Power factor measurement

[financecatalyst]

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.

[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.

[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.

[brettcgb]

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"

[Byte_Butcher]

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

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)

[brettcgb]

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.

[financecatalyst]

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.