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CT sensor too big

currenttransformer
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(Stephan Kluge) #1

Hi everyone.
Just today I started to install my EmonTX. Everything worked fine until I tried to install the clip-on ct sensors SCT013. Unfortunately there is not enough space for all 3 sensors either inside my in my house connection box or my fuse box. So I am asking myself if it is possible to use smaller sensors maybe like single core sensors. I found the HWCT004 sensors on Aliexpress?!
Are there any recommended singlecore ct sensors that could replace the big SCT013?
I would be glad to get some suggestions…

Regards, Stephan

(Robert Wall) #2

I don’t understand what you mean there. ALL current transformers must be fitted on a single-core cable. If you fit one onto a 2-core cable, it will not work.

Do you really mean a ring-core current transformer? You can only use one of these if it is safe and legal to disconnect the wire to thread it through the c.t.

What is the maximum current that you want to measure? Generally, the physical size of the c.t. depends on the maximum current (because wire size or busbar (Sammelschiene) dimensions also depend on the maximum current)?

(Stephan Kluge) #3

Aaah, my fault… I know that each live conductor needs a single ct sensor. So I need to fit 3 of them into my fusebox
I meant solid core sensors. They seam to be much smaller than the split core sensors. The maximum current I want to measure is 40A

(Robert Wall) #4

From the information available to me, this c.t would appear to be suitable, though I cannot see any dimensions, so I am not sure how you know that it is small enough.

I have found the YHDC TA1050, but it is designed for p.c.b mounting, and you will need to add a cable and a plug.

(Frogmore42) #5

You should probably provide a picture of your box.

The solid core sensor is a little smaller than the clip on one, but not a lot.

It sounds like you have a too full box already and adding to it is likely not safe. It might be time for a box upgrade.

(Stephan Kluge) #6

Thank you Robert,
I have the datasheet of the hwct004 which says that its diameter is 25.7 mm and the width is 13.5 mm. But I will definitely have a look at the other sensor you have pointed me.
Are there any modifications in the firmware that I have to make when I‘m using a different sensor?

(Robert Wall) #7

Indeed there are - and possibly the hardware also.

The burden resistor in the emonTx (c.t’s 1-3) is designed to give 1.1 V rms into the ADC at 50 mA secondary current. If your c.t’s secondary current is not 50 mA (or reasonably close but less) at the maximum current that you want to measure, you need to change that resistor. It is easy to remove the surface mount one on the circuit board, and there are holes ready to accept a wire-ended resistor to replace it.
You also need to change the current calibration constant (Ical) in the sketch. The number you need is the current that gives 1 V rms at the ADC input (i.e. across the burden resistor).

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(Stephan Kluge) #8

Hey Frogmore,
indeed the box is quite small but a replacement isn’t possible because it is property of the electricity grid operator and simultaneously the meter board. There is only a small part inside the box above the electric meter I can use for my project. A picture of the box I will provide later. I am currently not at home…

(Robert Wall) #9

Frogmore is in the USA, I believe. The rules are different there, and their “load center” looks a lot different to yours (and indeed to the “consumer unit” we have here). It’s always dangerous to make assumptions about the standards and practice in a different country.

(Stephan Kluge) #10

Thank you very much for your quick help. I just ordered 5 HWCT-004 and I think I dont have to make too much hardware modifications because the ct’s output is 50mA as well. So the internal burden resistor of 22 ohm should be good enough. The only modification might be to extend the cable and and mounting a plug.
I think I will give a quick review, when everything is installed and works properly.

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(Robert Wall) #11

If you need to extend the cable by more than a couple of metres, there are details of the proper way to do it in ‘Learn’. But if you’re only extending by less than that, twisted wires should be good enough unless there’s a particularly strong source of interference close by. You wire to the plug tip and sleeve, there is no connection to the ring. If you get it wrong, swap the wires, or flip the c.t. round on its cable, or make the calibration constant negative. You need to change the calibration constant in the sketch to ~45.45 (that also corrects for component variations in the emonTx - but it should be close).

If you want to use the 4th c.t. channel for more than about 9 A with one of those, you’ll need to change the burden resistor in the emonTx. (I mention that as you have 2 spare c.t’s.)

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