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We are having random noise issues on PC controlled data acquisition where one is related to VFD drive which is controlling the AC motor.

The thing is that, sometimes we observe continous large noise around 0.3V and right now I cannot say the nature of the noise. But when the VFD is switched OFF the noise disappeared and when turned ON it appeared again for 5 seconds and disappeared in 10 seconds. This is strange becuase it is somehow VFD realted but also doesnt appear all the time.

About 12 meters far away we have a data-acquisition computer with various DC inputs and the daq board also controls the VFD through an isolation amplifier. All input signals are floating, so no ground loop. VFD ground is isolated from the daq side but my guess is somehow VFD's switching noise might be injecting noise to power lines.

If this noise is VFD related my question is:

If I daisy chain some electrical power outlets and power all the daq system(PC, transducers. power supplies) thhrough the same outlet. And if I also use a filter such as this one after the mains outlet, would that be worth to try? This filter says it can filter both common mode and differential mode signals. Or should I first find the FFT of the noise and then find a filter?

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  • \$\begingroup\$ You could use such filter, it won't do any harm aside from extra earth leakage. But you have not yet determined if the noise is conducted or emitted. (by cable or electric field) The VFD is properly grounded right? \$\endgroup\$ – Jeroen3 Nov 14 '17 at 13:33
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    \$\begingroup\$ Please show the example of a signal with noises. Is noise observed at all input? \$\endgroup\$ – AltAir Nov 14 '17 at 13:46
  • \$\begingroup\$ Look at this question: electronics.stackexchange.com/questions/334382/… \$\endgroup\$ – Charles Cowie Nov 14 '17 at 14:04
  • \$\begingroup\$ @AltAir Yes the noise is observed at all input. The daq system is single ended. But I cannot say much about noise nature they say white like noise but they sample at 500Hz so I need to check next time with a scope. \$\endgroup\$ – Genzo Nov 14 '17 at 14:39
  • \$\begingroup\$ @Jeroen3 Yes VFD properly grounded by a company ABB technicians. But the strange thing is the nosie appears sometimes. I dont think it would be radiated noise. But Im not sure \$\endgroup\$ – Genzo Nov 14 '17 at 14:40

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