SRA Lab Open Source Code

13Oct/108

Current Tranformer

At home or at work, you probably have spotlights which indicate only 20W or 35W of power consumption, but sadly these spotlights are powered by a lower voltage and so they require a transformer (hidden in the ceiling, a cupboard or a hood) which in turn consumes around 40W.

I've recently designed an application (with libmodbus, glib and Django) to measure the power consumption from various sensors built by LEM. By doing some tests at home, I've confirmed my suspicions about a transformer permanently plugged into the mains (0.18 * 224 = 40.3W for total of 80W when both spotlights are turned on).

So, I've replaced the shunt plug with a switch between the main and the transformer and once again I have a perfect 0W when off!

Do you know how many electricity transformers do you have around you?

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  • NiTr0

    Your transformer consumes 40VA, not 40W – you forgot about power factor, which is far away from 1 for ‘electronic transformers’ (that are used mainly in such applications), and is close to 1 for classical transformers.

    • http://sralab.com Stéphane Raimbault

      The measures have been done before the transformer so we don’t need to take care about the energy efficiency of the transformer to measure the consumption!
      To be even more precise, I’ve supposed a theorical cos phi of 1 because the real cos phi of my electricity provider is around 0.93 (EDF), so the final result is really near the theorical cos phi, so 40 VA is P=0,18*230*0.93 = 38.5W (only 1.5W of difference). Doesn’t really change anything? I don’t think so.

      • jpc

        I do not understand what the cosphi of your electricity provider has to do with anything.

        Btw. AFAIK this is a normal transformer (why “current”?) and the lamps do not need “continuous” current. They just need a voltage step down from 220V (or 110V) to 12 or 24V.

        • http://sralab.com Stéphane Raimbault

          Yes, the cos Phi is useless as I’ve explained!

          For `direct current’, you’re right I’ve used a multimeter to measure the transformer output and it’s just a voltage step down (AC) in this case.

      • Jim Kyner

        Electronic ballasts for some lights can have relatively low power factor (VARs as well as KW are being drawn) which means some power is fed back to the grid (which consumers aren’t charged for). A simple current transformer (CT) measurement can be misleading. Get a power meter (e.g. Kilawatt, WattNode etc.) that can give you a better idea of your true power consumption.

    • Jim Kyner

      Good point. Real energy (watt-hours) can’t be measured by a CT alone. A power meter that measures voltage and current simultaneously that can distinguish watt-hours from VAR-hours is needed since consumers are charged for watt-hours, not VAR-hours.

  • http://www.eems.at Armin

    Hello Stephane,
    we’re using WiLEM to monitor our Power Consumption.
    http://www.lem.com/hq/en/content/view/279/218/
    The MeshGateway offers a serial Port (RS232) where the Modbus RTU data stream is hookes up to the computer.
    How can we use your driver within our C++ application – all in Win32 (sorry).

    • http://sralab.com Stéphane Raimbault

      Yes you can use libmodbus to talk with your device, you need v2.9.3 which offers a Windows support.

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