|
Poynt should clearly preface this thread, I think, with the statement that this method only works in the case where the power factor on the input is known to be 1.00
This will always be the case in a constant voltage DC-powered circuit or where it is known for sure that PF=1.00 (by other means of testing) in a circuit powered by a source which is totally, primarily or even significantly AC.
In other words, the averaging of individual voltage and current measurements and then multuplying the averages to find input power is only acceptable if we know for sure that the power factor is 1.00. In any other situation, the method will only report "apparent" power and cannot make any distinction between real power and reactive (reflected) power. Take the case of hooking a capacitor across the AC line, for instance. You'd measure an average voltage of 120VAC and some average current depending on the capacitor value. You'd mulltiply them and get a positive number. But the real power is close to none. The averages would show the reactive power, a.k.a. reflected power. The only real power would be whatever is dissipated as heat in the wiring and the capacitor's internal resistance, a much smaller number than would be shown by multiplying the two averages.
That common knowledge was the basis which, sadly, was used by many here to convince each other and a certain Miss Ainslie that all the multiplying had to take place before any averaging...a misconception that she now clings to tenaciously despite the fact that hers is truly a DC-operated circuit where the actual input DC voltage is a constant with quite negligible AC ripple. She now is convinced and is trying very hard to convince others that Poynt keeps changing the rules regarding this; earlier being a strong proponent of "multiply before averaging" and now arguing the opposite.
The reality is that none of us "old pros" had the foresight to realize that Rosemary was, of course, going to hook up her battery stack using 30-odd feet of wire and no bypass or local energy storage caps anywhere and then measure the "battery voltage" at the wrong end of those long wires so that the scope would be fed a gigantic AC waveform as the voltage argument of the multiply. Nor did we much think about the inductance of the shunt causing that signal (the current argument of the multiply) to be shifted in time, amplitude and waveform as well, by almost 90 degrees and fourfold amplitude.
In defense of the poor foresight of everyone advising her at the time, one of the big reasons we all underestimated the importance of these things at the time is that none of us knew that Rosemary had changed the ballgame from using a 2kHz primarily hard-switching circuit to using a full-blown continuous linear feedback mode oscillation at 1.5 MHz, where the importance of these inductance effects becomes overwhelming.
Essentially, Rosemary has ended up feeding mainly di/dt (shunt) and -di/dt (battery) signals into her scope on both channels instead of sending true input voltage and input current signals to be multiplied. She has the scope multiplying the rate of current change times -1 times the rate of current change as provided by the voltages taken across shunt and battery wiring inductances that are overwhelmingly dominant over the shunt and load resistances and hundreds of times larger than any battery internal resistance.
And some of us on this forum and on prior forum discussions elsewhere inadvertently reinforced the idea in her mind of the correctness of that method because we agreed in the general case that the real-time multiplying had to be done before the averaging. This was correct advice ONLY if both the voltage and current were time-varying quantities (and they are not) and (as we assumed wrongly) if proper precautions were taken to assure that the actual battery voltage and a correct analog of the actual current were the terms being multiplied.
In retrospect, we all knew that batteries were being used and that therefore, the voltage argument of the multiply was going to be essentially a constant with possibly a very small AC ripple, so the whole argument over whether to multiply first or average first becomes moot when one term of the multiply is a positive constant. We should have focused our collective "educate Rosemary" efforts on getting her to understand that her battery voltage is not 158VAC at 1.5MHz. 20/20 hindsight. Oh, well...Rosemary has proven to be uneducable anyway.
Now that that is off my chest, the real reason I'm posting this time is to show you something I have just tonight realized for the first time in all of this:
IT IS EASY AS PIE TO COMPENSATE FOR THE INDUCTANCE OF A SHUNT IN REAL TIME AND OVER AN ENORMOUS BANDWIDTH!
So, in the event we ever actually needed to do real-time "multiplying before averaging" using a scope-sampling math function (say in the case where the power supply is a 1MHz square wave by chance or a DC source with high impedance and lots of ripple or even 60Hz sinewave mains power with heavy harmonics) we can fairly easily get our inductive current shunt to give us true-to-life real-time waveforms that are in-phase and amplitude correct and waveshape-identical no matter how complex the waveform.
How? Simply use the RC filter technique I've already shown many times but with one difference: Instead of pounding to a damn-near ripple-free DC average by heavily filtering (RC time constant hundreds or thousands of times longer than the waveform period), we simply set up our R and C such that the product of R*C is equal to the quotient of L/R (shunt inductance over shunt resistance). Now the inductive shunt's time constant is exactly compensated by the filter's time constant. The result is exactly the same as if we were somehow able to eliminate perfectly the shunt's inductance. And it's broadband. All the signal differentiation done by the unwanted inductance is exactly undone by the reciprocal integration of the RC filter.
Below, I show a +/- 1A current trace of arbitrary shape with lots of rich harmonics and sudden slope changes and asymmetries to represent a signal with energy over a wide bandwidth. You can see how the scope is hooked up. Both traces are exactly identical and superimposed perfectly at all points. So the effect is just as if we had an ideal pure resistance for our shunt. No phase shifts, no distortions. Ready for real-time sampling and multiplying.
Humbugger
« Last Edit: 2011-04-06, 19:52:09 by humbugger »
|
Author
Topic: Measuring INPUT Power Accurately and with no Oscilloscope (Read 33133 times)
