Olin Lathrop wrote:
> Chen Xiao Fan wrote:
> 
>> Maybe FFT is the way to go for PF analyzing.
> 
> 
> FFT would not be very useful.  You are looking for relative phase
> relationship between two signals, not the spectra of the signals 
> separately.
> 
> Think of the case where the voltage is a pure sine and you have three
> different loads, resistive, capacitive, and inductive.  If the magnitude of
> the load currents are the same, an FFT will give the same result in all
> cases even though one has a power factor of 1 and the other two of 0.
> 
> To compute power factor, compute the RMS voltage and current, and the 
> actual
> power.  Power factor is the ratio of the true power divided by the product
> of the RMS voltage and current.  You can see that this can be at most 1,
> which is what you get only with a resistive load.  The extremes are pure
> capacitive and inductive loads where the delivered power is 0 and the power
> factor therefore is 0.  Technically an active load could produce negative
> power factors, but this means the load is actually delivering power back to
> the AC line.

So calculating the power factor could be done quite easily on an 18F 
however THD would require a FFT to calculate the sum of powers at 
frequencies above the fundamental.

-- 
Martin K
http://wwia.org/
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