Do I need one voltage sensor for each current sensor

Dear all,

I did my first tests with one CT sensor and it is working fine concerning current measurement.

But when I started to install the voltage sensor and reading more about about it i feel that I need one voltage sensor per CT sensor (especially for inductive loads).
Could you please confirm ? I there is a way to avoid it that would be great because the installation will get bulky and expensive.

Thank you

Probably, you do not, but I can’t answer the question accurately because you have not filled in your country in your personal profile, nor have you told us what you are using. There are too many unknown factors for anyone to guess.

I see you are new here - take a look at the FAQ page. That gives you some hints about what we need to know to be able to help you.

Dear Robert,

Sorry for that. I just added my country of residence to my profile. I did it during the registration but maybe did not validate :cry:
I’ll use the following vocabulary (i do not find the correct english equivalent sorry) :

  • PCS : Power Control Switch
  • SAS : small automatic switch

I am leaving in Spain in a flat with the following electrical architecture

  • 1 main PCS
  • 1 SAS for washing machine
  • 1 SAS for water heater
  • 1 SAS for kitchen
  • 1 SAS for air-con
  • 1 SAS for plugs
  • 1 SAS for lights (moslty led)

For the current transformer I use the current clamp from YHDC. I want to put one on each line.
For the data recording, computation and analysis I am working with a raspberry pi3 + MCP3008 to convert analog signals to digital.
I also want to try to use the LM358 following your advice to improve measurment accuracy.
At the moment the only thing I did was to :

  • realize electrical mountage on a breadbord to measure 1 CT sensor a visualize it on the raspberry

The AC-AC transformer I have has this description : Original fuente de alimentación AC Adaptor AA de 091abn 9 V ~ 1 A para router teledat 431 LAN
Traduction : Original power source AC adaptor AA of 091abn 9V ~1A for router teledat 431 LAN.

I hope this is better and thank you for letting me know if any other information is missing.

Best regards

One more question: Do you have a 3-phase supply? My guess is you do have. Please look here Learn→Electricity Monitoring→AC Power Theory→3-Phase Power→Identifying a 3-Phase Supply at the photographs. If you have a meter and the terminals look like picture ( c ), or your main switch is like picture ( d ), or if there are 4 main wires coloured brown, grey, black and blue, then you do have a 3-phase supply.

If you do NOT have 3 phases, then you only need one voltage transformer. You can use the same voltage with each current transformer to calculate the power.

If you do have 3 phases (and I think this is likely) then you must have one voltage transformer for each phase, and you must use the same voltage transformer for all the current transformers that use that phase.

Thank you the time you take to answer.

So I confirm that my installation is with just one phase.

I was wondering about using several voltage sensors because of the possible voltage shifting due to resistive load, see section below :

Learn→Electricity Monitoring→AC Power Theory→Introduction→Partially Reactive Loads

The question that rise from this part of the documentation is : resistive loads is shifting the current or the voltage ? Or Both ?
If it shifts only current, the 1 voltage sensor is fine. But is voltage is shifted too, then I thought I would have to install one voltage sensor on each line.

By the way, for the voltage sensor I installed a plug inside the electrical board like the one below. This way I just plug the AC-AC transformer in it and drive the output toward my breadboard using the electrical scheme recommended in the “learn” section.
[The image shows a DIN-rail socket]

So the question is : how resistive loads impact voltage?

Thank you.

Good. That makes everything much simpler.

I think you are confusing the words “reactive” and “resistive”. For a purely resistive load, voltage and current are exactly in phase. It is only when you have a reactive load, like an induction motor, that you have a phase shift between voltage and current. can help you with electro-technical words.

Phase is relative. There is in general no such thing as absolute phase. Therefore, you need a reference and then you measure the difference between the wave you are interested in and the reference. Because all your appliances are wired in parallel and see the same voltage - which is the same throughout your house, to within a very small amount, we normally use voltage as our reference and say that it is the current that phase shifts because of the reactive component of your load. And then we use the same voltage measurement with all the separate current measurements. It works for us in the UK where almost everyone has a single phase supply.

Most loads will reduce the voltage very slightly because the extra current they take will cause a voltage drop in the cables in your house. But because the electricity supply is generally inductive, a large capacitive load might actually increase the voltage due to the effect called “resonance”.

Dear Robert,

Thank you very much for this extensive answer.
Indeed in mixed up “reactive” and “resistive”.

So it is good news, I started some more set up yesterday and will do all my testing soon.
I understand then that it should be fine with one voltage sensor.
I will do the test for some time and compare approximately with the meter of the electricity company (even if I know there will be some discrepancy due to CAN accuracy etc…).
By the way, I had an idea to “correct” the measurement during the time by using the “light pulse” signal of the meter (considering the value is the real one). If it gives somethings I’ll let you know.

Best regards.

Really? I was under the impression that using the same voltage transformer for all three phases is common practice and only leads to minor inaccuracies. Is this not correct?

Yes, really.

That is because of the relationship between the three phases. If you try to calculate real power and you use a voltage reference that is not correct, you get a power reading that is not correct.

If your load is purely resistive and the phase angle between all three voltages is 120°, then the real power calculation will give a result that is exactly -0.5 × the correct value. If the power factor of your load is not exactly unity, then it will not even be that. The apparent power calculation will however be correct.

The 3-phase sketch for the emonTx uses a single a.c. reference to give an approximate voltage for phases 2 & 3 by delaying the voltage of phase 1 internally. Then it is able to calculate the real power for all three phases with the only error being due to any imbalance in the line voltages (plus of course the usual measurement errors).

If you use anything that does not do the same sort of thing, then you do need a voltage from each phase.

That’s what I thought. But that’s not quite the same as

If you read all of that thread, from the top, I did not know at the time I wrote that, what equipment Adrien was using. It could have been anything. The initial question assumed that anyone answering was a mind-reader, and knew what was being used.

The 3-phase version of the Mk2PVRouter, which is mentioned in ‘Learn’, does use one voltage transformer per phase.

Yes, so if you don’t know, how can you make a statement about how many voltage transformers he needs?

But nevermind. I’m not here to quibble about this. I just wanted to make sure I had understood things correctly (otherwise this would have abruptly ended my experimentation with OEM as I can only use one voltage transformer.