Measuring power factor with microcontroller

You're not taking into account the non-linear effects of many loads.

loads.http://en.wikipedia.org/wiki/Power_factor

I actually read the Wikipedia article and was lost as to how to deal with the non-linearities. This is just a power-meter I'm building and really doesn't need to measure PF, so it may be easier to leave it out.

One trick is to just add a little phase lag into the voltage signal conditioning path, enough to correct for the sampling lag between voltage and current. Another is to alternate voltage:current samples with current:voltage samples.

No, that's not right. The apparent power calculation must be independent of the E:I phase angle.

What I usually do is square individual voltage samples, lowpass filter them, then square root the filter output to get true RMS volts. Then do the same for current. Now multiply those results to get apparent power, VA's.

PF = real power / apparent power.

You can simplify the math by lowpass filtering the absolute values of E and I, rather than doing the true RMS stuff. You'll get the same result for sine waves, just as an averaging AC voltmeter is as good as a TRMS meter for sine waves. You can also do block averages instead of lowpass filtering.

This technique gives a pretty good number, but loses the leading/lagging sign. We've been thinking about this lately, working on algorithms to do it better. But they turn out to be fairly compute-intensive, Hilberts or dsp pll stuff, or FFTs maybe, so we'll probably do that math in an FPGA.

What's your sample rate? That's a very interesting issue here.

There are lots of situations where the meaning of PF is debatable.

John

I'm using a microcontroller and its built in A/D to measure energy

You will at lest need to compute the " true rms " value of the current wave over one cycle and multiply this number by the similar voltage one.

The voltage wave could be assumed to be sine to a reasonable approximation for the purpose - then you only need to scale the peak value.

....... Phil

This is what I would recommend.

This would start working into what the customer expects, too -- do they just care about leading/lagging and harmonic content, or do they want a detailed spectrum (with phases!) of the various harmonics?

It can make a huge difference. I've scratched the surface here:

Or perhaps one should say "where one must define the meaning carefully".

The meanings in common use have certainly expanded since I was in school.

I sold roughly 12,000 16-channel power survey meters that were used for a lot of end-use load surveys, by utilities and research institutes. They sampled each channel at 26.9947 Hz.

My newer stuff, with more compute power, samples at 263.032 Hz.

One survey did a bunch of fast-food restaurants. The deep-fat friers used burst-mode zero-crossing temperature controllers for the heaters; you know, maybe 5 cycles on, 11 cycles off or whatever. There were heated (pun!) debates about what the pf might be in that situation.

John

Technique we used in the 70's to get PF - and in those days circuits were always inductive, but the basic idea will work for either leading or lagging:

Phase angle can be derived from a few op amps rather simply. Current and Voltage are put directly into two op amps with no feedback so you are using the full open loop gain of the amps (200K or more - but protecting the input with some diodes and resistors so the input doesn't exceed the supply voltages). (can also be done with logic IC's like CMOS inverters, since you just need a lot of gain to make square waves from sine waves)

Current is sensed with a current transformer on the power line to the device under test - voltage with a step down transformer or directly with dropping resistors.

You get a square wave out of both amps - the zero crossing on each is used to make a charge pump. One amp sends voltage through a resistor to an integrating cap the other terminates the charging until the next cycle. (basically an AND gate) The voltage on the cap reflects the power factor - at unity there is no charging of the cap at 90 degrees there's maximum charging.

We used a buffer amp to drive a meter to read PF in degrees directly.

"default"

Where there simply is no phase angle.

( snip drivel)

Even Wiki is way up on you.

....... Phil

My sample rate is a little over 13ksps; that's including separate voltage and current samples. I may raise it, but this might be unnecessary.

Good grief, I've done AC power meters at 27 Hz! But if you've got the CPU horsepower, why not?

John