How exactly is the "direction" of the flow of power measured in a alternating current system?

Submitted by Landhund t3_z1o0qe in askscience

(Quick info to start with, I'm from Germany, so I may make some mistakes with the technical vocabulary and some best practices may differ)

So this is a kinda awkward question for me to ask, considering I'm an electrical engineer planning the electrical systems for all kind of construction projects, and I like to think I'm quite good at it, including the technical side.

But during a conversation with a colleague yesterday, we noticed we both can't quite explain how exactly the drection of the flow of power is measured in AC power systems (compared to DC, where it's really easy). We both know it's possible, we both have watched old electricity meters (like this one) run backwards when either wired incorrectly or when the measured system is actually feeding power into the mains, for example with solar panels. And while I understand the basic principles of those old meters (it's essentially a finely tuned linear induction motor), what I can't figure out is how the direction of the powerflow is determined. Even worse with modern electronic meters, those don't even have moving parts where I could at least justify that they work the same as the old ones. And yet those to can determine the direction and don't measure currents running in the opposite direction (for example again with solar panels when you don't actually have a contact for receive compensation for the power you provide. The meter simply doesn't count up or down when you produce more than you need and thus are feeding into the mains. Mechanical meters solved this with a ratchet system.)

AC power doesn't have an inherent direction, that's its fundamental principle. So how do those meters do it? The only thing I can come up with is that it's basically more like "measuring" the voltage differential between one side and the other, or at least that that is what determines the direction. But how? What physical effects are used to electronically measure if power is flowing from A to B and when it's going from B to A?

Thanks for any explanations you can provide, the more detailed and in depth the better.

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FourierXFM t1_ixcqhdi wrote

There's one thing I think most people are forgetting. Let's assume a power factor of 1.

Voltage is alternating (it is a sinusoid). Current is also alternating (it is also a sinusoid). The power is NOT alternating.

If power factor is 1, the current and voltage are in phase, which means they positive and negative at the same time.

So when voltage is positive, current is positive. Power = voltage * current, and both are positive, so power is positive.

When voltage is negative, current is negative. Power = voltage * current, and both are negative, so power is still positive.

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_Tegridy_ t1_ixcv1by wrote

The real power is oscillating as well, it's just that the mean value of the real power is VIcos(theta).

There is a double frequency component of oscillating real power on top of it.

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TrappedInASkinnerBox t1_ixcxt6j wrote

When the power factor is 1 the power (only real power here because of the pf) oscillates but isn't ever negative.

There's some coefficients in there but basically you square a sinusoidal waveform.

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_Tegridy_ t1_ixcz2at wrote

Yes, that's what I was trying to get at. Power as such is oscillatory as well but when we talk of V I cos(theta), that's just the average real power over one cycle.

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VulfSki t1_ixcz90r wrote

This is true. But it is still said to be alternating. Alternating current includes anything that is not DC. Even it is positive during the entire cycle it's still AC.

The confusion comes from the fact that people don't understand what alternating current means.

It doesn't mean energy flows back and forth. It means the voltage and the current are alternating. The voltage is a potential difference. The current is charge per second.

The power is traveling down the conductor in a waveform. Like a wave. So even though the charge carrying particles are moving back and forth, it is still delivering energy to the electrons on the other side of the circuit.

Like a light bulb. You send energy to the bulb in sinusoidal waves. The waves delivers energy to the device that is used to create light.

You don't deliver the particles themselves. It is the particles that move from one voltage potential to another that deliver energy to a load across that voltage potential.

An electron has a fixed charge. The energy itself comes from the votlage potential. Volts =Joules/Coulomb. Or in other words energy/charge.

The electrons can deliver the energy be moving from a high potential to a low potential. Thats why we refer to voltages as a potential. Cause it is like moving from a state of high potential energy to a low one.

So even though the charge moves back and forth it is still delivering energy in one direction (assuming a matched impedance)

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FourierXFM t1_ixd0o2n wrote

I have never heard it that way. Alternate means to change direction. The power is not changing direction.

I have never heard someone call a DC voltage with a high frequency ripple as "alternating", even though that is kind of the same as what you're talking about.

People do say AC power a lot, but AC means alternating current, which is true. Not alternating power, which is not true (again, all assuming pf = 1).

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I_dont_have_a_waifu t1_ixd7lhq wrote

See, in that situation, I'd say that there was a DC and AC component of the signal.

The AC component is the high frequency ripple, with an average value of zero. Then the total signal is the AC signal added to the DC signal.

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VulfSki t1_ixd9umy wrote

Yes. I think in their example tho they are thinking about power supplies that rectify and the smooth ac into a useful DC signal. And on practical terms many people consider the ripple negligible and call it a DC power supply.

But I think they are coming from a pretty simplified explanation and then the added nuances don't exactly work with the simplified explanation

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I_dont_have_a_waifu t1_ixda5h0 wrote

You're right, and it's not often that I would bother thinking much about the AC ripple on a power supply like that. I think the only I really gave it much thought was when I was designing rectifiers, and switching converters back in power electronics.

Otherwise I'd just throw a filter on it and call it a day.

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VulfSki t1_ixd9hpq wrote

Then you learned it wrong. (Or just a simplified explanation) I have a bachelor's in EE, the standard convention for the terms is that any signal that is not DC is considered AC.

You can for example take any AC voltage and add a DC offset which then makes it so that the entire wave is positive through an entire period.

Nothing has changed about the signal, other than you have shifted the wave enough to no longer be negative, this is most certainly NOT a DC signal.

But it is entirely positive.

Take amplifiers for example as well, the transiistors in the output stage of a class A amplifier are biased in a way that the entire signal is positive. This obviously is not a DC signal.

The issue with thinking of AC signals as requiring to be both positive and negative is that all the meaningful conventions fall apart when you consider signals that are not centered around zero .

An ac signal with a DC offset will pass through capacitors, and then lose the DC component. The same way an AC signal, with a DC offset will not see an inductor as short.

What you refer to as having a little bit of ripple tells me you're thinking strictly in terms of power.

So yes when people rectify a signal and then try to smooth it to convert from AC to DC you can accept a small ripple in the signal. yes I definitely see how someone would call that a DC signal because that is what you are looking for in that case. And the ripple is small enough to not cause an issue. But of course depends on how precise your power needs to be.

So alternating having charge flows back and forth is not entirely wrong. But it's just overly simplified and in EE we consider any signal that it's not DC to be AC. Because that is a more useful convention in terms of how the laws of physics govern electro-magnetics. Because you can have alternating current signals that are entirely positive (or negative)

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FourierXFM t1_ixdbot3 wrote

>Then you learned it wrong. (Or just a simplified explanation) I have a bachelor's in EE, the standard convention for the terms is that any signal that is not DC is considered AC.

I don't mean to get into a pissing contest, but you're being rude, so I will. I have a masters in EE with a specialty in power electronics focusing on AC/DC conversion. I promise I did not learn it wrong.

Alternating means back and forth, or positive and negative. A full bridge rectifier with no capacitive filter at the end is still called DC even though it's oscillating up and down.

At some point of ripple you would be more right to say it's DC with an AC component, but nobody in industry calls that alternating power... because it's not alternating.

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VulfSki t1_ixdcxxb wrote

Yeah that's not what I'm talking about at all.

Yes I said it makes sense to consider that DC power. I never said otherwise there.

You seem to have misread my comment. What I said was that as a matter of convention, anything that isnt DC we called AC.

The phrase flowing back and forth can mean a number of things. My point was that it doesn't need to be negative to be considered AC.

When I work on power electronics, and still do, we often refer to the ripple at the output of the supply as an AC ripple as part of the DC. But yes of course we would never consider that an AC power supply. Of course we would still call that DC power. Sure.

Just to be clear. I said anything that isn't DC we refer to as AC. And you picked one very specific example to say "no everyone I talked to calls a DC power supply DC even if it has a ripple" which yeah or course they do. But that's not really related the point I was making

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feint_of_heart t1_ixemxax wrote

> The power is traveling down the conductor in a waveform. Like a wave. So even though the charge carrying particles are moving back and forth, it is still delivering energy to the electrons on the other side of the circuit.

Damn it, I was following along until you said that. Can you explain how power moves in a wave in only one direction when the current and voltage is alternating?

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VulfSki t1_ixepoh1 wrote

It has to do with the way that waves propagate through any medium.

The electromagnetic waves that travel down a conductor represent changes in the electromagnetic field across the charged particles. This does move the charge carriers. But it is the EM waves that are essentially transferring the energy.

For example, the rare at which the electricity travels down a copper wire is just about the speed of light. But the electrons themselves don't move that fast down the wire. They are accelerated back and forth and do drift down the conductor but the particles arent moving down the conductor at the speed of light. They move at what is called the drift velocity. Which is lower than it would take for you to walk. But that's because the energy is transferred via electromagnetic waves. It's not like a faucet or water where electrons flow like water.

And how you define how it flows in one direction depends on the scale. It does go back and forth but you can't violate the first law of thermodynamics. Power is energy over time. (Watts = Joules per second).

You can't have a passive load (the thing there needs electricity) sending energy back towards the generator (the thing that is making electricity) without violating the first law of thermodynamics. The one but caveat there is of course that the energy can be reflected back when it hits the load. That happens when the impedances don't match, which affects the power factor (which for the mathematically inclined is cosine of the phase angle between voltage and current.)

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feint_of_heart t1_ixerjwd wrote

Ah, it just clicked. Thanks for taking the time to explain it :)

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ImMrSneezyAchoo t1_ixe9a3w wrote

Power does not alternate only for 3 phase power (the math works out to have a literal constant power). In a single phase application p(t)=v(t)I(t) definitely can be an alternating signal, of varying complexity as the current changes its phase wrt voltage

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[deleted] t1_ixccj2g wrote

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bellsandwhistles t1_ixcn85t wrote

Fundamentally, the power is flowing in the fields rather than the voltage and current. The direction of power flow is determined by the Poynting vector.

Edit: misspelled Poynting as pointing

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Redingold t1_ixcq4k3 wrote

So named because it points in the direction of the flow of power (not really, it's the Poynting vector, not the Pointing vector, named after physicist John Henry Poynting, but it's an amusing case of a sort of nominative determinism).

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RobusEtCeleritas t1_ixh7w52 wrote

What you're looking for is the Poynting vector.

It's pictured here for a DC circuit. The red arrows are the electric field, the green are the magnetic field, and the blue are the Poynting vector (the direction of power flow).

In the AC case, the red and green arrows change direction sinusoidally, but the blue vectors always point in the same direction; from the power source to the load.

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YurtlesTurdles t1_ixfyspq wrote

Very interesting question that I have a followup question to that maybe someone can explain. First reading your question I thought the meter you meant was an electrical testing meter like a Fluke or a Klein. With those meters amperage is measured with a CT(current transformer) clamp, and it does not tell you the direction of flow. I install solar and batteries and during the testing phase it would be extremely valuable to have the direction of power flow. Sometimes it obvious, sometimes I'll have an imbalanced load and have phase A backfeeding and phase B drawing power, or maybe the battery will be involved but i cant tell if its charging or discharging. I know that permanently installed CT meters give pos and neg values because it has the reference voltage, turning the CT around or to the wrong phase will give opposite values.

How do I get that same data with direction of flow from a field testing meter like a Fluke? Any other meters someone can recommend? Gratitude to any who can help.

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