8 Replies Latest reply on Jun 15, 2020 4:19 AM by aswinvenu

    Voltage divider vs potential transformer based design

    aswinvenu

      Hi,

       

          I am designing a Voltage and Current data acquisition module for an industrial application. I was talking to different vendors about the analog front-end. I have studied TIs power-meter designs really well.

      But the question I have in my mind is for voltage measurement should I go with a simple voltage divider (More cheaper, compact and linear option) or a potential transformer based design.

      Class 0.5 TI reference designs are using voltage dividers. If I go ahead with the voltage divider will be problem to get IEC61010 ? Because of poor isolation ?

      I am looking for measuring 230, 460 and 690 volt systems.

       

      This is what something in my mind.

       

        • Re: Voltage divider vs potential transformer based design
          shabaz

          Hi Aswin,

           

          I think all the metering circuits I have seen have used a potential divider for the voltage measurement, but I have no idea what specs are relevant for meters or any such applications.

          Isolation is needed somewhere, but that could be digital isolation at the output of the energy meter circuit, or somewhere in-between. Power could be from each phase.

          Personally, if I was going to design it, I'd make it as three separate modules, so only one could be used for single phase, and have them all have an isolated output.

          But then I don't make energy meters, and if you look at existing meters, I doubt they will have such a design, and instead they will integrate/cost-reduce onto a single board.

          By the way, you mention 230, 460, 690V, but the voltages are not in-phase, so these will not be seen. The RMS voltage will be (say) 230V (single phase), or 400V (phase-to-phase)

          [or multiply accordingly for whatever country it is in]. In the UK, we tend to see higher nominal values, like 240V or 415V, or even higher since that's not a precise value from the grid.

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            • Re: Voltage divider vs potential transformer based design
              aswinvenu

              Hi Shabas,

               

              Thank you for your reply!

              In India we also have the same single phase 230 3-phase 240/415 (RMS).

              To be exact my DAQ systems goes infront of power delivery systems of motors and pumps. They are rated at 230 and 440 vollts.

              The reason I don't want any transformers is to make the design less bulky and less prone to noise. But I got into an argument with a CT vendor last weekend. They insisted us on designing the frontend with a stepdown transformer.

              I have don't have experience in designing industrial power/energy meters.

               

              According to your experience can I go ahead with the voltage divider model?

                • Re: Voltage divider vs potential transformer based design
                  shabaz

                  Hi Aswin,

                   

                  Measuring current with a current transformer is normal (although so is using a current sense resistor, at least my home (consumer) energy meter plug uses a current sense resistor for size and cost reduction), a current transformer is more usual for industrial I'm sure, since there will be less power loss.

                  I just don't see the reason to have to measure voltage with a step-down transformer if you don't want to. Series resistors will reduce the voltage and drastically limit current in a fault condition, and you can have isolation elsewhere.

                  I'm sure there must be dozens of example circuits (or could even tear-down existing energy meters) to prove this is done in real life and meets some specs of some sort! : )

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                  • Re: Voltage divider vs potential transformer based design
                    shabaz
                    They insisted us on designing the frontend with a stepdown transformer

                    There may well be a step-down transformer for powering the circuit, but as for measuring voltage with it, how accurate could it be? I'm not sure normal voltage transformers are designed for precision so then you'd need to calibrate : (

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