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true rms powermeter or energymeter

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Necessaryevil

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I want to build an true rms energymeter or powermeter (AC) some day. I want to use it for measuring power consumption of mains powered devices.
It's not the purpose to built this device soon, at this point I'm more interested in the general theory of operation and probably some recommended parts.

I came up with two ideas:
1)Sample voltage and current (ADC) with a relatively high samplerate and let an microcontroller calculate the power
2)get voltage and current, use an analog multiplier and sample the output of this multiplier with an relatively high samplerate




My question is, what is the best way to do this? How do commercial true rms energymeters do the job?



Stefan
 
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What application are you using this for? AC obviously? Measuring the power consumption of mains powered devices?
 
It may be more complicated than you think when power factors are brought into the equation. AC power has three components (Active, Apparent and Real) it just depends on what measurement you want. Have a look at some articles on power factor.
 
Yeah but you're just sampling the actual voltage and current values at a point in time. What about measuring the phase relationship between the voltage and current waveforms? In a purely resistive load they are equal but this isn't how things are in the real world.
 
If you sample the AC voltage and AC current at a sufficient rate (say 100 times per cycle), then you just multiply each instantaneous voltage sample with the current sample taken at the same instant. Then sum all these calculated results over one cycle. That will automatically include the affect of any power factor phase shift and will give you the true RMS power. No need to separately determine the phase shift or power factor.

Edit: Commercial energy meters use ICs built by Analog Devices and others to perform all the relevant calculations for determined power, energy, PF, etc. from the voltage and current measurements.

Edit 2: You do know that there are devices available for about $25, such as the Kill a Watt that do all these measurements and display the results on a digital readout. I have one and it works quite well. You just plug the meter into the wall and the appliance into the meter.
 
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Hello Crutschow,

Thank you for your information. I'll study the IC. Edit 2 is probably posted on the wrong forum ;-). To be serious: Thank you for this answer as well; but I'm interested in DIY. And I 've read some reviews of devices here on the Dutch market, most of them don't include the powerfactor and thereby give inaccurate results.
 
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And I 've read some reviews of devices here on the Dutch market, most of them don't include the powerfactor and thereby give inaccurate results.
The one I have measures PF and displays either PF, VA or W. It seems to work well and gives reasonable readings although I have no way to verify its accuracy.
 
I know one manufacturer claims that it is hard to sample voltage and current close enough in time to be accurate. They have a custom chip designed to do this, and claim that it is the only one that does an adequate job. I think this was Yokagawa.
 
I don't see any problem in sampling current and voltage with sufficient closeness in time to get good accuracy for the low frequencies involved in mains power. I believe Yokagawa is blowing smoke. ;)
 
Sure Cruts; I didn't see that you were measuring 60Hz. The Yokowaga is for high frequency PWM servo motor waveforms. That should work!
:D
 
I know one manufacturer claims that it is hard to sample voltage and current close enough in time to be accurate. They have a custom chip designed to do this, and claim that it is the only one that does an adequate job. I think this was Yokagawa.

Hmm.. It is hard to believe they are the 'only one' but I immediately believe that it is very hard to obtain accurate results. Maybe it is possible to store the data of one of the meters in a volatile memory so it is possible to process samples which were obtained at the same time.

A link to a test. They compared with a professional calibrated meter. (google translate):
https://translate.google.nl/translate?hl=nl&sl=auto&tl=en&u=http%3A%2F%2Fbe.hardware.info%2Freviews%2F1460%2F6%2Fvergelijkingstest-9-energiemeters-tabel-met-testresulaten

(it's actually 50 Hz here in Holland)
 
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