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| Types of AC power P, Q, S and PF |
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| RoGeorge:
Latest QUCS 0.0.20 simulator includes a type of probe for measuring power. I was trying to understand how it works, so I fed the power probe with two AC sources, one for current and the other for voltage. Both I and V sources are the same frequency, but with the phase angle between them adjustable. The AC current source I1 has 1.2A RMS and the AC voltage source V1 has 1.5V RMS. The phase of the voltage relative to current, Ph, is swept between -180 and 180 degrees. PF (dimensionless) - power factor, yellow trace P (W) - active power, red trace Q (VAR) - reactive power, blue trace S (VA) - apparent power, magenta trace I was expecting the apparent power S to change with the phase, too, just like the others. Why is S constant no matter the phase between V and I? |
| HackedFridgeMagnet:
--- Quote from: RoGeorge on June 10, 2021, 03:06:56 am --- I was expecting the apparent power S to change with the phase, too, just like the others. Why is S constant no matter the phase between V and I? --- End quote --- ? The definition of apparent power is Vrms * Irms if you ignore the phase difference. So it will be flat and not vary with phase. |
| WattsThat:
VA = Volts * Amps Phase angle does not mater, Cos/PF always equals one. Think DC. P = E *I. |
| RoGeorge:
Now it makes sense, thank you! At first I thought it was a software bug. ;D Easy to agree S should be constant even intuitively, without pencil and paper if I visualize the triangle of powers: Source: https://en.wikipedia.org/wiki/AC_power From the simulation plot, P and Q behave like two quadrature sinusoidal signals when varying the angle between I and V, but P and Q are always perpendicular, which forces S to stay constant. S will rotate while varying the angle between I and V, but the length of the S vector will stay the same. |
| T3sl4co1l:
Pythagorean identity. Take the magnitude of S = P + iQ. :) Tim |
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