Do non-galvanic isolated supply for 120 deg C paddle wheel flow meter will affected by VFD?

Hi engineers,

I currently have a high-temperature DIN25 paddlewheel flow meter in my machine. This flow meter uses a non-galvanic isolated supply which means I read the 4-20mA from the instrument. The flow meter builds up with a paddlewheel flow meter and an electronic board (frequency to current) converting to a 4-20mA signal. The flow meter also complies with EMC standards of EN61000-4-3, EN61000-4-4 & EN61000-4-6. The issue is that the flow meter reading will be abnormal (more than 22mA) when the 45kW VFD is running. I also have instruments such as PT probes and pressure sensors in the machine that do not have abnormal readings during VFD running. All the instruments, flow meter, PT probe and pressure sensor, installation radius 3-4 meters from the VFD. Thus, does the VFD create noise and affect the paddlewheel sensor? The flow meter supplier keeps explaining the noise problem but does not in detail. I need some experts who also face this similar issue and share some knowledge.

Thank you so much for your attention and participation.


Eddy Kee

  • It is quite possible.

    The EMC standards are very loose limits, set at a level to usually ensure that you are unlikely to cause serious interference to your neighbours in adjacent buildings. The levels of filtering and so on you would need to guarantee that you will never interfere with your own kit in the same room would need to be considerably tighter - closer to the levels that Mil Std and Def Stan certified equipment have to meet. For commercial products it is not cost effective to do  that, as in most cases it is not needed. Only when it is a problem, like this case, then additional measures need to be taken

    Now your problem is to determine how best to reduce the coupling between your victim the sensor, and aggressor, the VFD, and beyond the basics of spacing the sensor wiring from that of the VFD, and perhaps using screened cables, it then needs someone with some electronics knowledge to look with a 'scope and perhaps a spectrum analyser, to decide how the interfering signal is getting out of one place and into another. As a very first try, a medium wave portable radio may be able to hear the motor whine to identify which cables it is getting out on, but unless there  is one very obvious one then  a proper investigation will be a lot more scientific than that.

    Then it is likely to be a mixture of filtering at both aggressor and victim, to deprive them of coupling to which ever bits of the wiring are acting as accidental antennas and means of egress and ingress.

    If you have more description of the set-up and how the wiring is routed and how much is currently filtered, it may be possible to advise a few things to try.

    however, without seeing it such problems are hard.


  • Hi Mike,

    Thanks for the brilliant explanation. Yup, I agree that it is not cost-effective for commercial products to do that, Mil Std and Def Stan. The supplier does not revert to me of the EMC clarification further. The supplier only sent me the galvanic isolated supply transmitter. This transmitter has programmed some K-factors according to the paddlewheel flow meter. It seems the paddlewheel sensor and transmitter can give a good range of reading even the VFD is running. The current installation did not use screened cable on the sensor and aggressor, but I only managed to use screened cable for the VFD. I can determine that the galvanic supply method also helps in this noise issue. Since I can receive the proper range measuring, thus I did not propose to my management for the spectrum analyzer service. Cheers


    Eddy Kee    

  • Hi Eddy,

    I've encountered problems with VFD drives disturbing sensors before. The synthesis of the VFD waveforms tends to create a lot of harmonics (think of the spectrum of a square wave as a simple example) and I've seen these extend into the GHz region on drives with poor EM compliance. There's often coupling of this "noise" into the earth and/or the equipment chassis - sometimes through capacitive coupling into the motor casing. Mike is absolutely correct  in all his suggestions. Could I also suggest you explore eliminating AC current loops between the sensor electrical connections and the protective earth or equipment chassis. "Filter" capacitors connected between the sensor lines and the protective earth can cause more problems than they solve. Perhaps try "floating" the sensor with respect to the equipment chassis and protective earth and see if the problem disappears. An insulating plastic thread insert or spacer might be an inelegant but quick solution.

  • It may be related to either protective conductor current, or coupling of this through other mechanisms.

    Some VFDs have exceedingly high protective conductor currents (and need to be installed as "high protective conductor current" circuits).

    Some manufacturers of (and indeed the product standard for) VFDs recommends the use of isolating transformers to localise the effect of the protective conductor currents, and the solution is also recommended in BS EN 60204-1 ... where this is not done, due to cost considerations, the only option is to use "low inductance earthing" and "multiple common-bonding network" approaches described in BS EN 50310 ... but how much "earthing and bonding" is "enough earthing and bonding" is a bit subjective, and could be costly if the cable runs are extensive, the plant is spread out, or on long production lines.

  • Hi Philip,

    I have tried to use an isolation transformer for the instrument. The noise from the VF is still not yet solve. Then decided to use another flow meter with a floating connection and Safety Extra Low Voltage operating technology. The noise is gone. I do believe that the previous flow meters enclosure is using metal so the synthesis noise from the VFD affected the chassis of the machine. Since the changed PVC flow meter works fine, I do not make more testing to the metal digital flow meter with an isolation transformer. Thanks

  • Hi Gkenyon,

    Yes, you are right. The protective conductor's current is grounded to the machine chassis. Then the flow meter enclosure mounted on the machine chassis is also a metal enclosure casing. Thus I believe this is the main reason of the noise transfer even though I use an isolation transformer directly to the instrument. Now I changed to another paddle wheel sensor flow meter with a PVC enclosure casing. Thanks for the advise.