# Determination of direction in AC Power Flow

How does one determine the direction of AC power flow?
I understand how you can measure voltage and current with simple
instrumentation, but this is AC and the average value of those parameters is zero. How can you tell in which direction the power is going?
Let's put this in the form of a puzzle: There are three adjacent soundproof rooms A, B, and C. You are told that only one of the following conditions is true:
1. There is an AC generator in A feeding power through open buss bars in room B to a resistive load in room C.
or
2. There is an AC generator in room C feeding power through open bus bars in room B to a resistive load in room A.
You are shut inside room B and are to determine whether condition 1 or 2 described above is true. Remember, the rooms are soundproof so you can't tell from sound leakage whether room A or C has the generator. Also since the load is 100% resistive, assume that the power factor is 1.
Questions 1. Can you determine the direction of power flow just from measurements to the AC buss bars in room B ?
2. What sort of instrumentation would you need?
3. Do you need to break the circuit to make the measurements?
Any takers?
Beachcomber
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Assuming 2 bus bars: find the sign of the Poynting vector. In principle this can be done without breaking into the circuit. Use a FET or similar device to measure the electric field between the bars. Use a hook on meter or current transformer to measure the current flowing. After suitable amplification to get the amplitudes within linear limits, use one signal as a reference and the other as an input. Feed them into a synchronous detector or lock-in amplifier. With proper tracking of the signs, you will have yourself a noncontacting power meter.
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Yes.
Any meter that allows you to see both the current and voltage waveforms at the same time (Fluke 43, Dranetz-BMI 4300 or PX5, scope with appropriate voltage and current probes) or a good power meter (one that measures direction also).

No. You use clamp on CTs. Connect the voltage probes (we will assume single phase) from line to neutral. Connect the CT (the CT has an arrow on it for direction). Measure power. If it is negative, the power is flowing in the opposite direction from the arrow on the CT. If it is positive, power is flowing in the direction of the CT. Using a scope, if the voltage and current waveforms are in phase, then the power flows in the direction of the arrow. If they are 180degrees out of phase, then power flows in the opposite direction of the arrow on the CT.
Pretty easy actually.

Sure. What does it pay?

Charles Perry P.E.
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On 11/11/06 1:19 AM, in article snipped-for-privacy@news.verizon.net,

Assuming 2 bus bars: find the sign of the Poynting vector. In principle this can be done without breaking into the circuit. Use a FET or similar device to measure the electric field between the bars. Use a hook on meter or current transformer to measure the current flowing. After suitable amplification to get the amplitudes within linear limits, use one signal as a reference and the other as an input. Feed them into a synchronous detector or lock-in amplifier. With proper tracking of the signs, you will have yourself a noncontacting power meter.
One can use the output of the hook on meter's current transformer as input to the current terminals of a real (electrodynamometer, for example) wattmeter. Connect the voltage terminals to the busses.
The kicker is the "proper tracking of the signs." If you get that wrong, the power appears to go the wrong way and the meter needle goes against the zero stop pin. It is possible that the current transformer indicates, usually with dots, that indicates the relation between current through the magnetic core and the direction of current out of the transformer winding.
Bill -- Fermez le Bush
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Now the head of the EE department goes into the generator room when you are not looking and replaces the AC generator with a DC battery.
(This means no changing magnetic fields - no inductive coupling)
The DC battery could be in Room A or C, you don't know in advance.
What equipment would you bring into room B to determine the direction of power flow? This time you are also told you are not allowed to break the circuit.
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Beachcomber wrote:

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On 11 Nov 2006 13:07:19 -0800, snipped-for-privacy@gmail.com wrote:

No, but it is something simple though.
Beachcomber
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Hall effect CT. They know direction also ;-)
Charles
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On Sat, 11 Nov 2006 17:27:08 -0500, "Charles Perry"

My answer would be a DC voltmeter and a compass!
Beachcomber
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On 11/11/06 2:37 PM, in article snipped-for-privacy@news.verizon.net,

Good Show!
Bill -- Fermez le Bush
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Assuming 'bar buss bars', use the voltmeter on it's most sensitive scale to find the polarity of voltage drop through one bar as it passes through the room. If you know the properties of the bar (temperature, specific resistivity, cross-section, length) well enough, you can use this to determine current.
Even without an accurate current measurement, you can determine *direction*, and that (along with polarity of voltage between busses) is enough to determine which room is the source and which is the sink.
But need a DC voltmeter instead of an AC one.
daestrom
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On 11/11/06 12:23 PM, in article snipped-for-privacy@news.verizon.net,

Easy enough. You need a voltmeter and a magnetic dipole (compass). From that, you can determine the direction of the Poynting vector.
Bill -- Fermez le Bush
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------------ A compass needle can be used to determine the DC current direction (you may want to test it to determine the direction it swings when over a conductor carrying a known current). If you know this and the polarity of the voltage between the busses, you are home free. Otherwise- spend a bit more on a Hall device for current measurement.
--

Don Kelly snipped-for-privacy@shawcross.ca
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--------- Sorry, I gave my compass answer before checking other responses. Honestly, I didn't cheat. I have done this to determine whether my alternator was actually charging the battery in my car. Of course, as Daestrom has noted, the voltmeter has to be a DC voltmeter (or a dynamometer meter).
--

Don Kelly snipped-for-privacy@shawcross.ca
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Again, Kirchoff says current in=current out.
Gauss says voltage along the line is the same -
Electron drift in one bar is to the left, and in the other bar is to the right, so that tells nothing.

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wrote:

Well, electron drift is related to current flow. The important concept for this puzzle is current flow. Long ago in the 19th century before anyone even knew about electrons, it was decided that current flows from the positive terminal to the negative terminal.
That convention has stayed with us today, even though we know that electrons are moving from negative to positive and the actual speed of any one individual electron in this flow is very slow, the current is effectively traveling at/near the speed of light.
In the case of this puzzle, you need a dc voltmeter to determine the positve buss bar conductor.
In the positive conductor, power will flow in the same direction of the dc current, relative to this conductor. That is, current will flow from the (more positive) terminal at the battery side to the less positive terminal (at the resistor side). This DC current will produce a steady-state magnetic field. A compass can determine the direction of this magnetic field, either by a derivation from the theory or, has been suggested, doing a reference experiment under controlled conditions.
Thus, the most electrical engineeringly elegant and simple answer to the problem is: A DC voltmeter and a compass. These are two very common items.
Of course, the suggested Hall effect measuring devices and temp gradient probes (and super-sensitive voltmeters) might also work, but I should have added the real world condition that, the company you work for expects you to solve this problem with existing equipment and a spending budget of no more than \$50, if necessary. (Hmmm...Seems like I've worked for cheap outfits like that before...)
Thus, most of us already have dc voltmeters and we could buy a decent compass for under \$50 if we had to.
Beachcomber
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On 11/14/06 12:13 PM, in article snipped-for-privacy@news.verizon.net,

Tell that to a lead-acid battery that is being charged.

What happens if you happen to use bismuth conductors rather than copper? Or p doped silicon, for example? What if you use a conductor made of acidified water?

ill -- Fermez le Bush
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Gernerally speaking, current DOES flow from the positive to the negative terminal. You are bringing up a special case. Charging was not mentioned in the original puzzle.
But, even so, in order for the battery to charge, the charger + terminal needs to be even more positive than the battery + terminal. for charging current to flow.

Don't know. Unless you are a research scientist, your boss and co-workers might think you are nuts.
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On 11/14/06 5:47 PM, in article snipped-for-privacy@news.verizon.net,

True life is full of special cases.

I bring such items up because others tend to over generalize explanations. In acidified water, the primary carrier will be hydronium ions. In a proton beam, there are no electrons at all. Current still flows.
Bill
-- Fermez le Bush