Power Factor of Non-Sinusoidal Waveform? Possible using an oscilloscope? - Page 1
Forum section: electronics / projects

author: guscrown
author_rank: Regular Contributor
timestamp: December 16, 2012, 06:21:18 am
content: I'm trying to calculate the Real Power consumption of my device in Stand-by mode, but to do that I need to figure out its power factor due to:Real Power = Vrms x Irms x PFNow, my device like many other IT devices doesn't have a perfect sinusoidal current curve, so I can't just calculate the phase shift can do cos(theta).I read through some documentation for an Arduino application and apparently you can calculate Real Power by doing several instantaneous samplings of current and voltage and multiplying them and just get the average. So I took out my scope and decided to get 1000 samples.Here is the graph:I exported this data to an excel sheet and got the following values:Vrms = 118.96V (RMS)Irms = 0.02024A (RMS)S (apparent power) = 2.40792 VAP (real power) = 0.93713This gives me a power factor ofPF = 0.93713/2.40792 = 0.38919 <-- This is a very low power factor.I used my kill-a-watt device and it tells me my power factor is somewhere around 0.6 average.Can any of you guys with more experience point me in the right direction. What am I doing wrong?

author: jahonen
author_rank: Super Contributor
timestamp: December 16, 2012, 09:39:13 am
content: I have done this several times in the past (although I didn't try calculating true power manually using data samples, though), using Agilent MSO6034A scope. I see you have similar scope. Have you compared what your scope calculates internally for real power, i.e. set math trace so that it shows i(t)*u(t) and then use a math function to calculate an average from the math trace. This of course requires that you get the current waveform scaling correctly. Here is an example for such a measurement:Also, another important point is to set up the horizontal time/div so that you get integer number of mains periods to the screen. For my example time div is set to 8 ms/div so that total time per screen is 80 ms which is just 4 periods of 50 Hz mains (20 ms period). If that is not correct, it affects average and RMS calculations, especially if you do it manually without paying attention to the period.Regards,Janne

author: penfold
author_rank: Frequent Contributor
timestamp: December 16, 2012, 04:48:28 pm
content: How are you making your current measurement? On your scope trace it looks to have a touch of vertical offset, could that perhaps account for your phantom amount of apparent power?Regards,Dan

author: guscrown
author_rank: Regular Contributor
timestamp: December 16, 2012, 07:03:54 pm
content: Quote from: penfold on December 16, 2012, 04:48:28 pmI am using a current probe on my Agilent scope.

author: penfold
author_rank: Frequent Contributor
timestamp: December 19, 2012, 12:44:04 pm
content: Is it a hall effect type? I've known some very accurate ones prone to enough drift to ruin some measurements such as that

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 19, 2012, 03:52:40 pm
content: I have used the TCP202 from Tek, and these are indeed very prone to drifting offsets, there's a reason there's a zeroing knob on them. But on the, say, minutes scale they are stable enough for these kinds of measurements.If you want really accurate measurements, you better use a power meter, I used Yokogawa ones in the past. But they are really an odd piece of equipment for the average hobbyist

author: penfold
author_rank: Frequent Contributor
timestamp: December 19, 2012, 09:43:50 pm
content: I was basing mine on a LEM PR50, rather prone to drifting, its not a great deal of drift when you're getting up to the 50 amp limit of it but it a massive pain at the lower end of the range.Away from the immediate problem of stability I was suggesting more that the probe may not have properly been zeroed (or not very well zeroed or not a terribly good probe or had drifted off zero before the reading was taken etc).Even talking relatively low currents in the order of a few mA, over a cycle would be enough to weigh strongly against the peaks of 100mA for a fraction of a cycle and would cause enough positive current during the negative half cycle or visa versa to really mess up the p.f. calculation

author: Online Simulations
author_rank: Newbie
timestamp: June 03, 2013, 09:04:12 am
content: Active power is actually offset (DC value) of current and voltage product. If current and voltage are sinusoidal then your formula works.You can simulate it inhttp://www.cirvirlab.com/simulation/active_power_reactive_power_and_apparent_power_online.phpOtherwise, you can't calculate it as product of effective values and power factor, since for non-sinusoidal waveforms it is very hard to define power factor.

author: Harvs
author_rank: Super Contributor
timestamp: June 03, 2013, 11:25:21 pm
content: Quote from: Online Simulations on June 03, 2013, 09:04:12 amIf anyone has a good reference for that does define power factor for non-sinusoidal voltages and currents I'd like to read it. I tried to find an adequate reference some time ago but failed. For example how would you define power factor of a CFL being driven by a square wave inverter? My intuition would tell me that all power being drawn is inherently real, albeit almost entirely harmonic currents, but a tek scope we were using at the time that had all the power functions built in had the power factor as about 0.4.

author: penfold
author_rank: Frequent Contributor
timestamp: June 04, 2013, 08:05:46 pm
content: I seem to remember there is a reasonable discussion from the perspective of EMC on power factor in generalised form in 'EMC for product designers' by Tim Williams. Power factor is a funny term because its meaning has been stretched, it really only applies to linear systems. Its calculation can be extended for complex input waveforms (those with many frequency components) but it all falls apart when you start getting more current harmonics than you do voltage ones.

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 04, 2013, 08:42:49 pm
content: Quote from: Harvs on June 03, 2013, 11:25:21 pmDid you look at the obvious reference:http://en.wikipedia.org/wiki/Power_factor