Power Measurement - Misconceptions

[Omnibus]

Because there's the real potential for many people to learn from this.

Why are you trying to correct me?

.99

Oh, don't bother with @spinner_MP, @MrMag or @XS-NRG. They are just nuisance. What you're doing is great and I'm really looking forward to see your analysis especially of the AVG value in a RC circuit when the voltage (not the current) has an offset compared to the result with no voltage offset.

[poynt99]

Omnibus requested I perform some sim runs with something closer to 80º phase angle (I used 75º), and also a run with a DC offset in the circuit. See the following for the results:



rms_ave_07.gif illustrates the voltage and current with a 75º phase differential. Offset is 0V and 0A.

rms_ave_08.gif illustrates the instantaneous power trace at the top (green trace) and the resulting RMS and AVG computations. Note that the instantaneous power trace now deviates almost symmetrically about the 0 mark, but still exhibits a 2Wp-p swing. The bottom plot clearly indicates the discrepancy between the RMS and AVG computations, the RMS being 0.753W and the AVG being 0.259W. The reader may recall that the power factor for phase-shifted wave forms is: PF=COS (q), where "q" is the phase angle. In this case we have PF=COS (75) and PF=0.259, so once again the AVG computation provides a correct result, while the RMS does not.

rms_ave_09.gif illustrates the voltage and current again with a 75º phase differential. This time the offset is 2VDC and 40mADC (with a DC offset voltage, there will be a DC current in the circuit).

rms_ave_10.gif illustrates the instantaneous power trace at the top (green trace) and the resulting RMS and AVG computations. Note that the instantaneous power trace normally shows a symmetrical wave form that is double the voltage/current frequency, but it is now asymmetrical in form due to the offset in voltage and current.

In order to obtain the total dissipated power in the circuit, we simply add the previous power values obtained when there was no offset present (see above), to the power produced by the offset alone. This is easily accomplished because the offset is a pure DC value, and is computed to be:

2VDC * 40mADC = 80mW.

Therefore, the new plots should indicate 80mw + 753mW = 833mW for the RMS plot, and 80mW + 259mW = 339mW for the AVG plot.

The bottom plot not only indicates the discrepancy between the RMS and AVG computations, (the RMS being 0.904W and the AVG being 0.342W), but the RMS computation with the DC offset has produced an even worse error than before, whereas the AVG computation is nearly exact to what we would expect for the REAL power in the circuit. The RMS power computation produced an erroneous increase of about 151mW rather than the correct ~80mW increase produced by the AVG computation.

.99

[XS-NRG]

Oh, don't bother with @spinner_MP, @MrMag or @XS-NRG. They are just nuisance.

Just nuisance with working devices.
Mine doesn't need all this crap it just runs itself no calculation needed.
At least my name isn't Lawrence 

[Omnibus]

Thanks @poynt99. Will have to see these data later on my laptop because the iPad I'm using now can't unzip the downloaded files.

One thing, however, makes an impression. The data with the voltage offset also have current offset. Would it be possible to calculate AVG of data with only voltage offset and no current offset as is the case with an RC circuit and compare it with AVG without any offset (neither V, nor I offset)?

[poynt99]

Thanks @poynt99. Will have to see these data later on my laptop because the iPad I'm using now can't unzip the downloaded files.

One thing, however, makes an impression. The data with the voltage offset also have current offset. Would it be possible to calculate AVG of data with only voltage offset and no current offset as is the case with an RC circuit and compare it with AVG without any offset (neither V, nor I offset)?

Can you explain this further, I don't follow what you are asking in regards to an RC circuit possibly having an offset voltage but not an offset current.

With a series C and shunt R, the voltage across R will eventually (10 tau say) not have an offset.

.99