Noisy RTDs on VFD controlled motor 1

[nhcf]

A plant is reporting noisy temperature signals from motor RTDs when the motor is operating. Motor is 100HP and VFD controlled. The RTDs and associated cabling are in separate conduits/boxes from any motor conductors/cables. Motors are supplied with Belden shielded VFD cable, which appears to be installed consistent with drive and cable mfr instructions.

Has anyone experienced EMF induced 'noise' on motor RTDs due to effects of VFD?

 

[Skogsgurra]

"Has anyone experienced EMF induced 'noise' on motor RTDs due to effects of VFD?"

Yes! Many times. Especially if the RTD instrument is not an integral part of the drive. A 10 - 100 nF capacitor across the (receiving end terminals) helps very often. I would say always.

VFD:s are such heavy HF polluters that such things always happens. Vibration, temperature, pressure, speed and what have you are all victims. In severe cases, it may be necessary to bond motor and drive or move the drive closer to the motor.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[GroovyGuy]

I must admit that I have never personally experienced noise issues with RTDs on VFD driven motors. (Note that I have wired 1000's of RTDs into VFD driven motors).
What is make / model of VFD? I assume that it is a standard 6-pulse type.
Is there a dv/dt filter on the output of the VFD? (I always install a type TCI dv/dt filter on all LV VFD outputs.)
What type of RTD device (or motor protective relay) are you connecting these RTDs into?
Please confirm the type of instrument cable that you are using (ie twisted-triads c/w indiviudal and overall shields). Is conduit rigid steel or rigid aluminum? What other control circuits, if any, share the same conduit?
What is length of conduit?
GG

ps Note that I have never used "VFD Shielded" cable for any LV motor feeder in a VFD application. However this should reduce the likelihood of any noise being radiated to other nearby circuits.

pps If Skogs suggestion of adding a small cap to the RTD input device does not work, I would then consider installing a dv/dt filter on the output of the VFD.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)