Hi, I'm trying to better understand what is happening when using the emonTx on 3 phase systems:
3 Phase 208V L-L (120 L-N)
3 Phase 277V L-L (480 L-N)
The setup uses an emonTx with a slightly modified emonTx_CT123_Voltage sketch. The Voltage sensing uses an outlet connected to the 120V line to neutral which the AC sensor is plugged on. CT 1,2,3 each have a SCT 200A connected to each leg.
What I don't understand is why each CT needs a different calibration than what I had originally found using my single phase test bench. After all isn't the CT measuring the current in the wire, irregardless of whether it is single or 3-phase !?
I know Robert has extended this sketch to "emonTx_CT123_3Phase_Voltage", I gravitate towards this in the future, but first I need to understand what is going on the above!
Thanks!
Re: Understanding 3 Phase monitoring with emonTx sketch
You shouldn't be needing very different calibration coefficients. How different are they? Each will be different of course because of tolerances in manufacturing the c.t. and the burden, but they should be within a few percent. You can always put the 3 c.t's on the same wire to compare and calibrate.
Have you read "3-Phase Power" in Building Blocks? If you are reading the current and voltage on different phases, you can only measure VA (apparent power) and then it's only a 'best guess'. It makes no sense whatsoever to look at real power.
Re: Understanding 3 Phase monitoring with emonTx sketch
Thanks for the reply Robert!
I read the Building Block and I'm wondering if its because the readings are not taken ⅓ cycle apart.
Current Sensing device : SCT-019-200
Single phase calibration at 115VAC
3 Phase 208V L-L (120V L-N)
The 3 Phase calibration was done quickly on site to match the voltage and then power on each leg. I'm only using 1 emonTx with this setup.
Re: Understanding 3 Phase monitoring with emonTx sketch
What are those numbers - are they the current calibration constant that gives you the correct reading compared with what?
Because if they are, something is wrong, and the first thing I'd suspect is you're confused. The SCT-019-200 is a voltage output device, meaning it has its burden wired internally, and you don't need an external burden as well. But then, you might have the SCT-019-000 which is a 200 A device and which does need an external burden as it has a 33 mA per 200 A output (possibly! the SCT-013-000 says that too and it's wrong!).
I did tell you not to take any notice of real power. If that's what you're showing me, forget it. Go back and look at those phasor diagrams and think what real power means - it is the product of the component of current that is in phase with the voltage and that voltage. If you're multiplying a current that is 120 ° away from the voltage with the voltage, that's not real power. If you're doing what I think, you can only legitimately multiply rms voltage by rms current to give you apparent power.
And remember, if you're measuring current one side of a transformer and voltage the other, you need to know what the winding arrangement of the transformer is - in the UK I'd expect it to be delta-star, but American systems are illogical to me!
I need full, accurate data to help you.