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.

Mike.

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

Regards

Eddy Kee    

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

Regards

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.

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