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


"John Larkin"


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.


** But it need not be either.

Why does everyone keep forgetting this ???



(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.


** 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




.

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