSS not through the old style html tags). Plugwash 00:11, 25 August 2005 (UTC)

[edit] Math markup

I try and avoid png rendered math inside text because it looks so ugly inline. What do others think? Plugwash 23:22, 14 August 2005 (UTC)

Hehehehee. Are you sure you want to ask that? Wikipedia_talk:WikiProject_Mathematics/Archive4(TeX) I really don't think we should be using CSS for overlines, though, as it's not necessarily supported by all browsers.

[edit] Proposed mergers

Right now, the merges suggested look like this (i think):

Electric reactance -> AC power -> Alternating current

I'm okay with this, except may I suggest that we first merge Electric reactance -> Reactance, so that the mergers would look like this:

Electric reactance -> Reactance -> AC power -> Alternating current

Reactance appears to this layman as the better written article. The only reason for merging Reactance -> Electric reactance, is to distinguish this from the sense in which Reactance is used in chemistry. Just a thought. Vonkje 11:51, 3 October 2005 (UTC)

I consider the concepts of electrical current/voltage, resistance/reactance and power to be sufficiently different that they should not be merged, otherwise the combined article will become too long and very difficult to organize clearly. I note that there is inadequate discussion in these articles of non-sinusoidal waveforms, particularly of the substantial 3rd and 5th harmonics generated by the non-linearity of standard power transformers - that would make a combined article even longer. Then there are the square and 2-step waveforms of DC-AC power converters. I recommend keeping these three separate but consistent, with appropriate cross links. As a retired electrical engineer, I could try to start this process, but would do so only when agreement is reached on separation or merger. JohnSankey 15:31, 5 October 2005 (UTC)

I do not think it is a good idea to merge electric reactance with AC power. The concept of reactance is important in other areas of electrical engineering, such as RF circuit design. Merging would change its apparent position from a fundamental concept in EE to an apparent subtopic of power transmission. Future articles that refer to reactance would become extremely awkward. Mbset 07:23, 15 October 2005 (UTC)

Can we remove some of the merge templates then?

[edit] Watts, volt-amps, and power meters

This is a very nice article, but a bit more than I need to know. I'm trying to understand, something more basic, the difference between Watts and Volt-amps in the real world. I have a meter that measures both watts and VA when you plug in normal household appliances. If I plug in an incandescant bulb the watts and VA are virtually identical, if it is a compact flourescant bulb the VA are almost twice the watts. I understand why this is, but my question is more basic. Which is being measured by my electric meter? I'm under the impression that it counts VA hours but calls them watts. If this is so, why do all electric appliances come rated in watts and not VA? -- Samuel Wantman 06:35, 18 January 2006 (UTC)

I think at least here in the uk domestic meters do actually measure watts (meters for large commercial and industrial installs measure watts and VArs seperately). but i'm not entirely positive. Plugwash 14:16, 18 January 2006 (UTC)

VA hours would not be equivalent to watts. It's equivalent to joules. The difference between VA and W is real vs apparent power, as explained in the article. — Omegatron 17:20, 18 January 2006 (UTC)

[edit] Reactive Power

Reactive Power carries much more significance than what is highlighted in this article. I think that Reactive Power should not be combined with AC Power. A few major points about Reactive Power not discussed here, are:

1. Reactive Power - Power System Voltage regulation (Voltage Control)
2. Significance of Reactive Power in Power Industry today
3. Efforts on part of Regulatory/Governing bodies to ensure availability of reactive power for System Stability
4. Compensation of Electric Generators for providing Reactive Power in emergency situations

Other discussions will come up as soon as we open this topic. Therefore, I request you to create a separate topic for Reactive Power.

It certainly deserves mention in a general article on AC power but there is no reason there cannot be a seperate article on it if you belive you can produce enough content for one. Plugwash 21:25, 17 November 2006 (UTC)