Calibrating Vcal for 3 phase Sketch in UK


Thanks to help received in another thread I have my 3 phase monitoring working.

I am now trying to calibrate. I have a Fluke meter and that shows the voltage to be around 254V which strikes me a high as I understand UK voltage should be 230v (+10% -6%)

To get the sketch to show a similar voltage I end up with Vcal at 283.9.
Is this realistic or should I try to get my meter calibrated? I have another low cost meter and that shows 250V.

(As an aside, how close should you try to get this? Within a Volt? The meter is fairly steady but the volts displayed in terminal vary by about a volt)

How important is it to get this right? Am I correct in assuming I need to get Vcal right BEFORE I adjust Ical
and Phasecal?



Historically, the nominal UK voltage is 240 V, and as I’ve written in the calibration instructions (I think! No, it was the a.c. adapter test report.), I can’t see it changing any time soon. So anything under 254.4 V is OK. I’d be inclined to believe the Fluke.

You want to get it as close as you can. Obviously, an error in the voltage will give you an error in the power, and an error in the current will add (or subtract) from that. If you’re happy with a 5% overall error, you only need to get voltage and current within 2.5% each if the errors are in the same direction. Within 1 V is probably as close as you’ll get - unless you happen to choose a time of day when the voltage is particularly steady.

You might need to set the voltage calibration again after adjusting PHASECAL - for reasons explained in Learn, so I’d get it close and move on to current and phase, then recheck it.

Many thanks. After a little trial and errror I have got everything set apart from Phasecal3 . This is now down to 0.0. In the comments it states:-

If it not possible to keep Phasecal2 and Phasecal3 within the range 0 - 1 , it will be
necessary to change “#define PHASE2 6” and/or “#define PHASE3 14”

I understand that I will need to alter PHASE3. What is not clear to me despite reading learn and the sketch comments a number of times (Some of which I am beginning to understand) is should I increase or decrease PHASE3 which is currently 17 as I am only using 3 CT’s.



The voltage wave you measure and multiply by current to calculate power is synchronous (more or less) for phase 1. In order to have a voltage wave that is synchronous with the second and third phases, the voltage wave is recorded in an array, and used after a delay. The numbers attached to PHASE2 and PHASE3 define that delay in coarse steps (they are the index into the array and point to a pair of samples), and the corresponding PHASECAL interpolates between the two voltage samples. There’s a detailed explanation of this algorithm in the Learn section. If PHASECAL needs to go outside the range 0 - 1 (0 or 1 means it uses just the end samples exclusively, 0.5 means half way between the two), it’s better to move to the next or previous pair of samples, by changing PHASE2 (or 3).

The value you need for PHASEx and PHASECALx depends entirely on the errors in your particular a.c. adapter and current transformer, and these depend on the voltage and current that is being measured. So without having your units to test, I can’t tell you the numbers that you need. You’ll just have to work by trial and error until you get the closest you can to unity power factor with your test load.