Ideas for inexpensive True-RMS Metering?
in [Design]
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From: Nobody on 4 Jul 2010 07:55 On Sat, 03 Jul 2010 13:30:14 -0700, linnix wrote: >> Nope. You can compute power factor for waveforms which aren't even >> periodic, and thus don't *have* a "phase". >> >> Therein lies the problem with #5, i.e. "phase difference" may not even be >> a meaningful concept if the waveforms are far enough from sinusoidal. >> >> Actually, #3 suffers from the same problem. > > But we are talking about AC power, not just random waveform. We were talking about the stuff that you find on real-world mains supplies, not the theoretical concept found in text books. The voltage waveform may not be *that* far from sinusoidal; if you're metering it, it's probably coming from the grid rather than e.g. a "modified sine" UPS. While the current waveform probably won't actually be "random" in the mathematical sense, it may well be closer to that than it is to a sine wave.
From: Paul Keinanen on 4 Jul 2010 08:31 On Sun, 04 Jul 2010 12:55:42 +0100, Nobody <nobody(a)nowhere.com> wrote: >On Sat, 03 Jul 2010 13:30:14 -0700, linnix wrote: > >>> Nope. You can compute power factor for waveforms which aren't even >>> periodic, and thus don't *have* a "phase". >>> >>> Therein lies the problem with #5, i.e. "phase difference" may not even be >>> a meaningful concept if the waveforms are far enough from sinusoidal. >>> >>> Actually, #3 suffers from the same problem. >> >> But we are talking about AC power, not just random waveform. > >We were talking about the stuff that you find on real-world mains >supplies, not the theoretical concept found in text books. > >The voltage waveform may not be *that* far from sinusoidal; if you're >metering it, it's probably coming from the grid rather than e.g. a >"modified sine" UPS. With a rectifier+capacitor load without PFC, when the rectifier starts to conduct it is actually driving a short (a partially charged C) and the only thing that limits the inrush current is the source impedance of the electric distribution network (mainly source resistance R). This current can be several times the fuse nominal ratings, causing a significant voltage in the source resistance, significantly dropping the voltage at the load from the unloaded sinusoid voltage waveform. If the RC time constant (network source R, storage capacitance C, with power transformer impedance transfer ratio correction if used) is longer than 1-2 ms, the rectifier will conduct well after the nominal sinus voltage peak, causing voltage drops past the nominal voltage peak. When the open circuit voltage after the peak drops below the nearly fully charged capacitor, the rectifier finally stops to conduct and no further voltage losses will occur in the source impedance. If the voltage is measured close to the load, the highest voltage might be measured significantly after the nominal (unloaded) voltage peak, just slightly before the point when the rectifier stops to conduct. Thus, do not assume that peak voltage measured close to the rectifier load would represent the peak voltage of the source or that the timing of this peak could be used for phase difference measurements. For voltage phase (reference) measurements, the zero crossing is a better place, but this can also be polluted by harmonics and other high frequency noise. >While the current waveform probably won't actually be "random" in the >mathematical sense, it may well be closer to that than it is to a sine >wave.
From: John Larkin on 4 Jul 2010 12:06 On Sun, 04 Jul 2010 10:37:03 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Sat, 03 Jul 2010 13:11:45 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >>The cheapest way to do this is to buy a small uP that has an on-board >>ADC and mux. Scale down the line voltages with resistive dividers. > >Remember to put sufficient number of resistors in series so that the >combined resistor voltage rating is 1.5 to 2.5 kV depending on the >national standards, so that voltage peaks can be handled without >flashover. > >>A >>single-phase meter or Y-connected meter can let the uP ride on >>neutral, and sense neutral current(s) with cheap shunts. > >That is a horrible idea when used with TN-C (or measuring on the TN-C >side of a TN-C-S system) wiring system, in which Protective Earth (PE) >and Neutral are interconnected at the load. That would make for some interesting ground loops. Using safety ground as a current carrier is illegal here. A test equipment wiring >fault or a blown shunt resistor could cause the full phase voltage on >the cases of multiple load equipment. As would any open in the single ground wire. Scary, especially at 240 volts. > >>Other >>configurations will generally need current transformers. > >Putting current transformers on the phases is the safe way of doing it >and works both with wye and delta loads. But they're big, expensive, and nonlinear. John
From: John Larkin on 4 Jul 2010 12:15 On Sun, 4 Jul 2010 00:41:33 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: ><krw(a)att.bizzzzzzzzzzzz> wrote in message news:mnjv269firfs4rdvb5kr8pi0ql06v2pppi(a)4ax.com... >> But "Nobody" is still right. The phase angle and the power factor (as defined >> as W/VA) are *very* often different animals; if you calculate #4 you may well >> not have any meaningful answer for #5. ...if #5 has any meaning, or is even >> measurable. > >Not really. Take the DFT of a series of samples, find the fundamental. Compare phase components of V and I. Done. Also, cos(phi) = DPF, the classical (sine wave) power factor that you learn about in Power Systems. You don't need a full Fourier transform if you just want the fundamental components. Just S = sum(current_samples * sin(377t)) C = sum(current_samples * cos(377t)) and you can do this over many cycles if you want. and you can get the phase angles from them. The amount of math is reasonable on a small, busy uP, where a real DFT is likely not. I have assembly code around here somewhere... But the "power factor" that you get wouldn't be very popular for silly loads. John
From: Winston on 4 Jul 2010 12:48
On 7/4/2010 5:31 AM, Paul Keinanen wrote: (...) > With a rectifier+capacitor load without PFC, when the rectifier starts > to conduct it is actually driving a short (a partially charged C) and > the only thing that limits the inrush current is the source impedance > of the electric distribution network (mainly source resistance R). The biggest current limiters here are capacitive reactance plus the equivalent resistance of the rectifier, yes? --Winston |