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So I recently bought 3 current transformers with the goal to measure the current of 3 washing machines. All I want to measure is if the machines are running or not so accuracy isn't really a concern. Now I ordered them a bit hasty and didn't think of the actual working logic of these guys. From what I read, I thought I can just clamp them around a power cable and read the current.

Which apparently is not the case because I can only measure one phase with a CT as far as I understand. I live in a country with a 3 phase system and I don't understand that much about alternating current. I tried to read into this 3 phase system but its quite a bit over my head.

Now from what I think I understand, this means if I clamp this around a standard power plug I wont get any output voltage because the phases cancel each other out and I don't get any magnetic field that the ct could sense, is that correct?

Question: Would it be possible to remove the outer isolation of a power cable and clamp the CT to just one of the three cables/phases? Or would they be too close and interfere and still cancel each other out? Or is it a stupid idea all together because I totally didn't get how this 3 phase alternating current system works?

This is the CT sensor I'm using: http://www.seeedstudio.com/depot/noninvasive-ac-current-sensor-30a-max-p-519.html Datasheet: http://garden.seeedstudio.com/images/b/bc/SCT013-030V.pdf

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Now from what I think I understand, this means if I clamp this around a standard power plug I wont get any output voltage because the phases cancel each other out and I don't get any magnetic field that the ct could sense, is that correct?

Absolutely!

Would it be possible to remove the outer isolation of a power cable and clamp the CT to just one of the three cables/phases? Or would they be too close and interfere and still cancel each other out?

It would work, as the CT "measures" only current flowing in wires passing through the aperture, and is not influenced by anything else happening outside.

However, there are a few questions:

  • You have 3 machines, each having a single-phase feed? If yes, then you can go on, and attach one CT to the Live wire of each machine. If you have three-phase feeds for each of the machines, then you can measure a single phase, or you'll need more than a single CT per machine. Measuring a single phase could work if the machines load the phases symmetrically - a three-phase motor does, but the controlling electronics may not. In the latter case (and assuming you're not satisfied with recognizing only when the motor runs), you might still succeed if you can figure out which phase to use for the CT (the one which feeds the controlling electronics).

  • What else do you have to do to keep the installation safe (and fully in accordance with the local electrical code)? To answer this, you'll need someone with knowledge about the local electrical code. Generally, stripping the outer insulation layer from a cable would decrease its safety level, so some measures will have to be made to keep the system safe.

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  • \$\begingroup\$ Thanks for the answer, thats very helpful! As far as I know I only have 3 phase feeds for electronic devices, so far I haven't seen a cable with less than three wires. Though I might be wrong. I'd have to check the manual of the washing machines. As for the safety issue, I'm aware of that. I wont remove the isolation myself as I'm not sure if the cables are easily replaceable if I cut too far. A friend of mine is quite savvy with these safety regulations and does AC cabling in houses, I will definitely ask him before doing anything. \$\endgroup\$ – PTS Jan 15 '14 at 0:58
  • \$\begingroup\$ Another question, any good way to test this before trying this on the actual machines? Any consumer electronic device that would probably also just use one of the three phases? \$\endgroup\$ – PTS Jan 15 '14 at 1:01
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    \$\begingroup\$ If the machine has 3 wires in its cable, it's 99.999% not a three-phase machine. It would need 4 or 5 wires to be a three-phase machine. You can test the CT on any equipment you like. Preferably an equipment that you can open and find the wires already separated inside so that you don't have to strip the cable insulation for the test. \$\endgroup\$ – Laszlo Valko Jan 15 '14 at 1:44
  • \$\begingroup\$ So far I only stripped IEC 60320 C13/14 type cables. Those with 3 female pins that typically go into PC PSUs, TVs and other home electronics that doesn't have an external PSU. Are these 1 phase cables then? So the neutral and or earth/ground wire is not optional for AC I recon? Which of the two is mandatory then? \$\endgroup\$ – PTS Jan 15 '14 at 2:01
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    \$\begingroup\$ Yes, those are single-phase cables. For single-phase, both are mandatory, so a 3-wire cable (or a 3-pin connector) can only be single-phase. Earth/ground is always mandatory, and you can leave out neutral only if you have more than one phase. \$\endgroup\$ – Laszlo Valko Jan 15 '14 at 2:14

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