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If I have a AC network with two black box components connected with each other and I can tap into all voltage and current measurements, how do I determine which component is the source and which is the load.?

In case of DC, I can identify this by looking at the direction of current. In AC, current is alternating, so the information I can obtain is if the network is lagging or leading. Not able to say which block is source or load.

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  • \$\begingroup\$ If these are two port boxes and one port from each is joined. I would look at the other ends and try to inject a signal if possible AC coupled and look for a response \$\endgroup\$
    – D.A.S.
    Commented Jan 7, 2020 at 6:13

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Hi: I guess it is really up to you, you could decide what to call them arbitrarily.

Or, you could compute the average power flow into each box. If the power in is positive, the box is absorbing power, you could call that the load. If the power in is negative, the box is supplying power. You could call that the source. All physically existing circuits should obey conservation of energy, so the sum of the two average powers must be zero.

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  • \$\begingroup\$ Shoud i use power meters to compute the sign of average power flow? \$\endgroup\$
    – jrvinayak
    Commented Jan 7, 2020 at 15:47
  • \$\begingroup\$ You are missing an massive part of the explanation, the sign of the power doesn't mean anything if you don't mention the measurement convention used. \$\endgroup\$
    – MathieuL
    Commented Jan 8, 2020 at 16:19
  • \$\begingroup\$ The convention in circuit theory is that current flows INTO the POSITIVE terminal of a device. Thus, current*voltage is the instantaneous power flowing INTO the device. To find the average power, average the instantaneous power over time. (For repetitive signals, over one period is good) \$\endgroup\$
    – user69795
    Commented Jan 8, 2020 at 20:58
  • \$\begingroup\$ @user69795 If you connect amp meter in the load convention. However if i connect the amp meter in a generator convention, the current will have a Negative sign on the measurement equipment. here the problem is literally just an analysis of sign on a multi meter, there is no need to compute the power. \$\endgroup\$
    – MathieuL
    Commented Jan 9, 2020 at 15:45
  • \$\begingroup\$ @user69795 I will remove my down vote if you update your answer with proper schematic of the measurements. Your explanation is correct for someone who already understand the concept of power flow. However, your question is not clear for some one like OP who need a basic explanation of power flow in AC. \$\endgroup\$
    – MathieuL
    Commented Jan 9, 2020 at 15:49

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