Hello,
i monitor the three mains with Emon Tx V3 and Rasp-Pi. I supply the Emon with AC/AC adaptor. I have the following strange problems:
- line 1 reads about -15W when there is no consumtion
- i noticed on line 2, on which the deep freezer is connected, the consumption grows about 70W when the freezer turns off an shrinks 70W when it switches on. Other consumtion on this line (lights, etc.) let the value grow as expected
Do you have any explanation for that? How can i correct this and/or how can i calibrate (necessary?) the Emon Tx?
Thank you,
Florian
Re: (Once more) Negative meter readings and strage behavíour...
So that I can better understand your installation:
Do you have a single phase electricity supply?
Where do you measure the voltage in relation to the mains distribution panel?
What loads do each of the "lines" supply?
Re: (Once more) Negative meter readings and strage behavíour...
I have a three phase supply with the AC/AC adaptor connected to phase three.
The three main fuses should carry up to 63A each from which i am far away. Actually i consume about 10 kWh per day with peak loads while cooking or washing etc.
Re: (Once more) Negative meter readings and strage behavíour...
With a 3-phase supply, only one voltage monitor and especially as you are some distance from the main fuses, then I think it is certain that you will not be able to make precise measurements. This is because the voltage drop in the common neutral conductor will change the voltage that appears on all three phases. When you switch a load on one phase, it affects the voltage on the other two phases as well as its own phase. You might like to read 3-Phase Power.
Now because of the time difference between the voltage wave that you are measuring on phase 3, and the current waves on phases 1 & 2, you cannot calculate correctly the real power (which is the value that the emonTx V3 sketch prints), you can only calculate apparent power and that will be in error because of the changing voltage of the neutral conductor. You are seeing the peculiar effect with your freezer because the motor is an inductive load and it is changing the phase angle of the current. The power you are reading on phases 1 & 2 will be wrong by approximately 1/√3 = cos(120°).
There is probably very little, if anything, wrong with your emonTx. You cannot correct this with calibration because it will change depending on how the three phases share the load and even what types of load (heaters, motors, etc) you have on each.
Line 1 reading 15 W is a little higher than I would expect, however it is normal to read a very small current when no load is connected - I think it is noise from the digital circuits getting into the analogue part.
OpenEnergyMonitor shop does not sell a 3-phase emonTx. You can use the emonTx V3 3-phase sketch (on Github, in Examples) to get a better approximation for the real power measurements on phases 1 & 2, or you could have 3 x emonTx V3, one on each phase, so that you accurately measure the correct voltage and calculate the real power correctly for each phase separately.
[Note: if you use the 3-phase sketch, phase 1 is the one where you measure the voltage, phase 2 is the one that follows next.]
Re: (Once more) Negative meter readings and strage behavíour...
A simpler version of that voltage drop story is also a common source of error even in single phase systems. With the right combo of heavy duty kitchen appliances plugged in at the same time, I can get close to a 5V drop down that leg, which represents ~2% error if your V-sense circuit is plugged in on the wrong side of that voltage drop.