apparent .vs. real power - appliance standby oscilloscope chart analysis - Page 1
Forum section: electronics / general

author: tester
author_rank: Contributor
timestamp: February 23, 2023, 11:00:41 am
content: Hi,I have appliance, on whichhttps://www.emporiaenergy.com/how-the-vue-energy-monitor-worksreport ridiculously high standby.Factsstdby. real power measured on utility company meter is negligible (due to p. fluctuations unmeasurable). Method - hob breaker on/off, diff.Emporia Gen2 Smart home monitor reports 70Wclamp meter Bside ACM91 reports 342mAel. system is European 230V 3-phase/line distributionFirst I did attribute this to extremely bad power factor cos(Φ)What I measured on dedicated power line for the hob with Micsig oscilloscope is 90W RMS and phase shift 32° (attached image)90*cos(32)=76W - thats ± what Emporia shows.What the problem is CERTAINLY not- other appliance on the measured line- presented facts are incorrect- current clamp orientation, sign (+,-) on the Φ - degree angle. I always boils down to PF=.86 one way or anotherQ1. is my oscilloscope measurement flawed ?Q2. any idea whats going on here ?Many thanks in advance to anyone who willing to go down this rabbit hole.

author: richard.cs
author_rank: Super Contributor
timestamp: February 23, 2023, 11:52:49 am
content: When the waveforms are so far from sinusoidal it's not especially useful to measure the angle. Power factor is only really equal to cos(phi) for sinusoidal waveforms, a more useful general definition is power_factor=Preal/(Irms*Vrms)Assuming your red math trace is instantaneous I*V then true power is equal to themean(not rms) of the red trace, over an integer number of cycles. I would not expect this to correspond exactly with any phase angle measurements.

author: tester
author_rank: Contributor
timestamp: February 23, 2023, 01:40:44 pm
content: yes - the red trace is instantaneous V*AI've made one more measurement with Micsig including the mean(V*A), centered the red trace on Y-axes zeroResult 64W.Thats pretty far from expected (and likely true) <~3W stdby.The Hob stdby. I-curve in its essence is not THAT far from SIN waveform. What is measured by the utility meter (sum of all appliances) is quite far from SIN, because lot of other switching power supplies, next to none pure resistive load.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 23, 2023, 01:42:57 pm
content: AC theory is complicated, and easily misleading.Forget all phase angles, RMS values and other mental models. They are tools, not the truth. One needs to know when they apply.Thankfully, real world is simpler than that. P = U*I is the only fundamental physical truth here, and it always applies, at any point in time. in other words, multiply the U and I traces for instantaneous power trace. Average power is then simply the average of that. Do this at high enough sample rate and you have a power meter that shows correct values."High level abstractions" are useful when they aresimplerthan the underlying (more widely applied) basic math, but in AC line theory, this is usually not the case; the abstractions are harder to understand and get right. This is not to say they are useless. But they have confused shit out of beginners and even more experienced people. In the era of cheap microcontrollers which can easily perform tens of thousands of analog-to-digital conversions and calculations per seconds, there is no reason to abstract things to phase angles and RMS valuesjust to reduce number of calculations.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 23, 2023, 01:50:19 pm
content: Quote from: tester on February 23, 2023, 01:40:44 pmAre you sure your units are correct (probe settings, etc.)? Current trace looks suspiciously bad to me.

author: jonpaul
author_rank: Super Contributor
timestamp: February 23, 2023, 01:58:38 pm
content: the monitor devices curr3nt sense transformers are not précis and dépend on the installation technology.The scope waves and calculation are not accurate.power = energy consumed per second eg 1 December is a joule per secondapparat power = Vrms * IrmsPower factor = power / apparant power.eg resistive load, PF 1.0 real= apparant = V*Icapacitive load PF = 0,Most electronic devices have either capacité input rectifier and filter PF ~0.7 or PFC PF 0.99Standing power is hard to measure as it can be just a fraction to a watt.jon

author: tester
author_rank: Contributor
timestamp: February 23, 2023, 02:34:32 pm
content: It absolutely makes sense what you say. And its good to realize it.In this case, ∫(V*A) over one AC cycle should equal true power, assuming stable V,I curves (and that IS the case - I have persistence ON, they don't fluctuate)The cardinal problem is still - why such such discrepancy between measurements with 2 devices (Bside clamp A-meter, Micsig oscilloscope - they agree within 20mA) and the reality. Its not decimal point (multiplication factor on oscilloscope for probe) problem either. I double checked on other circuits with known loads that I'm setup correctly.It also can't be that meters are right, because there is no waste heat produced by the hob in stdby. - everything perfectly cold. Its modern German product. They are very Eco minded. and I'm pretty sure they would not let product with 2x70W stdby. be sold. Also utility meter suggests negligible stdby.I struggled with this for several days before I posted here and I'm still

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2023, 03:55:43 pm
content: Can you describe precisely how your oscilloscope is connected to measure current and voltage?It would be nice to work all the way through the problem, which would involve deskewing your probing system and measuring interference and noise with a resistive load, but I can probably tell you how it will turn out right now. Here are my guesses:Your utility power meter is right, your power monitor has some flaw or limitation that is giving you the erroneous result.Your oscilloscope measurements have a similar flaw, probably either because you are using the CT from the power monitor or one like it. The types of flaws that you may have include skew (time difference) and frequency response differences between V and I measurements.Your appliance has a nearly pure capacitive load in standby and the recirculating or 'reactive' current is indeed ~350mA as you measure it. However, the phase angle of the current is actually very close to 90 degrees instead of what you measure.Where things turn into a mess is when you have a noisy, distorted mains waveform.Siwastajawas right earlier when he said that there's no point in using AC theory tomeasurepower when you have the capability to rapidly measure V and I, but the two problems with that are first you have to actually have the capability to actually do that properly and second it doesn't help you understand the reaction of the circuit. In this case, your distorted and noisy mains waveform will have a large number of different frequency components and the capacitive load will react by admitting a current that is ~90 degrees out of phase (leading)of each frequency component.That means that the phase difference in absolute time units, rather than degrees, will be different for each frequency.Consumer-grade power meters like the Kill-A-Watt that use a resistor for current measurments avoid a lot of these issues and generally work pretty well with situations like yours. It looks to me like your power monitor system is struggling for some reason. Is that one appliance the only one with an issue? If so, the first thing you might check is whether my assumption that its standby load is mostly capacitive is actually true.

author: tester
author_rank: Contributor
timestamp: February 23, 2023, 05:11:51 pm
content: Quote from: bdunham7 on February 23, 2023, 03:55:43 pmYou nailed it.What a blunder on my sideSo -what happened is, my voltage probe was hooked on conveniently exposed wire on L2 instead on L1around which was clamped current probe. Just overlooked one position in the line of breakers..L1-L2 natural phase shift played in favor of the appliance PF, bringing it to rather expected 0.86The real PF is nearly 0 in stdby. and the angle Φ~=90°When hooked correctly, theoscilloscope had hard time determining the Φ because its almost perfect 90°.BTW - itsinduction hobThe Emporia monitor is using clamp meter (but is also monitoring all 3 line voltages) and iseither not at all able to deal with phase shiftor certainly not with such extreme angle. I will look at its limitations in detail later.The Bside is also clamp meter and has no way to determine phase shift, because doesn't see voltage and the fact that current just flows between capacitors, inductors and the grid back and forward..Problem solved. You have beer on me pleaseBTW - richard.cs also nailed it with mean instead of RMS. You can see in the attached pic 3.9W mean power.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 23, 2023, 06:07:22 pm
content: Yeah, RMS is like absolute value in a sense it ignores direction. Because reactive power is all about using energy, storing it, and then returning it back, average power is the correct metric. I think it's quite intuitive to look in time domain, positive power and negative power alternates.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2023, 06:29:27 pm
content: Quote from: tester on February 23, 2023, 05:11:51 pmDon't let yourself be fooled into thinking that because your result appears to be correct that your testing methods or theory are correct!It may be an 'induction hob', but that doesn't mean it necessarily presents an inductive load to the mains, especially in standby. For the standby to be accounted for by an inductor and yield that ~350mA current at 90 degrees lagging would require approximately a 2.2H inductor. I suppose that could be the primary of a transformer with an open secondary, but that would be a pretty inefficient design for a heating appliance. Of course 350mA of standby current seems pretty large in any case, so anything is possible.I would challenge you to reexamine your original assertion that it doesn't matter which way the current clamp goes. And your current trace still looks pretty terrible, but at this point I couldn't tell you why.

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2023, 06:36:25 pm
content: Quote from: bdunham7 on February 23, 2023, 06:29:27 pmI'm confused what you are trying to say here?When the hob is on standby the heating elements are turned off and not powered, therefore the question is not about inductors.Most likely the electronic control circuit contains a microprocessor and is powered by a simple capacitive dropper supply. The standby power consumption is going to come from that supply.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2023, 06:54:27 pm
content: Quote from: IanB on February 23, 2023, 06:36:25 pmIf that is directed at me, what isn't clear? We seem to agree?

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2023, 07:39:18 pm
content: Quote from: bdunham7 on February 23, 2023, 06:54:27 pmFor the main part yes, but this comment had me scratching my head:Quote from: bdunham7 on February 23, 2023, 06:29:27 pmI saw no suggestion in this thread that the standby current would be connected with an inductor, nor that the induction hob might imply an inductive load of 350 mA. Hence my confusion.The important aspect of an induction hob is that unlike a simple heating element, it will have electronic controls and therefore will have a non-zero standby current to run the electronics. As compared to a simple resistive heating element where off means off and the standby current would be expected to be zero.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2023, 07:59:09 pm
content: Quote from: IanB on February 23, 2023, 07:39:18 pmIn the OP's reply #8 he says:Quoteand then he posted screenshot of his 'corrected' measurements that show the current lagging the voltage by ~90 degrees. This to me implies that he might have the mistaken impression that the standby load is not capacitive in nature.I'm pointing out that a) it being an induction hob isn't relevant and b) he may still not be getting an accurate picture of the situation due to the current clamp orientation, among other things.

author: tester
author_rank: Contributor
timestamp: February 23, 2023, 08:54:24 pm
content: The puzzle is solved, but if anyone curious, I made 4 new measurements- 2 measurements for L1, current clamp either direction- 2 measurements for L2, current clamp either direction- 5ms timebase instead 2ms- 0.5s persistence- cursors on V*A trace- checked hob L3 circuit, nothing thereFew observations are sort of interesting1, With 5ms timebase the scope identifies the Φ angle much more reliably then on 2ms timebase. If +-7 degree fluctuations can be called reliable. But for our calculations its largely irrelevant as mean(V*A) is the way to get the real power2, I hear the relay click on the hob when power is set and pot is present => the inductors or their power regulation circuits aren't active until then.3, both L1, L2 have current flow back and forth in stdby. One would assume UI would be powered from one only.But none of above concerns me much. In next days when time permits will check the Emporia monitor limitations and once will post review on YouTube.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2023, 10:03:04 pm
content: So you have current on L1 and L2, which seems to indicate device is using the ~400V phase-to-phase power instead of L1 to N? Can you check for current on the neutral to verify that?Now if that is the case, I'm assuming from what you've posted so far that you are measuring voltage L1 to N. If that is all true, then your V*A=W calculation is badly flawed because you are using the wrong voltage and phase.

author: tester
author_rank: Contributor
timestamp: February 24, 2023, 01:13:48 pm
content: The hob is drawing from L1 or L2 against neutral depending whether right(L1) or left(L2) side is used.The apparent V*A power on L2 remains the same regardless whether hob is in stdby. or right side is in use.Beside that, I've newly installed mini solar system on terrace - that's why I dove in these details and measurements and got Emporia energy monitor in first place. But so far - it doesn't look very good for Emporia. Pretty much unusable.Even on main L1 line coming to the house, Emporia is reporting RMS power value, not the mean.When solar kicks in, I'm pushingtothe grid 100W after satisfying my appliances consumption. Emporiahttps://www.emporiaenergy.com/how-the-vue-energy-monitor-worksreports consumption 300WWith my type of loads - easily 300-400W difference real .vs. apparent power.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 24, 2023, 03:25:14 pm
content: The current you are seeing is likely the capacitive current of the X capacitors in the line EMI filters.

author: f4eru
author_rank: Super Contributor
timestamp: February 24, 2023, 09:27:19 pm
content: 1) Calculating a RMS calculation on instant power makes no sense and has no physical meaning.RMS calculation is basically a substitute for a power measurement, when you have only current, or only voltage, and you make assumptions.RMS is an extrapolation that gets meaningless very fast when your load is not resistive.2) When measuring real power on very low power factors, you very quickly get to the limits of of the test setup, be it power meter, or oscilloscope.Any small phase shift on the current and/or voltage measurement will put a huge error to your measurement.Current clamps typically have some phase shift built in, and will scramble your measurement.A good way to check that is to measure a purely resistive load, and check that Q~0, then use a purely capacitive load (film capacitor without bleeder for example), and check that P~0.The deviation of phi from 0/90° will show the quality of your measurement setup(Assuming pure loads)