Inductive cooktop gives wrong power readings

pieterdg · 2023-01-02T18:27:22.521Z

My induction cooktop seems to give erroneous readings of power use, even while turned off. I have an Siemens induction cooktop on a separate breaker. The emonpi is just connected to the mains.

Emonpi reads about 350W of ‘background’ electricity use (no big users turned on), the cooktop is at that moment turned off (no cooking, but still connected to power). But when i kill the breaker of the cooktop, the power drops to about 250W (see at around 16:45 in the image, at around 17:25 i turn the breaker back on). When i measure with a current clamp i have more or less the same readings as the ones form emonpi. (not so erroneous after all?!)
However the counter from my electricity provider is more close to the 250Wx24h. The difference between my bill and emonpi is in the range of 1,6-2 times higher for emonpi (My provider installed an analog meter, i don’t have exact numbers, but 350W during 24h, is surely not what i pay!)

I am no expert in electricity but could this be corrected in emonpi? Is this normal behavior for induction cooktops? My installation is only 2 years old, nothing fancy i believe, no exceptional appliances etc… (the emonpi is older, 2018? not sure)
I have been scratching my head over this since a while now, any help is appreciated!

(image of my emonpi dashbord, in the beginning and the end i remove the current clamp to check for irregularities, at 16:45 and 17:25 i switch the breaker from the cooktop off and back on)

image1133×650 40.1 KB

Robert.Wall · 2023-01-02T20:07:40.098Z

Welcome, Pieter, to the OEM forum.

I see you are in Belgium, what is your electricity supply? Is it 230 V, and is it single phase or three phase? Are you using the ac-ac adapter (as well as the 5 V USB power supply) with your emonPi?

Have you updated the software in your emonPi recently? I’m not sure which version is the present standard, I’ve been running the “CM” version for about the last 2½ years. The reason for asking is I’m not sure when the ability to make fine adjustments to the calibration of the “emon” part of the emonPi became available, or even if it has. Because I think your problem is most likely linked to this, because induction cooktops are difficult loads to measure.

I am basing this on

pieterdg:

When i measure with a current clamp i have more or less the same readings as the ones form emonpi.

which I interpret to mean that the current read by a clip-on clamp ammeter agrees reasonably closely with the current read by the emonPi.

pieterdg · 2023-01-02T21:05:43.655Z

Hello, thanks!

Yeah, i am in Belgium. Its 230V single phase, there is 3 phase in the building but my apartement only has 1 phase.
I have the AC-AC adapter, from the emonshop back in the days, but i dont get a reading from it anymore, at least the vrms reading is zero all the time. Its plugged in, maybe the arduino still reads it, but de raspberry doesn’t
I could do fine adjustments in the emonhub config file, i believe. But i would not call it ‘fine’, its more a factor 1,5-2

To be precise, i had to measure the current with a clip-on clamp ampmeter and multiply it by the volts, it would give my a power that is only a few percent away from the emonpi reading.
I cant read current directly from the emonpi right? only the power is exposed, i believe?

But just after the breaker for the cooktop i measure a constant 0,6 amps while its turned off. About 130W, i that possible? But only apparent power, there is no real power component to that, since i don’t seem to pay for it (i might be using the words wrong, i have been reading the emon resources )

Robert.Wall · 2023-01-02T23:19:23.086Z

pieterdg:

but i dont get a reading from it anymore, at least the vrms reading is zero all the time.

I think this could be your first problem. If your a.c. adapter has failed, your emonPi will be using a default 240 V and multiplying by the current to give you an approximation of apparent power - exactly like your ammeter does. If you have a meter that can measure 12 V a.c. check the output of your a.c. adapter.

Note that for the emonPi to use the a.c. adapter to measure the voltage, it must be powered at the same time as the emonPi or before, because if no voltage is detected when the Atmel '328P “emon” part starts (almost immediately after power is applied, a long time before the Pi has finished booting), it will use a constant 240 V and guess the apparent power.

The other place where a large error could creep in is due to the way that your induction cooker switches on and off in bursts to regulate the power, and the way your “emon” software works by taking discrete samples for 200 ms every 10 s. What this means is if the emonPi samples at the same time as your cooker is taking full power, it will assume it takes full power for the next 10 s. Likewise, if your cooker is taking almost no power when it samples, it assumes no power for the next 10 s. The software I am running (“emonPiCM”) monitors continuously, so will always correctly read the average power over the 10 s. If you have only the emonPi, you could change to this. If you have any other sensor nodes feeding by radio into your emonPi, then those will need new software too, because the format of the radio message had to change to allow the emonPi to work.

pieterdg:

But just after the breaker for the cooktop i measure a constant 0,6 amps while its turned off. About 130W, i that possible? But only apparent power, there is no real power component to that, since i don’t seem to pay for it

This value is a bit higher than I might expect, but not very much higher - given what I think might be happening. And it’s entirely reasonable that it is almost all reactive (not active) power.

pieterdg:

i might be using the words wrong

No, that is correct.

pieterdg:

I cant read current directly from the emonpi right? only the power is exposed, i believe?

That is correct. Current is available inside the ‘emon’ part, but in your emonPi software version it will not be not passed down to emonHub and emonCMS. In my version, the current can be made to appear on the LCD display and in the monitor in Admin to help with calibration, and it’s possible to fully calibrate the “emon” part as you would an emonTx.

dBC · 2023-01-03T20:51:23.556Z

pieterdg:

But just after the breaker for the cooktop i measure a constant 0,6 amps while its turned off. About 130W, i that possible? But only apparent power, there is no real power component to that, since i don’t seem to pay for it

My induction hob draws 0.66A while just sitting there waiting for someone to push a button. You can see a picture of the waveform in this post.

Without that green Voltage trace to reference it against, there’s no way to know how much real power is flowing. In this case it’s about 3W Real and 165 VAR Reactive, plus a little bit of Distortion power.

pieterdg · 2023-01-08T18:14:56.742Z

dBC:

My induction hob draws 0.66A while just sitting there waiting for someone to push a button. You can see a picture of the waveform in this post.

Yep, we seem to have the same numbers! mine is a 4-burner build-in cooktop.
I had no idea it could be this much. And i was getting afraid emonpi would not be able to manage this.

Robert.Wall:

Note that for the emonPi to use the a.c. adapter to measure the voltage, it must be powered at the same time as the emonPi or before, because if no voltage is detected when the Atmel '328P “emon” part starts

I could not believe it at first, i thought the ac adapter would only make a few percent of difference…
But I managed to get a reading from it again! Unplugging the whole thing and plugging it back in, in the right order seems to have done the trick!! (simply restarting the raspberry pi is clearly not enough )

(i am still getting used to these concepts Real vs Reactive etc…i must have been sick that day at school )

So after 2 days of testing, i can confirm that i have a difference of about 3,5% compared to my utility meter. Seems about right!

Thanks a lot for all the help!!
(oh and the CM version is a bit over my head, maybe someday!)

Robert.Wall · 2023-01-08T20:40:13.168Z

pieterdg:

simply restarting the raspberry pi is clearly not enough

Correct - there are two processors in the box, and restarting the Pi doesn’t restart the “emon” Atmel 328P that measures the voltage current and hence power. It’s in the FAQ, amongst many places.