Ideas for inexpensive True-RMS Metering?
in [Design]
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From: whit3rd on 4 Jul 2010 13:21 On Jul 4, 9:06 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >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. Big? If you are sensing dozens of amps and the alternatives are shunt resistors and current transformers, there aren't any 'small' candidates. Expensive? Not a problem in onesies, I'm not sure what a few thousand DOES cost. OK, probably that's true. Nonlinear? That only happens with large core field, and current transformers with active burdens solve that problem neatly. It doesn't HAVE to be a small-value burden resistor, that was a simple example. Actually, the 'big' and 'nonlinear' are connected; you need the size to keep the nonlinearities down, and the use of an active burden reduces both the size and the nonlinearity.
From: linnix on 4 Jul 2010 13:26 On Jul 4, 4:55 am, Nobody <nob...(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. > > 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. The wave form may be distorted, but the base frequency (from the power generators) would not change. What's important (for load correction) is the peak voltage and current phase difference, which should not be difficult to measure with a simple micro.
From: m II on 4 Jul 2010 13:34 John Larkin wrote: >> You could not possibly jump into a game such as this with such nievete. > > What does that have to do with designing polyphase wattmeters? 'Nievete' seems to be a cheap Chinese clone of better quality instrumentation, hence the warning. michael the naive
From: Nico Coesel on 4 Jul 2010 14:10 whit3rd <whit3rd(a)gmail.com> wrote: >On Jul 4, 9:06=A0am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >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. > >Big? If you are sensing dozens of amps and the alternatives >are shunt resistors and current transformers, there aren't >any 'small' candidates. Hall effect sensor? -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico(a)nctdevpuntnl (punt=.) --------------------------------------------------------------
From: Richard Henry on 4 Jul 2010 14:22
On Jul 4, 9:06 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 04 Jul 2010 10:37:03 +0300, Paul Keinanen <keina...(a)sci.fi> > wrote: > > > > >On Sat, 03 Jul 2010 13:11:45 -0700, John Larkin > ><jjlar...(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. It used to be legal (or at least common practice) to connect the neutral to both the neutral and ground terminals when "upgrading" residential outlets from 2-prong to 3-prong. Is that not now (or not ever) legal? |