Measure direction of power(current)


I was reading up on the AC power theory in the learn section and I am having a hard time understanding “Determining the Direction of Power Flow” section. because I was under the impression that AC power has no polarity but it says that the power is reversed can you guys explain that to me.

I think I can see how your mind is working - you’re confusing the “alternating” part that’s applied to voltage and current with power.

By your thinking, the power is alternating too - but it isn’t. If it was, your electric oven would never get hot because the power/energy/heat would flow into the oven and heat it on one half-cycle, then immediately flow out and cool it on the opposite half-cycle.

Look again at that page. In the very first graph, voltage and current are both alternately positive and negative, but power (yellow) is always positive. OK, it has big ripples on it, but it’s always positive. And if you think about it, that must be true: P = V × I, and P is positive when V & I are the same, whether they are both positive or both negative.

Now if you invert one of those, say the current (because we usually use the voltage as our reference), you get diagram 4. Using exactly the same maths, power is always negative.

The only piece missing now is to relate the direction of current flow to positive and negative. Let’s say: if the current is flowing into my house when the voltage is positive and out of my house when the voltage is negative, then power is flowing into my house. If the current is flowing out of my house when the voltage is positive and into of my house when the voltage is negative, then power is flowing out of my house.

Robert thanks for the explanation

I have a couple of more question. The current is not physically inverted (shifted 180) due to power flowing in the opposite direction is it just the CT senor picking up the current inversion?

if so how is the current inversion detected?

My understanding is that the CT sensor outputs an analog signal but then it is converted to a digital signal with an Arduino. How would the Arduino know the current was inverted unless it had a reference voltage or current?

Yes, it is exactly that. It is inverted. That’s because we use the convention that current flowing in one direction is positive, and in the opposite direction is negative, and on a graph - on those diagrams - positive current is upwards.

As I wrote, by comparing it against the voltage wave.

It must have a reference voltage to do the maths on. It does the maths of multiplying voltage sample × current sample roughly 50 times per mains cycle for me (rather less for you, as your mains runs faster) for about 300 ms, then averages the power values to take the ripples out.

Without the reference voltage, you’re in exactly the same position as you are with an ordinary ammeter measuring alternating current - you have no means of knowing.