 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?
Don't average them. Measure voltage and current at common instants
letting each instant figure its own power level and direction. Then
average those to get real power and direction. If that averages out
to zero, then you have no power flow (power factor is 0).
 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.
I think being shut in a room with open buss bars violates safety rules :)
Of course you mean, the challenge is to figure it out entirely from inside
that room with only the intrument(s) you figure in advance (the topic of
the question) you will need brought in with you.
 Questions

 1. Can you determine the direction of power flow just from
 measurements to the AC buss bars in room B ?
Sure.
 2. What sort of instrumentation would you need?
Something that measures voltage and current at an instant, gives you a
power reading from that instant, and averages the power readings from
many such instants across a cycle.
 3. Do you need to break the circuit to make the measurements?
No. A clamp on ampmeter would do. But for direction alone, I don't
think you even need that, as you can measure the direction of the
magnetic field between the conductors to determine polarity.
I suspect there is some simpler answer. A coil of wire sufficient to
handle the voltage applied (maybe with its own resistance to restrict
current) that can fit between the buss bars could show field alignment
by its orientation.

/
 Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below 
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