Ideas for inexpensive True-RMS Metering?
in [Design]
|
From: John Larkin on 4 Jul 2010 15:52 On Sun, 4 Jul 2010 10:21:33 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >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. > I did the multiple-outle demo with two 1206 low-ohm resistors, in parallel, per channel. Gain is essentially free. >Expensive? Not a problem in onesies, I'm not sure what >a few thousand DOES cost. OK, probably that's true. We punch or photoetch our own current shunts out of manganin, for maybe 20 cents each. They'd be pennies in serious volume. The 1206 low-ohm resistors are cheap, too. > >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. The linearity problem isn't at large fields, it's at low ones. Silicon steel permeability drops seriously at low fields, enough that an affordable CT won't meet Ansi C12 accuracy requirements without messy compensations. An active burden doesn't help the copper loss problem; it's easy to use a passive shunt that's ting compared to winding resistance. Active burdens burn power, too. > >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. In real life, no. John
From: krw on 4 Jul 2010 15:56 On Sun, 4 Jul 2010 11:22:12 -0700 (PDT), Richard Henry <pomerado(a)hotmail.com> wrote: >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? I don't believe that was ever "legal". The practice now is to use a GFCI on ungrounded outlets and label them as such.
From: Paul Keinanen on 4 Jul 2010 16:11 On Sun, 04 Jul 2010 09:06:57 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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. Take a look at http://en.wikipedia.org/wiki/Earthing_system While a pole pig in the USA feeding a single house might actually be a true TN-S system, larger systems are most likely TN-C-S systems, so tampering with the TN-C side grounds is a bad idea. > 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. A sane person would not even think about tinkering with the neutral/ground with some test equipment :-). Typically individual grounding electrodes are required at each house in Europe, but typically the 230/400 V feeder cables and overhead lines are 3L+PEN only.
From: Grant on 4 Jul 2010 16:26 On Sun, 04 Jul 2010 18:10:48 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >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? That's what AC/DC clamp meters use ;) Grant.
From: whit3rd on 4 Jul 2010 16:43
On Jul 4, 12:52 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 4 Jul 2010 10:21:33 -0700 (PDT), whit3rd <whit...(a)gmail.com> > wrote: > > >On Jul 4, 9:06 am, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >> >Putting current transformers... > >> But they're big, expensive, and nonlinear. > >Nonlinear? That only happens with large core field, > >and current transformers with active burdens solve that... > The linearity problem isn't at large fields, it's at low ones. Silicon > steel permeability drops seriously at low fields... I don't understand that. The B-H curve doesn't have low slope at low fields... is this about hysteresis? Not sure about prices, but better cores (like metglas) can help, and it can't be terribly expensive in small quantities (you see all those strips glued in DVD packages). Silicon iron is favored for power transformers because of low losses due to eddy currents at high flux; if you are making a low-flux transformer, it's not the best material. > An active burden doesn't help the copper loss problem; > it's easy to use a passive shunt that's tiny compared to winding > resistance. Active burdens burn power, too. To treat for copper loss, design with two secondaries; one for the burden, one for sensing. The active burden DOES burn power, no way around that. Most ADC+ processor solutions will have low V high-ish current supplies available, though. The main benefit of an active burden is that the core can be made quite small. A secondary benefit is that it fits nicely with the amplifier stage into an ADC. |