Hi,
Two questions on the emonTx V3 CT1234 + 3-phase Voltage sketch :
1. Imagine that the voltage monitor can only be on L2 (physically speaking). In that case, I guess that CT1 must be round L2, CT2 round L3, CT3 round L1 and CT4 round L2. Am I right ? or it doesn't matter which voltage is monitored ?
2. is it possible to modify that sketch to be able to approximate several circuits on the same phase ? As an example, being able to have CT1 on L1, CT2 on L2, CT3 on L2, and CT4 on L1.
Thank's
Eric
Re: emonTx V3 CT1234 + 3-phase Voltage example
1. You are correct. The voltage is by definition L1, the others must follow in the correct sequence. You don't actually know, and in fact you can never know, which phase really is no.1. (What happens if the transformer up the street is delta-star, as it will be? You have a 60° phase shift, so is L1 coming out 60° before or after L1 going in?) But you do know the sequence 1-2-3, and that's all that matters.
2. That should be possible. The principle is you choose the delayed voltage sample that matches the phase that the CT is on. Also beware PHASECAL also acts as a fine tuning control, interpolating between adjacent samples in the voltage array.
Re: emonTx V3 CT1234 + 3-phase Voltage example
Robert,
Thank's for your reply. Some clarifications needed to be sure to understand.
1. If I can only plug the AC/AC adapter on L2. Which configuration below is OK ?
- a) CT1:L2, CT2:L3, CT3:L1
- b) CT1:L1, CT2:L2, CT3:L3
a), b) or both ?
2. I understand that it should be possible but not straightforward. It's doesn't simply require few modifications in the sketch but it means configuring the sketch by trial and error to find the right calibration constants. Is that correct ?
3. One more question : it looks like a continuous sampling sketch. Would it be possible to calculate the kWh continuously on board the emonTx ?
Eric
Re: emonTx V3 CT1234 + 3-phase Voltage example
1. Only (a) With (b) you'd get a pf=0.5 and apparent generation on every phase, even with a resistive load on each, because everything would have a 120° phase shift between voltage & current. [I think you need to read up about 3-phase systems ;-) ]
2. Yes, it will require some work. The underlying assumption when I wrote it was that you'd want to monitor all 3 phases, and then the V3 came along, so I tagged CT4 onto L1 because you can always rotate the phases. All these are legal without changing anything:
&npsp;Voltage on L1, CT1 on L1, CT2 on L2, CT3 on L3, CT4 on L1
&npsp;Voltage on L2, CT1 on L2, CT2 on L3, CT3 on L1, CT4 on L2
&npsp;Voltage on L3, CT1 on L3, CT2 on L1, CT3 on L2, CT4 on L3
But then the power that's reported as "power1" is the one measured by CT1, and the others follow in order.
If you only want to monitor two of the three phases, then you must change things around. Whatever you do, you'll need to follow very carefully the calibration procedure in the comments in the sketch.
3. It's not continuous, so basically if you do try to calculate Wh, you will have all the inherent errors of the discrete sampling method. One day ( ! ! ! ) I'll convert MartinR's PLL to do 3-phase continuous monitoring (or at least try). I think there will be enough processor capacity for that, based on what Robin had achieved. That will reduce or remove many of the sources of error that come from timing/frequency changes and interpolation. But it can never change the errors resulting from voltage imbalance.