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.

Cheers

Eddy Kee

Parents
  • 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 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.

Reply
  • 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.

Children
No Data