Power factor measurement

Has anyone does any project on power factor measurement?
I am struggling with hardware components for this project, I was previously thinking of putting 2 hall effect sensors for current and voltage, but didn't knew what to use for real power measurement.

NOW, after some research I have seen that few ics are there which can actually compute these three things (and more) and can pass the info to a PIC over serial or I2C interface. Has someone used any such IC? and how correct are the results. I am planning to take on a project for power factor correction but it's all down to these measurements being correctly done. I need to display these results on an LCD screen (Vrms, Irms & P.f), but that I can do that part, I need help with measurements.

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

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Originally Posted by Demon

.... how to use their products.

Robert

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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.

Try MCP390X...

There are three routes you can take:

1) Use a chip that measures the real and apparent power. You can probably get Vrms and Irms as well. SPI or I2C communications are likely necessary, but the chip will likely also generate a pulse for each unit of energy (example: 1 Watt x Sec) and a direction. Direction is which way power is flowing (to or from load). Increment or decrement the pulse sum depending on the direction. A highly reactive load will generate lots of energy pulses, but the direction will cause most of them to be averaged out. A resistive load will always have the same direction.

Divide Ptrue by Papparent to get Power Factor.

2) Write code to take lots of V and I measurements across a single cycle. Interleave the V and I measurements. Think >16 pairs of V and I measurements per cycle.
(This is a good point to measure frequency.)

2A) Compute the RMS values for V and I across a cycle. Here's how:
Square each V and I, add to sum for one second (Vsum, Isum).
At end of second, compute the square root of Vsum and Isum to get the RMS values Vrms and Irms.
Multiply Vrms anf Irms to get apparent power (Papparent).
(Vrms and Irms can be reported as the voltage and current measurements that a meter would indicate.)

2B) For each pair of V and I measurements, multiply them to get instantaneous power (I can be negative in reactive loads, so power may be negative). Sum instantanious powers across a cycle to get true power (Ptrue).

Divide Ptrue by Papparent to get Power Factor.

3) Hack a Kill-A-Watt. This may be very similar to #1 above.

P.S. I wrote "AN220 - Watt-Hour Meter using PIC16C923 and CS5460"

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

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

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.

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.

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Originally Posted by brettcgb

P.S. I wrote "AN220 - Watt-Hour Meter using PIC16C923 and CS5460"

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Ahhhh... do we have a Microchip engineer lurking in our midst? :)

Greetings Brett!

AN220 looks like a good read. Thanks!
(Hmmm... I think I've seen your name on other Microcips AN's as well)

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.

Former applications engineer with Microchip. I've since moved on.

I've written a few apnotes, not as many as would have liked. I'd heavily revise my oscillator apnote, but the testing section is accurate.

I can't thank enough for the knowledge you have shared.

I would like to know how useful this thing will be to learn more and put my next step forward: http://www.microchipdirect.com/ProductSearch.aspx?Keywords=MCP3905RD-PM1

I am really relying on this forum to complete this challenge. I thank once again for the inputs. More questions will be on the way soon.

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Originally Posted by dhouston

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

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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.

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Originally Posted by Charles Linquis

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

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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.

I still don't understand why they sample so fast. The power factors of most loads don't change over thousands of cycles. You can sample every single part of a waveform even if you sample it once a second. You just need to take a lot of samples.

I have built two wattmeters over the past few years. One did have to sample fast (about 10KHz). I needed the value of each half-cycle as soon as it reached zero-crossing.
I started the A/D, then did the calcs for the previous sample, then grabbed the result and started the A/D again.

The other sampled at a 2 mSec rate (500Hz). I sampled for 3 seconds (1500 samples). The waveforms I measured were definitely
not sinusoidal. The results were the same in both cases, and I actually used a KILL-A-WATT as a calibration reference.

I've seen a PC based utility that does this with an interseting approach; it uses the 2 sound card channels to track the voltage and current readings, does the math and displays all the parameters on the screen. The simple coupling hardware is also described in the help section. The company's since changed its name so it took some searching, but here it is. http://www.infinitespectra.com/freew...400/index.html

Since you can calibrate the software for your own setup, I'm sure the components arent too critical either.

And did I mention its free?!

Regards,

Anand

Neat!

If you do this, make sure you use an isolation transformer between AC line and your test circuitry. Otherwise, you caan make the case of your PC hot.

Dave, the circuit does isolate both the channels already, doesnt it? The voltage sensing is by way of a 12 volt step-down, and the current sense is a coil wound around one of the power carrying cables. This might be the only difficult to procure component for those having no access to Digikey, but since there is an option to calibrate the unit, I suspect a home made hand wound coil should be fine.

Anand, sorry - I didn't look at the circuit. The transformer is fine.

Moved from Schematics.

Robert