Monitoring 13 circuits over three phases

@Gwil Did he order two extra VT’s?

Only these adapters (the cables with built-in resistors) were to be included free of charge, not the sensors themselves.

I can’t recommend that. I’ve had a chance to calibrate the phase shift of the OEM adapters, albeit at 60Hz, but nevertheless a reasonable starting point. It’s important to get that as accurate as possible.

All right, where and how shall I get the additional VTs from? From what I understood, for accurate three phase monitoring I need the original VT supplied with IotaWatt for one phase, and I need two additional VTs for the other two phases. I also thought the special cable with the burden resistor is used to put some load on the voltage output and thus transform the voltage output of the reference VT to current input expected by the IotaWatt. Am I missing or misunderstanding something?

You have the general idea. The resistor in the adapter, in series with the burden resistor in the IoTaWatt, create a voltage divider that produces a reference voltage across the internal burden resistor. That’s half the battle, but the VT output is out of phase with the AC line, so to be accurate, IoTaWatt needs to know how much the phase shifted so that it can compensate for best accuracy. Also, I don’t recommend using those adapters if the voltage out of the VT is greater than 11.5V.

I know the OEM VTs conform to those requirements, but I can’t recommend any other VT because I don’t know the phase shift and voltage.

There is another less accurate method of measuring three phase power with only one VT that you may be interested in trying. Basically, you set up the IoTaWatt as if it’s a single phase system and specify a 120 or 240 degree additional phase shift for the CTs on the other two phases. Depending on the accuracy that you need, it may work fine for you. In any event, it’s something you could try out while waiting for two additional VTs.

I did not want to use a single VT because in our place the three phases have different voltage levels, this would create a great deal of inaccuracy. I thought about the adapter producing 9 VRMS, because it is less than the 11.5V. What power output is needed? In the drop down menu of the IotaWatt I saw quite many predefined VTs. I presume they contain the proper phase shift value preset, don’t they? If not, can I use the calibration method described using the RMS voltmeter for all three VTs the same way?

Now you want to do something different, which may work fine, but given the confusion so far, I can’t recommend you do anything other than what was originally proposed. I am halfway around the world. There is a simple solution, which is to stick to the plan.

Sorry if the lack of my deeper understanding causes confusion, but I want to do exactly the same as originally proposed. Would you have a little sketch to help me assemble the parts and fill the missing pieces? In the package I got only one VT (9 VAC, 6 W) that plugs into the IotaWatt. I have two special cables with the resistors. I have plenty of CTs for the current measurement.

You will need a total of three standard VTs. I am still a little confused about what you actually have. It seems as if you only have one right now.

The two additional VTs will have 5.5mmx2mm barrel plugs. Those plugs insert into the barrel connectors on the adapters, and the 3.5mm stereo jacks on the other end of the adapters go into any input on the IoTaWatt. You then configure those IoTaWatt inputs as VT instead of CT and select the model VT that you bought. Calibration is optional but recommended in this case.

Once the three voltage references are working and calibrated, you can connect CTs. As you configure them, there will be a dropdown menu to select the VT that corresponds to the phase of that circuit:

Ok, great, thanks! This was what I’ve been trying to describe earlier (not very successfully though, sorry about that), so I’m happy we’re on the same track again. Indeed I only have one VT instead of three. Probably it was the wording what confused my because when you said adapter I thought it was a special thing containing a VT plus the cable with the resistor. When I realised it was the cable solely, I tried to express the need for extra VTs. Now it’s clear I need to buy two additional VTs. should it be the same type as the one I already got (the one that came with IoTaWatt), or does not matter so much, but it rather be on your list to choose from.

All I can say is that I’ve tested the OEM VTs to work ok with IoTaWatt and I have a phase shift reference for them. So I can say they will work as planned. I don’t have any way of qualifying anything else.

That’s fine! I ordered the remaining VTs, however I realized I need another IotaWatt for monitoring individual circuits (not only the mains), I just ordered that as well. This time I included two aditional VTs both for the existing and the new device. Now it looks complete, except for the adapter cables. Would you be so kind to include that for my new order as well? (order number #16562)

I don’t have any additional adapters available right now. I don’t have all of the parts to make more and they come from China, so the lead time is several weeks to me, a week to build, a week to the UK, a week to you… That’s not gonna work.

Anticipating more three-phase interest I’ve designed a VT adapter PCB that is out for prototype. I should have them end of this week. The new adapters have two channels, so they can be used to adapt either two VT channels to two IoTaWatt input channels, or to split one VT into two IoTaWatt input channels. It sounds like you would be interested in the latter. In any event, you should not need more than three VTs to run two IoTaWatt.

With the new adapters, you could setup two three-phase IoTaWatt in on eof two ways, assuming three phases A, B and C:

  1. Connect the phase A VT to the VT input of IoTaWatt 1, connect the phase B VT to the VT input of IoTaWatt 2, Using the new splitter adapter, connect the phase C VT to inputs on both IoTaWatt 1 and 2. Now you can monitor CTs on A and C using the remaining 13 channels of IoTaWatt 1, and phases B and C on IpoTaWatt 2.

  2. Using a CCTV type splitter ($3 - $4), connect the phase A VT to the VT inputs of IoTaWatt 1 and 2. Using 2 new splitter adapters, connect the phase B and C VTs to inputs of IoTaWatt 1 and 2. Now you can monitor any of th ethree phases on any channel of either IoTaWatt.

So you need only three VTs total, and either one or two of the new adapter/splitter. I will post here when I have received the prototype PCBs and assembled and tested one. If they work (high confidence), then I will make a batch (ordered 48 PCBs) and send to OEM. I have not discussed this with them, but I intend to sell the adapter/splitters to make it worthwhile to build, stock, and advise.

So I’d recommend you stick with a total of three VTs, get the additional IoTaWatt (they must be nearly sold out), and wait a few weeks until the new adapters are available to order one or two, depending on which of the above methods you decide.

Update: corrected the phases for IoTaWatt 1 in example 1.

That’s great news! I have to use the second setup, because my two IoTaWatts are gonna be in different switchboards so I need to keep the flexibility of monitoring all three phases in each switchboard. I need to check if there is physical feasibility of placing shielded cables reaching from one to the other. If yes, I’m gonna make use of your new splitter adapters. I’ll let you know and I’ll try to modify my order I placed earlier today.

What’s the distance?

The distance is 6-8 m, but I’m not sure if the there is enough physical space.

That’s a fair distance. If I were wiring that up, I think I’d try to employ CAT-5 wire because it’s economical, is twisted pair, and is a reasonable gauge (22 I think).

I use Cat6a usually (AWG 24), which has foils around each pair plus an overall shield around the four pairs. Would you suggest to use the pairs for the voltage references (as it is a balanced signal) and just leave the outer shield unconnected?

Does the same apply for the current sensors, in case I want to get them further away from the IoTaWatt than the 1 m lead cable?

Don’t have a lot of practical experience in this area. It’s been pointed out that I don’t know much about electrical engineering. I know that twisted pairs with balanced line transformers make noise disappear from long microphone cables, but that’s about the extent of my experience.

My approach with stuff like this is to try it and see. Both the output of the VT adapters and the CTs have the same current sensitivity. The difference might be that the VT adapters are always using full range while the CTs are usually in the low range. I think that might be what is called signal-to-noise ratio. So my sense is that the VT adapter output would be less affected by noise.
All that said, I think the cat6e will probably work fine for both. It’s probably more important to make sure the transition to connectors is solid.
I would let the shielding float or connect it to ground if available. Specifically I would not connect it to either of the signal lines.
There is a discussion in the Learn section about ct extensions, and I believe Cat5 is mentioned there as an option.

Hi Bob,

Just confirming that all that is required is a barrel adapter to a 3.5mm stereo jack for this to work?

I have a three phase setup here in Australia, the circuits i absolutely wanted monitored (home office) are on the same phase so I have it work with that but would be great to expand it to cover my main air conditioner and main usage.