Power factor measurement

[Demon]

I would start with checking out Microchip's site. They have application notes that explain how to use their products.

Good luck!

And report back how things progress.

Robert

[financecatalyst]

.... how to use their products.

Robert

I tried searching microchip's website for such IC, but could not find anything.

It doesn't seem they have this kind of power measurement ICs.

[pedja089]

Try MCP390X...

[brettcgb]

Look at:
Analog and interface / Mixed Signal / Energy Measurement
Six devices are listed. One is MCP3906.

[financecatalyst]

Thanks a ton. I surely have more knowledge than with what I started with. I have read AN220, very interesting indeed. This project idea came to me while doing experiment in the LAB last week (doing my HND in electronics currently).<br>Taking readings there didn't seem much complicated then, we had Voltek PM1000 to do this job. Now that I want to do it myself, I can see the hurdles ahead, mainly of understanding properly terms like gain, harmonics etc. But I have hope that I will be able to find many answers in the forum only.<br><br>So after my first attempt in reading everything from AN220, and little bit about MCP3906 I have the following questions:<br>1) With MCP3906 what is the MAX load I can put?<br>2) What does this statement mean: "32:1 PGA - MCP3906"<br><br>My main aim is to do this calculation on inductive loads (motors) only running on single phase 230Vrms<br>That's it for now. As I will go along, I will be posting more questions. Thanks

[dhouston]

You need to take several thousand instantaneous readings of both current and voltage per second and average their product over the time period to calculate true power (Kill-a-Watt does it this way). You can measure the current using hall-effect sensors and the voltage using an unregulated DC wall transformer (you'll need a calibration procedure). Apparent power is just Vrms * Iavg. TP / VA gives you PF.

Doing it this way will also handle loads like switching power supplies, CFLs, etc.

There used to be a PIC based example on Dr. Ed Cheung's website. He's a NASA engineer who built a device to measure the power used in his house. I'm not sure it's still there but he will probably respond to email, if not.

Of course, the easy way is to spend $25 for a Kill-a-Watt meter. I think I was the first person to buy one and review it (on comp.home.automation). Now you can buy them at every corner store.

[dhouston]

There's also an excellent discussion of PF on Wikipedia. I would have just added this to my earlier post were it not for the puerile editing rules used here.

[Charles Linquis]

You need to take several thousand instantaneous readings of both current and voltage per second

Not really. In most cases, you can do it slowly. Consider a 60Hz sine wave at first. You could statiscally get a sample at every degree if you sampled one time in each cycle for 360 consecutive cycles, as long as your measurement interval was not synced to the 60Hz. You could obtain those 360 samples in 6 seconds. So, you could measure voltage, then current, then wait for 15msec and do it again. Plenty of time for calculations. Of course, you don't get cycle-by-cycle results, but I don't think a KILOWATT does either. If the update rate is too fast, humans can't read it.

And you don't need to grab every degree, either. If the crest factor is not too high, you can get really good results by sampling every 10 degrees or so. If, by chance, the input was a pure sine (I agree that isn't too likely, but it is a starting point), and you sampled 5 degrees before peak and 5 degrees after peak, sin 85 = .996 That is a really small error. You might say that the waveform isn't changing very fast at the peak, and you would be right, the rate of change (COS) is greatest at zero-crossing. The sampling error would be highest there. But, the energy at zero crossing is low, so it doesn't contribute much to the RMS values.

[dhouston]

... I don't think a KILOWATT does either.

The Kill-A-Watt sampling rate is about 2kHz which is on the low side. I exchanged email with one of their engineers way back when it was introduced. And, I don't recall what the sampling rate was in the Microchip AN220 (I did read it long, long ago.) but believe it was even higher. With electronics, you can get some very non-sinusoidal loads. Harmonics are why you need high sampling rates. AN220 says, "The CS5460 power/energy measurement IC measures instantaneous voltage and current four thousand times a second."

IIRC, there were also some excellent articles on the Rensselaer Polytechnic Institute site and there was a really good article on non-linear loads written by a fellow in New Zealand whose name escapes me.

[brettcgb]

The max load that can be measured depends on several factors.

1) The sampling circuit gain. For a 1A load, does the circuit output to the MCP3906 1.0V, 0.1V or 0.01V? The maximum full scale current currents could be 5A, 50A and 500A, respectively. In other words, the full scale measurement depends in part on the sampling circuit.

2) PGA = Programmable Gain Amplifier. The gain of the input amplifiers can be programmed at any time. Options are likely Av=32, 16, 8, 4, 2, 1, 10, 5. The input voltage is multiplied by 32, 16, 8... before being applied to the A/D converter.

So, choose what the full scale current and voltage SHOULD be measurable, then build your circuit and code around that.