Power factor/ frequency / true rms measurement of a 230V AC 50hz signal

Start by not writing this like a text message - use grammatical English.

Then tell us what you actually need to accomplish.

Its called a earch engine. Learn how to use it.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

With questions like these, are you sure you're a "engg final year" student? (by-the-way, or BTW, "Engineer" is spelled with one 'g') These are fundamental principles. Maybe you should have spent more time listening to the instructor rather than texting your buddies. Power factor should be defined in your "final year" text. Figure it out or don't bother trying to be an "engg"...

Dave

If you still remember the way to high school try to ask the math teacher about "vectors".

HTH

Stanislaw

** Maybe YOU need to go look up the definition of " power factor " - Mr Flatto.

Cos vectors have NOTHING to do with the task the OP has set himself.

........ Phil

First of all, I would like to start by apologizing for my casual use of language, I always thought language was just a tool of communication but if the way I am using it is offending somebody then I am really sorry for that. Anyway, I am reframing the problem I am facing.

Basically I am set out to build a power analyzer, which will include :-

-1 phase supply monitoring-

-Surge detection capabilities of the order of 10 microseconds

-Data logging at configurable intervals

-Current harmonics distortion calculation

-Measurement of parameters -True rms I & V, KVA, KVAR , cos(phi)

I need to find out how to measure the power factor from the AC mains supply using a DSP microntroller and MATLAB for demonstration purpose. So far I have reached a stage where I can acquire data using a conditioning circuit which includes a CT and a PT, followed by Instru. Amp. I tried looking for algorithms for measurement of these factors over the internet but didn't get any threads on the PF front. I would be really grateful if somebody could help me out with this. Thanking you in anticipation.

Chetan.

If you periodically, simultaneously digitize the instantaneous voltage and current (or nab them sequentially closely in time, 10's of microseconds apart) and scale each into a signed integer or float, you can multiply sample pairs to get instantaneous power points. If you average or lowpass filter these, you get true, signed power in watts.

If you then separately compute the RMS voltage and the RMS current, and multiply them, the result is apparent power, in volt-amps. Power factor is then just true power divided by apparent power. Note that we lost the leading/lagging phase information here, so we know the pf but not whether it's in the capacitive or in the inductive sense.

(To get RMS volts, just square the voltage samples, average a bunch of them, and square root the average. Ditto current.)

A more fun way to do this is to do real power like in paragraph 1, and compute imaginary power by phase-shifting the voltage waveform 90 degrees before multiplying by the current samples. So we get signed real power and signed imaginary power, and we can compute the true phase angle vector, hence lead/lag power factor. The 90 degree shift can be done in a DSP or FPGA as a Hilbert transform, as an IIR allpass network simulation, or you could be really tacky and just use an integrator.

All that's basic. What's interesting is implementing it with any sort of accuracy. Doing this the most obvious way will tend to be mediocre. Getting as good as a $20 mechanical electric meter is hard work.

The 10 usec transient thing is another issue, as is harmonics.

John

Since voltage and current are not always sinusoidal, this strikes me as a difficult project. I would guess that you are also going to need some sort of high-power, low-resistance, precision resister, like a short steel bar or something. In addition, I suppose you would need an expensive A/D converter and timer.

My advice would be to select an easier project.

"John Larkin"

** But it need not be either.

Why does everyone keep forgetting this ???

** Computing cos(phi) can be easily done by comparing the time interval between consecutive current and voltage maxima (or minima ) - but it is important to know that the current waveform is not a distorted one.

Comparing zero crossings would result in erroneous readings with rectifier /capacitor loads.

...... Phil

P'eff' = VxI cos(angle) reminds me of vectors. How to calculate other derivatives of this is trivial.

Which, nowadays, it usually is.

The classic steady-state definitions of things like power factor become problematic with "modern" loads. Back when I was in the power measurement business (alas, 60 Hz eventually becomes boring) I had debates with users about the power factor of zero-crossing triac heater controllers. The load is resistive, and while the triac is on the pf is 1.00, but the triac cycles on/off in bursts, so longterm the pf seems to be below unity. When they argued about cos(phi), it got messy. True power/energy can be computed unambiguously, but other measurements can be messy to define.

The line voltage waveform is usually pretty much a sine. If it is, the only component of the current waveform that produces true power is the

50/60 Hz fundamental. So I guess we could always extract the 50/60 Hz component of the current waveform and compare that to the voltage waveform to get our best guess as to the sign of pf, if indeed we agree that it has any meaning for nasty waveforms. That's exactly what the Hilbert thing will do.

John

I did a decent residential meter design using a manganin current shunt, a couple of opamps, and a 90-cent MC68HC05 uP with onboard mux'd 8-bit ADC. It passed the ANSI C12 electric meter accuracy specs.

This sounds like an excellent project to me. Mediocre proof-of-principle results are easy to come by, and there's fertile ground for insights and improvements.

Oh, steel makes a horrible current shunt.

John

Can somebody explain me what this steel current shunt stuff is ?? And how is it related to the topic?

Also .. I guess I can measure power factor by a method I thought of. Of course, based on only on the principles that I know. In MATLAB, the stepped down and conditioned signal is aquired and stored in an array. The voltage and current arrays are then phase plotted and the phase plot is stored in another arrays. Then the difference between the fundamentals of these are phase plots is calculated. This difference when operated with cosine function should give the normal PF. Please suggest any inconsistancies or flaws in the method. As for the implementation on the dsPIC itself I think the same principle can be modified somehow and the task can be achieved.

"chetanthegreat"

** For f*ck's sake, GO LOOK UP the **GENERAL** definition of Power Factor.

It has got NOTHING to do with phase.

First of all, you gotta KNOW what you are trying to measure.

PLUS:

Try reading the info already supplied AND this URL.

Note - the bit about the amplitude of the fundamental frequency of a square wave being 0.9 of the peak is wrong - that is the rms value of the fundamental.

...... Phil

Yah, I'm not surprised--that was just off the hip. With a 3-minute search, I found an example shunt at:

I wonder if you have to rectify the current first? Oh, never mind. I think I'm getting to deep into this :> :>

How would one measure true power, though? Maybe they sometimes do it based on heat. Here's something I found on the internet:

"A thermal converter consists of a resistive heater in close thermal contact with one or more thermocouples. When current flows through the heater, the temperature rises. Thermocouples give an output voltage proportional to the temperature difference between their junctions, in this case proportional to the square of the current, and so make suitable transducers for the construction of thermal wattmeters."

In any case, I imagine you could get a lot of information by simply putting "wattmeter" into Google.

You can buy a watt meter now days, by the way, for about $25.00.