OpenEnergyMonitor Community

3 Phase Power Monitoring at an individual CNC Machine level

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Hi, and I apologize if I have missed the correct FAQ. I did put this on the ‘Hello World’ area, but it seemed to attract no interest - but maybe because I am too wide of the mark!!
My question is do you have any support for monitoring 3 phase power in a light engineering environment?
We have a very advanced in-house Delphi based system that covers all aspects of our operations, and we would very much like to be able to extend this to monitor more accurately the amount of power each machine - rarely over 32 amps - uses, ideally using a Pi based collecting network, and even better extending to XBee nodes for each machine!
Probably rather fanciful, but I am sure someone has trodden this path to some degree, it is just finding them that is the challenge!
Thanks in advance for any response, Richard.

The EmonTX will do that with the appropriate sketch loaded. @Robert.Wall is better placed to advise.

There are a number of options to get the data from the EmonTX to an install of emoncms. Emoncms, whilst largely RPi based, can be run on any Linux environment (personally I run it in a ProxmoxVE container). You can transfer the data via the inbuilt RFM but, my suggestion would be to use Wi-Fi via either the EmonESP (simpler, available from the shop), or serial link an RPiZW to each EmonTX (more flexible and from experience more reliable).


As Brian mentioned, the emonTx can be used to give a good measure of 3-phase power, the shortcoming is that it must assume that the voltages are reasonably balanced, because it was designed for the UK domestic market and so only measures one voltage. Other than that, we think you can get, with careful calibration, to better than 1% accuracy.

I perhaps should mention, assuming you have the in-house expertise, that there is a wide selection of DIN-rail 3-phase meters available that communicate via Modbus, so those might be worth investigation too.

Brian, many thanks for this response, and it is indeed most helpful.
So that I do not waste your time, could you point me to a ‘beginners guide’ article that shows how the EmonTX and CT units will be physically structured, and how I can arrange for the Data to ‘flow’ into our Delphi / MySQL system so that it can be ‘paired’ with the operations each machine is doing which we have knowledge of in Real-time?
Robert, your post has also now come in, so I will follow through on the DIN rail idea - which I had noted in other posts as I had looked around. The only problem is that the feed into the machines is often direct to their own terminals, but I can start having a look when I am back at the Factory. We only have about 10 machines that use enough power to warrant all this, but we need the information both for costing, but also because we are pushing the boundaries of our inbound supply and may have to start staggering their concurrent use.
Thanks so you both meantime.

I don’t think there is one, I’m afraid.

I presume you’ve looked at the Shop page and the pictures. The emonTx is a box with four input sockets, 3 for c.t’s and one for an a.c. adapter that measures one phase voltage and serves as a (limited) source of power. If you go with Brian’s suggestion of Wi-Fi either with the ESP8266 or the RPi Zero W, then you’ll need a 5 V d.c. supply too. Depending on exactly what the control cabinets for your machines look like, you’ll either need to have panel space for these inside, or you may need an external (plastic) housing - especially if the Wi-Fi needs to get out of a metal cabinet, and of course 240 V, 13 A sockets.
Fitting the c.t’s is a matter of finding a place where the 3 phases are in separate cables. If you’re talking of under 32 A, I’d suggest the Talema AZ-0750 or the AZ-0500 from RS Components. They will likely require the leads extending and a 3.5 mm plug fitted, but they are smaller and being ring-core, should offer better performance than our “standard” split-core one. I haven’t tested one, though I can’t see RS stocking them if they’re not up to the job. The emonTx will need its calibration adjusted, but that’s a relatively simple matter and can be done in the 3-phase software before you load it (the emonTx comes with single-phase by default).

I must leave it to others to advise on getting the data into your system.

That depends in what format those systems expect the data and what mechanisms you have to inject it.

In simple terms, you can get the readings as a JSON object sent to an MQTT Broker (with a Pi connected directly to the EmonTX).

I’m not eactly sure ehat you mean by this but I think these meters are designed to measure single appliances (rather than everything connected to the supply in one go). Might need a bigger DIN rail and the whole layout modifying though.

The advantage of the EmonTX is it is non-invasive but that can be a disadvantage as well.

Brian, Robert, thank you both for some really useful information, and I have now found my way through to the ‘Learn’ section which somehow I had missed in my first trawl through the site, so that will keep me quiet for a while! I am lucky enough to have a superb IT programmer who I know will be able to pick up on the threads you have mentioned as well as a qualified ‘Sparks’ who can deal with the CT side of things; it was just the ‘Low voltage Electronic’ interface side of things that we were missing and which I am seeking to fill. Anyway, as and when we have made some progress, or I have got stuck again, I will be touch!
Best regards, Richard.

A bit of fill-in information, I did my student apprenticeship at a firm making thyristor drives and general industrial automation, but I’ve worked on everything (or so it seems!) from microphones upwards, the largest being a 3 MW d.c. drive pulling coal out of the ground, for which I was the project engineer. CNC machines however, are not something I’ve had anything to do with.

Hi, have done something similar in industrial environment that might fit your requirements.
Used off the shelf professional energy meters that talk modbus and a custom gateway to transmit the information via a wireless link from each device to a central custom gateway that has internet connectivity to log on emoncms.
The comunity might help you on this as a diy project.
If you opt for a custom turnkey project, i can help just pm me.

Chaveiro, thanks for this update, always appreciated.
Am about to buy one of the emonTX kits to experiment with, but could you send me the link for one of the ‘off the shelf professional energy meters’ that you refer to in your post?
Still very much feeling my way on this one!
Best regards meantime, Richard.

Used the one from this post Ebay 3-phases modbus energy meter

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