Hi,
I am newby, so please bear with me...
I have a EmonTx (3xCT + Voltage) transmitting to Raspberry Pi in Gateway mode. The sketch I am using is CT123-Voltage. I have calibrated the voltage on the AC-AC adaptor to reflect the measured voltage. The AC-AC adaptor is on the same phase as CT1. Some questions:
- I initially got negative values on CT1, which I ascribed to the polarity of the AC-AC adaptor. Since I have a European plug version, I turned it around in the plug, which then made the readings positive. My logic says this is the same as switching the polarity of the DC plug, correct?
- I am getting positive readings for realPower on CT1 and CT3, but negative on CT2. I am not generating. The direction of the CT clamps is identical for all three phases, so unless there is a wiring difference in the clamps (using the SCT-013-000 clamp from the shop), it seems more likely to be a software or calibration issue.
Any ideas?
Thanks
Willem
Re: Negative power when not generating
The sign of power (positive or negative) depends on the direction of power flow and that in turn is detected by the relative phase of voltage and current. First, as the AC adapter is a transformer, it makes no difference whether you change it round on the primary side or the secondary side, so what you did is OK.
As for one CT being different, it could easily be a manufacturing error. What you can do to test this is try the same c.t. in each input, where it should give exactly the same power (both in amplitude within a few percent, and sign). If the sign changes, you must look at the software. If that does not reveal a problem, put all 3 c.t's on the same cable, when all should read the same. If one is negative, the easiest solution is label it and reverse it on the cable (but the proper solution would be to change the wires round in the plug).
Re: Negative power when not generating
Thanks for the comments Robert, but I think I have figured it out:
CT123_Voltage appears to assume that all CTs are on the same phase, and (at least for real power) the Voltage is in phase with the current, which it would normally be in a single phase setup. This also requires the output of the AC-AC adapter to be in phase with the supply voltage. This is why reversing its polarity made a difference - it changed the AC phase by 180 degrees. Apparent power will never be negative, because it is calculated from a sum of squares, so this only applies to real power.
I have 3-phase, and so I now use the CT123_3Phase_Voltage sketch, which stores voltage values and does a phase shift before applying it to calculation for phases 2 and 3. I still had negative values on phases 2 and 3, but again it turned out to be a phase error - I had them the wrong way round, and once I had the 120 and 240 degree offset phases correctly identified, all is now well.
Now I still need to calibrate phase error (and current, but that's a little harder since I don't have a clamp meter).
Willem
Re: Negative power when not generating
You didn't say you had a three-phase supply. It does make a difference, as you realise.