Ac directional power flow. How do we measure the direction of ac power?

Question

The direction of electron flow changes direction with the change in polarity from an ac power system. The power flow remains in a constant direction. How do we measure this. How do we know what side the power is being put into, and what side it is coming out of. An electric motor and a generator are magnetically coupled and very similar in design, what clue exists in the power flow that indicates which direction power is flowing?

I have been told that it has to do with voltage leading or lagging current, which are vars. You can however have a positive or negative power factor, you can voltage leading or lagging current, but the power still flows in the same direction.

I have been told that it has something to do with the phase angle of the power. Like the power is out of phase a certain amount depending on the direction, but I'm not sure that's it either. That's just like vars again I thought. I have searched everywhere, can someone please explain this to me?

Doug has a good point, w/o the resistor in series with the wire there would be no potential difference in the wire to measure, forgeting the fact that wire itself has resistance.

Lets say we have 2 wires a load, a source, and a resistor in between.

-----x----resistor----y----
AC source load
-------------------------------

The first half of the sinusoidal wave form starts out, we have a higher voltage on the x side of the resistor, perfect we know the load is on the y side. Now we go into the next half of the ac waveform and now the voltage is higher on the y side. Did our load and source switch sides?

Also AnswerMan, I beleive the component that does work is watts, the vars are unusable power. Both reactive and capacitive power is unuseable vars. Completely reactive or completely capacitive power is where the volt and amp waveforms are peaking 180 degrees out of phase, and that's a 0 power factor

In my little drawing the source and the load were supposed to be seperated, this typing tool does not see multiple spaces entered in a row. I could have drawn it like:

Source ------x--resistor--y------load
-------------------------------------------

but it's basically supposed to show a loop with all these things in series, even though the x and y are just reference locations.

Wait a second. I am using the wrong terms. VAR stands for volt amps reactive. All vars are reactive, they are either capacitive reactance or inductive reactance. I get the two confused because we call capacitive reactors "Capacitors", but we call inductive reactors "Reactors" lol. Capacitors and inductors are all reactors, and capacitive and inductive load are both called reactive. Also, the reason why I mentioned electron flow was to point out the fact that the electron flow switches direction while the power flow stays in the same direction. Electron flow has nothing to do with the direction of power flow. It is common electrical convention to understand that electrons travel from the positive pole to the negative pole. Electrons are negatively charged particles, so the pole with the excess electrons is called negative pole, they then travel towards their source, the positive pole, which is lacking electrons.

Kaksi, we measure power like a vector, that's why it's called a quasi-scalar or something like that. I don't remember the exact verbage, but even though there is no such thing as negative power it still has a directional flow. It has to because we measure it and rely on those measurements to protect our power system. We say mw in or mw out on out transmission lines, and we use a + or - to denote wether it's feeding in our out. That's what is a mystery to me. I thought like you do, about power, but in everyday practice it is measured in one direction or the other.

The mw or megawatts generated by a power producer go out on the line to either a capacitive or inductive load. When the capacitive or inductive load receives the power it stores some of it, that power is released during the next half of the waveform and causes opposition to that source. So it is load that does no work. Some people call it power that returns to the source. This is only in ac systems.

Answer 1

Sure. Assume that you're standing in the middle of nowhere with two wires stretching to infinity in both directions. You have any type of measurement equipment you want (but *only* measurement equipment). The fact is, there is no way in which you can determine which direction power if flowing. (And this is true for DC as well as AC)

However, if you have a low-value resistor, you can now measure with certainty which side goes to a generator and which side goes to a load.

I'll leave it to you to figure out how (HINT: Remember Ohms Law ☺)


Doug

Answer 2

I disagree with the answer above. You can determine the direction of current flow, for example by examining the polarity of the magnetic fields surrounding the wires. You can determine the potential difference between the two wires. If you determine which wire is at higher potential, then the direction of current flow in that wire reveals which way the consumer of power lies. The source of power is opposite to the direction of current flow. Voltage and current are in-phase for a power consumer and out-of-phase for a power generator. (Bear in mind that in the universally-respected electrical convention, the positive direction of current flow is opposite the physical direction of electron flow. I wouldn't even mention it except that your question starts out with the direction of electron flow.)

As long as the load is resistive, it makes no difference whether the power is AC or DC, because both the potential and the current reverse in step every cycle.

For a p.f. less than unity, you need to be careful to look at the component of current that is not in quadrature with the voltage. For the consumer of power, that component is in-phase; for the generator it is 180° out of phase - exactly the same as the DC case. That is the component that does work; the reactive power ("var") has voltage and current in quadrature.

Now if the load is 100% reactive, you have a problem. You don't know whether the load is inductive (lagging) or capacitive (leading) - you can only measure that the voltage and current are 90° out of phase. However, in this case the real power is also zero, so in that sense your question has no answer - there is no source and no consumer of power. If you can characterize the load reactance then you will be able to use the phase shift to identify in which direction the generator and the load lie.

Answer 3

Stop, stop, stop panicking Jimmy. First let’s clear this mess! Power is not a vector it is a scalar so it cannot have a direction. Power is like bread, you can take it and transport to a place you like, the difference being the means of delivering them. Power does not depend on direction of moving electrons, it depends on their absolute speed, kinetic energy of electron being m*v^2/2, where m is mass and v is speed of electron; thus there is no substantial difference to receive power either in a/c or d/c form; as for leading-lagging stuff I’d suggest you study electromagnetism!, the understanding will come later.

Answer 4

hmm, you're all incorrect :-). permit me attempt to describe; electrons are not getting used, certainly they simply circulate the skill. it fairly is allot like water in a water skill plant. The water interior the lake, while it drops transfers its skill to the turbines. this won't bring about loss of water, basically displacement. In electric powered cutting-edge there's a ability distinction, making the electrons shuttle. (a ability distinction ability the electrons prefer to circulate someplace, like the skill distinction between the lake and decrease than (the water needs to circulate down.)). and since the electrons shuttle they CARY some skill which may well be extracted. so each and all of the skill corporation has to do is look after the skill distinction on your socket. via how, somebody electron in a typical skill line travels very slowly (i think of it replaced into approximately some centimeter according to minute). you could image it as a team of marbles in a tube. in case you slowly push one in at a million component, they'll all shuttle slowly, yet one will go out on the different end precise away. this replaced into all ideas for DC, in AC issues are slightly extra complicated because of the fact between the lines now cary a alternating ability, ensuing in an alternating ability distinction. concepts stay an identical however. wish that helped. stable luck