Potential Scale Factor Error/Configuration Problem
I have being using IotaWatt since installing it in February and things have been going fine. Today I noticed a major scale factor problem Today, when using the dryer. I am seeing 9000 W from the dryer when there is only 5700 W total reported for the mains connections! Obviously something is wrong here. Also, I am almost 100% sure that the power numbers used to make sense so something has changed.
First my system:
Standard US split phase 220 Vac
ECS25200-C2 current transformers on mains
SCT013-000 current transformer on 220 Vac dryer outlet setup checked “Double”
Now some data. Using outputs assigned to current for these inputs I am seeing:
Main 1: 27 A
Main 2: 21 A
Dryer: 38 A
Given normal dryer currents and a 30 Amp 220 Vac breaker circuit I don’t think 38 A is correct for the dryer current. Also, current in a 220 Vac load can’t be higher than both mains current readings with only standard utility interface/no solar, etc. It is as if the IotaWatt is doubling the measured current figure.
Because the dryer is a 220 Vac load, the setup option “Double” is checked. If I set the channel to not double the current (uncheck the box) the historic data from before the setup change doubles to 76 A and the present current stays as 38. When I put it back to “Double” checked the old current goes back to 38 and the current from when “Double” was not checked goes to a believable 19 A.
I thought the “Double” check mark would only change the power calculation by a factor of 2. That is, power = 2voltagecurrent. I don’t see why it would change the measured current value at all. Whether the “Double” check box is checked or not checked and whether it should be or not I would assume that the Amps reading would always be the same. Am I missing something here or is there a bug in the FW?
I note in the log that 3 weeks ago IotaWatt FW on my device updated. Line in log says:
IoTaWatt 4.x, Firmware version 02_04_02 to FW
and the previous verions was 02_04_00. There are no error messages in the log.
Is it possible that FW 02_04_02 introduced an error in the current reading when the setup option “Double” is checked?? If not that, what could be going on here?
Bingo…you got it in one. The loss of resolution of a 30 A breaker circuit being measured with a 100 A CT bothered me so at some point in the past I changed it from 1 wire/one turn to 1 wire and the return 220 Vac wire in backwards to give a net 2x gain on the current value. I feel so foolish that I did not update the setup accordingly right away…and then forgot I made the change.
Even so, I am still confused about the Setup “Double” option check box. The pop up info help on that check box says “Double measured power. Use with balanced 240 circuit with CT on one conductor.” So, I assumed that meant that it doubles the power calculation and does not change the reported current. However, my experiment on changing the setting yielded changed current readings. Not only that, but changing the “Double” setting changes the historically saved current readings.
See attached graph picture “Dryer Current Graph.JPG”. Before the change on the left it shows 38 Amp and after the change shows 76 amp. Over that entire time while recording that data the setup had “Double” checked. Looking at this historic data with “Double” checked these current values read 19 and 38 A respectively. Due to scale/sampling the graph looks ragged, but zoom in a little and they go to these levels flat topped.
So, what does “Double” really do? From the behavior it looks like it is a live value and affects the interpretations of the saved current data old and new.
Going forward, when 2 turns are in a CT would it be best to change to a custom CT and set the turns ratio to half the value so that reading current gives the right value? Then, for 240 Vac circuit check the “Double” value so that power is correct?
Even though solved, just to be complete I attached the requested screen shots.
Everything is “live”. The IoTaWatt doesn’t have any institutional memory. It assumes everything in the datalog was as it is now. I’d like it to be smarter, but it lacks the neurons (processing power/memory).
So what the “double” really means, as implemented, is “double the reference voltage” - which of course doubles the reported power. Now as you look at history it gets more complicated. IoTaWatt saves two values directly for each power channel. Cumulative Wh, and Cumulative VA. It doesn’t save the voltage used for that particular VA, instead the corresponding voltage channel in the same datalog record has the voltage.
If you want to get Amps, the IoTaWatt fetches the particular average VA, then divides it by the voltage in the present corresponding voltage channel. If the power channel is designated as “double”, it divides by twice the corresponding voltage. This approach will give correct Amps on a “double” channel, and if you think about it, when it gives you that doubled Amps historically, it’s because as far as it can tell, that is the Amps because the wire was doubled.
Not a perfect approach, but an efficient way to squeeze a decade worth of history into an 8GB SD card.
