# VFD Induced Temp Spike



## Big John (May 23, 2010)

Have you actually put a process meter on the 4-20 circuit to see if you can watch the current? Have you put a scope on the same circuit to look for non-DC interference?

The VFD may not be interfering with the conductors themselves, but it may be somehow interfering with the transducer or PLC proper.


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## JRaef (Mar 23, 2009)

Big John said:


> Have you actually put a process meter on the 4-20 circuit to see if you can watch the current? Have you put a scope on the same circuit to look for non-DC interference?
> 
> The VFD may not be interfering with the conductors themselves, but it may be somehow interfering with the transducer or PLC proper.


I think that's an excellent point, not all that uncommon either.

Also, you described the connection from MCC to drive, what about from drive to motor? That's the really nasty one, the output of a VFD is like a locally powerful radio transmitter. If you didn't use VFD shielded cable, did you at least use steel conduit? If it is shielded power cable for the motor leads, did you make sure to ground BOTH ends of that? Or if you used steel conduit, is BOTH ends of the conduit solidly grounded? The NEC doesn't require it, but prudence does.


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## maddhatter (May 7, 2012)

PLC code on the shonk?


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## oliquir (Jan 13, 2011)

already had this problem with pressure transducer, only way i found to solve problem was putting an isolated linear supply (the old ones with transformers, not switching units) to power that transducer.


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## Fixastang (Sep 4, 2012)

Big John said:


> Have you actually put a process meter on the 4-20 circuit to see if you can watch the current? Have you put a scope on the same circuit to look for non-DC interference?
> 
> The VFD may not be interfering with the conductors themselves, but it may be somehow interfering with the transducer or PLC proper.


Big John, I think we put a Fluke in line with the process meter at the PLC and saw the current increase. I will double check with the guy helping me troubleshoot this to determine if I am correct in what I am thinking. We have not used a scope to look for non-DC interference.


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## Fixastang (Sep 4, 2012)

JRaef said:


> I think that's an excellent point, not all that uncommon either.
> 
> Also, you described the connection from MCC to drive, what about from drive to motor? That's the really nasty one, the output of a VFD is like a locally powerful radio transmitter. If you didn't use VFD shielded cable, did you at least use steel conduit? If it is shielded power cable for the motor leads, did you make sure to ground BOTH ends of that? Or if you used steel conduit, is BOTH ends of the conduit solidly grounded? The NEC doesn't require it, but prudence does.


JRaef, The VFDs are mounted on the wall adjacent to the MCC. We originally had the contractor run the power and motor leads from the MCC to the VFD in PVC conduit. We then had the same contractor remove the PVC conduit and install metal conduit. We are NOT using shield VFD rated cable at this time. The Engineer who looked at the project did not think we would see this type of issues. 

The confusing part to me is there are about 10 other 4-20mA signals coming back to this PLC cabinet from the same building that are NOT affected by this VFD running. There are approximately 20 other 4-20mA signals located in the same PLC cabinet that have their shields connected to the same grounding point that are not being impacted by the VFD. I will see if we have a recording ammeter that I can install in line with the process meter and track the issue over time. Perhaps there is something else going on I am not seeing.

Thanks for all the help thus far. Hopefully, we will be able to fix this issue.


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## JRaef (Mar 23, 2009)

Fixastang said:


> JRaef, The VFDs are mounted on the wall adjacent to the MCC. We originally had the contractor run the power and motor leads from the MCC to the VFD in PVC conduit. We then had the same contractor remove the PVC conduit and install metal conduit. We are NOT using shield VFD rated cable at this time. The Engineer who looked at the project did not think we would see this type of issues.
> 
> The confusing part to me is there are about 10 other 4-20mA signals coming back to this PLC cabinet from the same building that are NOT affected by this VFD running. There are approximately 20 other 4-20mA signals located in the same PLC cabinet that have their shields connected to the same grounding point that are not being impacted by the VFD. I will see if we have a recording ammeter that I can install in line with the process meter and track the issue over time. Perhaps there is something else going on I am not seeing.
> 
> Thanks for all the help thus far. Hopefully, we will be able to fix this issue.


Shielded cable is usually not necessary if you use steel conduit. But to be precise, you said metal, which could mean aluminum (far fetched, I know), and that doesn't work as well as steel in shielding against radiated EMI/RFI.

But assuming it is steel, it is also key that it is GROUNDED properly. That means not gust terminated on the VFD enclosure and the motor frame, but that the enclosure and motor frame itself is connected to ground solidly and that bonding bushings are used on the conduit connectors. An often made error is in someone ASSuming this is the case, then later I come to investigate a problem they are having and find that the motors are on rubber vibration isolators and there is no ground strap. Or in one case, they used PVC, I told them to change to steel, they did, but they didn't use grounding bushings, and there was a PVC j-box in the middle of the run with no bonding strap through to the EMT going into each side, because the NEC didn't require it. 

Seemingly minor details can be important, especially if you know you already have a problem somewhere.

Also, aside from this, did you verify that it is only the transducer FALSELY showing a temperature increase, and the temperature is not ACTUALLY increasing? Because it might be that the VFD is not affecting the TRANSDUCER or it's signals, but is in some other way causing an actual increase in the gas temperature.


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## Big John (May 23, 2010)

Fixastang said:


> Big John, I think we put a Fluke in line with the process meter at the PLC and saw the current increase...


 Then look at your transducer. Interference from a VFD isn't gonna take the shape of normal 4-20mA signal. I'd bet a lot of money the transducer is changing impedance because of interference it is receiving.

What model temperature sensor is this? Where is it getting power from?


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