Ideas for inexpensive True-RMS Metering?
in [Design]
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From: linnix on 3 Jul 2010 15:59 On Jul 3, 10:28 am, "k...(a)att.bizzzzzzzzzzzz" <k...(a)att.bizzzzzzzzzzzz> wrote: > On Sat, 3 Jul 2010 08:37:38 -0700 (PDT), EnigmaPaul <enigmap...(a)gmail.com> > wrote: > > >Hi Guys, > > >I'm looking for the best ideas for implementing an AC power > >instrumentation front end for a microcontroller or FPGA. > > >What I need to accomplish is to measure 50-60HZ AC in the range of > >120V upwards of 600VAC with about 1% accuracy, TRUE RMS. I need to > >extract: > > >1. Voltage > >2. Current (via current transformers) > >3. Frequency > >4. Power Factor > >5. Phase difference > > All of these are easy to do except the last one. The waveforms can be pretty > ratty and it may be difficult to measure this. Why phase difference? Although > in getting the rest I suppose you could calculate this. 4 is related to 5. If you can do 4, then you can do 5.
From: John Larkin on 3 Jul 2010 16:11 On Sat, 3 Jul 2010 08:37:38 -0700 (PDT), EnigmaPaul <enigmapaul(a)gmail.com> wrote: >Hi Guys, > >I'm looking for the best ideas for implementing an AC power >instrumentation front end for a microcontroller or FPGA. > >What I need to accomplish is to measure 50-60HZ AC in the range of >120V upwards of 600VAC with about 1% accuracy, TRUE RMS. I need to >extract: > >1. Voltage >2. Current (via current transformers) >3. Frequency >4. Power Factor >5. Phase difference > >This needs to work with reasonably distorted waveforms, like what >might be found in small generators driven by engines. I also need >something that will work for single phase or for 3phase and be >scalable to instrument up to two 3-Phase sources. > >There are a variety of ways to go about this but I'm looking for >something that is very flexible and can adapt to a variety of voltage >configurations (line to line, or line to neutral) and is very low >cost. > >I would guess that it might be similar to the best practices that >would be used in the front end of True RMS DMMs or DAQ cards with AC >inputs? I know there are also energy metering chips out there, but >they are rather expensive. > >Thanks for any ideas! > >PW 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. A single-phase meter or Y-connected meter can let the uP ride on neutral, and sense neutral current(s) with cheap shunts. Other configurations will generally need current transformers. A 10-bit ADC works pretty well, 12 is even better. Add some dithering noise to the current signals. Sample voltage and current inputs and do the math. You can even acquire waveforms. The trickiest math is phase angles. One of the voltage dividers can also be used to run a timer channel to get frequency. The algorithms to get metering-quality power measurements are a bit tricky. RMS volts and amps are pretty obvious. You should be able to do a pretty good 3-phase power meter with a $4 uP, or maybe even a $1 one. John
From: Nobody on 3 Jul 2010 16:21 On Sat, 03 Jul 2010 12:59:37 -0700, linnix wrote: >> >What I need to accomplish is to measure 50-60HZ AC in the range of >> >120V upwards of 600VAC with about 1% accuracy, TRUE RMS. I need to >> >extract: >> >> >1. Voltage >> >2. Current (via current transformers) >> >3. Frequency >> >4. Power Factor >> >5. Phase difference >> >> All of these are easy to do except the last one. The waveforms can be >> pretty ratty and it may be difficult to measure this. Why phase >> difference? Although in getting the rest I suppose you could calculate >> this. > > 4 is related to 5. If you can do 4, then you can do 5. 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.
From: linnix on 3 Jul 2010 16:30 On Jul 3, 1:21 pm, Nobody <nob...(a)nowhere.com> wrote: > On Sat, 03 Jul 2010 12:59:37 -0700, linnix wrote: > >> >What I need to accomplish is to measure 50-60HZ AC in the range of > >> >120V upwards of 600VAC with about 1% accuracy, TRUE RMS. I need to > >> >extract: > > >> >1. Voltage > >> >2. Current (via current transformers) > >> >3. Frequency > >> >4. Power Factor > >> >5. Phase difference > > >> All of these are easy to do except the last one. The waveforms can be > >> pretty ratty and it may be difficult to measure this. Why phase > >> difference? Although in getting the rest I suppose you could calculate > >> this. > > > 4 is related to 5. If you can do 4, then you can do 5. > > 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.
From: AM on 3 Jul 2010 16:38
On Sat, 03 Jul 2010 13:11:45 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 3 Jul 2010 08:37:38 -0700 (PDT), EnigmaPaul ><enigmapaul(a)gmail.com> wrote: > >>Hi Guys, >> >>I'm looking for the best ideas for implementing an AC power >>instrumentation front end for a microcontroller or FPGA. >> >>What I need to accomplish is to measure 50-60HZ AC in the range of >>120V upwards of 600VAC with about 1% accuracy, TRUE RMS. I need to >>extract: >> >>1. Voltage >>2. Current (via current transformers) >>3. Frequency >>4. Power Factor >>5. Phase difference >> >>This needs to work with reasonably distorted waveforms, like what >>might be found in small generators driven by engines. I also need >>something that will work for single phase or for 3phase and be >>scalable to instrument up to two 3-Phase sources. >> >>There are a variety of ways to go about this but I'm looking for >>something that is very flexible and can adapt to a variety of voltage >>configurations (line to line, or line to neutral) and is very low >>cost. >> >>I would guess that it might be similar to the best practices that >>would be used in the front end of True RMS DMMs or DAQ cards with AC >>inputs? I know there are also energy metering chips out there, but >>they are rather expensive. >> >>Thanks for any ideas! >> >>PW > >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. A >single-phase meter or Y-connected meter can let the uP ride on >neutral, and sense neutral current(s) with cheap shunts. Other >configurations will generally need current transformers. > >A 10-bit ADC works pretty well, 12 is even better. Add some dithering >noise to the current signals. > >Sample voltage and current inputs and do the math. You can even >acquire waveforms. The trickiest math is phase angles. > >One of the voltage dividers can also be used to run a timer channel to >get frequency. > >The algorithms to get metering-quality power measurements are a bit >tricky. RMS volts and amps are pretty obvious. > >You should be able to do a pretty good 3-phase power meter with a $4 >uP, or maybe even a $1 one. > >John > The cheapest way to get an inexpensive true RMS multimeter is to simply go buy an inexpensive true RMS multimeter. You could not possibly jump into a game such as this with such nievete. --- news://freenews.netfront.net/ - complaints: news(a)netfront.net --- |