I understand a lot of what you guys are saying about symmetry etc. I know that that RMS is a point in magnitude that does not change in time, as long as the AC waveform remains constant. What I want to know is, well the book says:
- Average AC power and DC power are equivalents.
- AC power calculations are always performed with RMS values.
- Because power is a primary concern with any electrical or electronic system, RMS values are used more often than any other magnitude-related measurements.
- When a magnitude related measurement is unidentified, it's RMS.
This tells me that RMS values are used most of the time. So im thinking that if average power, calculated with RMS values, is the DC power equivalent, than V-RMS and I-RMS are DC equivalents too, but I'm thinking this not right, because my professor said that the half-cycle average values are DC equivalents. Im so damn confused.
Question: What are half-cycle average values used for?
Draw yourself a graph with a perfect DC voltage of whatever positive voltage. Colour fill the area between 0 and the curve (actually a straight line in this case). That coloured area is what can contribute to power.
If you draw the same graph with the same voltage, but now negative, you'll notice that the area is the same. In other words, the amount of contribution to power is the same.
Now draw yourself a symmetrical square wave without offset and half the wave positive and half negative. Use the same peak value as above for the positive and negative waves.
If you fill the areas between 0 and the curve, you'll notice that if you add them together, the total area be the same as with the DC wave above, despite half being negative and half positive.
Imagine taking the square wave and flipping over all negative parts to the positive side. That's what the rms calculation does and why the terms are squared.
With a symmetrical square wave, you're ready, with a different shaped wave, you need to take that shape into account.
Draw in your previous square wave graph the sinewave with equal peak values and you'll see that the corners of the square are "chopped off", diminishing the area.
But still, area is just area, whatever the shape. Hence the DC-equivalence if you total those areas.
The half cycle value is because in the calculations you only use V
peak, i.e. the value between 0 and the positive peak. Of course, the real signal varies between the positive and negative peak, or the V
peak-peak value. But that negative peak is flipped over to the positive side when it's squared in the calculation. It's a mathematical thing.
As said before, this works only for symmetrical shapes around 0.