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My electrical montage

On the assembly shown, I have built a current transformer with a toroid coiled with 30 turns of magnet wire. The brown wire is the phase going through, blue is neutral.

When I connect a fan onto the phase and the neutral wire, I don't measure anything with my coil. I measured using an oscilloscope on my voltmeter.

Is there a reason why it doesn't work? What am I doing wrong?

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  • \$\begingroup\$ What sort of copper wire did you use? \$\endgroup\$
    – Andy aka
    Apr 21 at 19:53
  • \$\begingroup\$ It's totally unclear what this is supposed to do, so it's even less clear how this doesn't work. Please 1. state a purpose. 2. State the design process, as simple as it might seem, 3. (necessary) actually draw a full schematic to go with your picture! \$\endgroup\$ Apr 21 at 19:53
  • \$\begingroup\$ I've used magnet wire. I have saw on wikipedia something called the rogowski coil and i'm trying to do one. but the current problem i have is that on the schematic found on wikipedia, it is supposed to pick some voltage to some extent at the output of the coil and be later amplified. But here i don't pick anything besides noise \$\endgroup\$
    – Vincent
    Apr 21 at 19:58
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    \$\begingroup\$ you have to plug an appliance into the power outlet and turn it on \$\endgroup\$
    – jsotola
    Apr 21 at 20:05
  • \$\begingroup\$ It is hard to tell whether the core of your coil is ferrite or something non-magnetic like plastic. What is your core made of? \$\endgroup\$ Apr 21 at 20:06
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Since you have created a current transformer, you need to measure current, not voltage with your meter. If the current drawn by the fan is (for example) 3A, you would ideally measure about 3A ÷ 30 turns = 0.1A on your meter.

If you don't have an ammeter, you will need to measure voltage across a very small shunt resistance. Your homemade current transformer cannot likely handle a large resistance on the secondary. (In US English, this is called "burden".)

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  • \$\begingroup\$ Thanks for sharing the US English term Theodore, did not know about it (I am not a native English speaker). \$\endgroup\$
    – ethcz
    Apr 21 at 20:05
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    \$\begingroup\$ Note that the current is AC, so measure AC 0.1A (not DC, that will always show zero for AC current.) \$\endgroup\$
    – rdtsc
    Apr 21 at 20:33
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    \$\begingroup\$ Coil is not soldered? I would measure the coil resistance; it should be zero. If not, then your orange/white wires are not actually connected to your coil. \$\endgroup\$
    – wbeaty
    Apr 21 at 20:38
  • \$\begingroup\$ @Vincent most current transformers use at least 100 turns to get the sensitivity up. \$\endgroup\$
    – Aaron
    Apr 21 at 22:09
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With a few assumptions on the geometry of your toroid, its permeability (µr=1000) and the current in the circuit (1 A), I find a voltage of approximately 10 mV RMS. Are you sure you don't measure somehing close to this value? I give you the calculation, you can change the values of the parameters with the actual ones.

enter image description here

In real Rogowski coils used for 50 or 60 Hz, the number of turns is much larger than 30. There are also two other differences when comparing to your design:

  1. The way the toroid is wound is a little different (I think it won't make much difference here).
  2. You have en electronic integrator after the winding to get a signal proportional to the current, and not to its derivative.
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  • \$\begingroup\$ I think my load is too light. It draws only 22W from the phase ( P=VI therefore 22/230 = I ) which is way too low considering your calculs. What should I do to get a better measurement. Knowing that I have a 230V phase at 60Hz basically an infinite amount of windings available, different cores available ( Microwave oven for the most part ) \$\endgroup\$
    – Vincent
    Apr 21 at 23:24
  • \$\begingroup\$ @Vincent: you may increase the number of turns. But what is your final objective ? There are certainly easiest ways to measure a current. \$\endgroup\$ Apr 22 at 8:59
  • \$\begingroup\$ i want to basically measure the power consumption of a server and send it via an USB port to the server and use it into an app I will code \$\endgroup\$
    – Vincent
    Apr 22 at 19:17

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